Market Sizing Questions with Answers: 20 Examples + PDF Practice (2026)

Market sizing questions ask you to estimate a market's revenue, units or customers from stated assumptions, such as "What is the US market for telehealth?" Below are 20 market sizing interview questions with worked answers, each a short sample case study with assumptions, math and a sanity check. Try each prompt cold, then compare your structure with the solution.

Each case study stops at a final answer and a sanity check, so you can compare your formula before reading the math. Then use the market sizing drills to practise choosing a framework and review the explanation.

Use the worked examples here for review. Use the market sizing question tool for timed reps or the free consulting case book PDFs for longer partner cases.

Start with the five-step method below. Then test the method in the guided framework-choice drill before Question 1.

Market Sizing Questions and Answers: Quick Path

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1

Learn the format

17 steps

• Learn how to caseGuided learning · 35 min
• The Case Interview MethodLesson · 8 min
• Profitability & Turnaround: AfrihealSkill drill · 5 min
• Profitability & Turnaround: Berty BankSkill drill · 5 min

How to Approach Any Market Sizing Question

Before the questions, a fast recap of the five steps. The method itself, including when to use top-down vs bottom-up market sizing, is taught in the market sizing framework guide. Keep the free market size calculator open beside these questions if you want to check your arithmetic without breaking the flow of the estimate.

With the framework locked in, the 20 worked questions below cover top-down and bottom-up problems across six industries.

Tech and SaaS Market Sizing Questions

Question 1: What is the US market for project management software?

Framework choice: Top-down, revenue-based. Start from the number of US companies that use collaborative software tools, segment by size, and apply average contract value.

Key assumptions:

• ~6 million businesses in the US employ at least one person
• Companies with 10+ employees are the realistic buyers: ~1.5 million
• Penetration of dedicated project management software among these: ~55%
• Average annual contract value varies by company size; use a blended ACV of ~$2,000/year (SMB pulls this down from the ~$8,000+ that enterprise deals command)

Math walkthrough:

• Eligible companies: 1.5 million
• Penetration: 55% → 825,000 paying customers
• Blended ACV: $2,000/year
• Market size: 825,000 × $2,000 = $1.65 billion

Final answer: ~$1.6-1.7 billion annually

Sanity check: Industry reports peg the US project management software market at roughly $2-3 billion in 2025. Our estimate is slightly conservative because we excluded enterprise mega-deals (Salesforce, ServiceNow, and similar platforms with $50K+ contracts). Directionally correct: within one factor.

Question 2: What is the global market for cybersecurity software?

Framework choice: Top-down, revenue-based. Start from global enterprise IT spend and carve out the security allocation.

Key assumptions:

• Global IT spend is approximately $5 trillion/year (analyst firms such as Gartner publish this figure annually; share it as an estimate, not a memorized fact)
• Security as a share of IT budgets: historically 5-8%; use 5%, the low end, because much security spend sits inside other budget lines
• Coverage: this misses consumer antivirus and government spend; add a 20% uplift for those segments

Math walkthrough:

• Global IT spend: $5 trillion
• Security share: 5% → $250 billion
• Add consumer and government uplift (20%): $250B × 1.2 = $300 billion

Final answer: ~$300 billion globally (lower than a 6% share on $4.5 trillion would give, because current IT spend is closer to $5 trillion and we used the low end of the security share)

Sanity check: Published estimates of global cybersecurity spending vary with the definition used (software only, or software plus services and hardware), and commonly land in the low hundreds of billions of dollars. Our estimate of about $300 billion sits comfortably inside that range. If the interviewer challenges the IT spend figure, note that Gartner publishes annual global IT spending data that you'd reference in a real engagement.

Question 3: What is the US market for cloud storage (consumer)?

Framework choice: Top-down, demand-side. Anchor from the US adult population, filter by smartphone and laptop ownership, and apply paid tier penetration.

Key assumptions:

• US adults: ~260 million
• Own a smartphone or laptop and generate enough data to consider cloud backup: ~85% = 221 million
• Already using a free tier (iCloud, Google Drive, Dropbox): ~70% of those = 155 million
• Convert free to paid: ~25% = 39 million paying subscribers
• Average monthly spend: ~$3/month ($36/year): most users are on the lowest paid tier

Math walkthrough:

• Paying subscribers: 39 million
• ARPU: $36/year
• Market size: 39M × $36 = $1.4 billion

Final answer: ~$1.3-1.5 billion annually (US consumer segment only)

Sanity check: Cross-check from the largest provider. If about 130 million Americans use iCloud (an assumption: roughly one in three people) and 15-20% of them pay, that is 20-26 million payers. At $36/year that is $0.7-0.9 billion from Apple alone. Add Google One, Dropbox and Microsoft and ~$1.4 billion is plausible. We're in the right ballpark.

Question 4: What is the US market for corporate video conferencing tools?

Framework choice: Top-down, demand-side. Start from the US workforce, filter to knowledge workers, then apply paid-seat penetration and license pricing.

Key assumptions:

• US workforce: ~160 million
• Knowledge workers (office-based, remote, hybrid: roles where video conferencing is standard): ~60% = 96 million
• Seats per knowledge worker: one seat per person, but company contracts are priced per seat so this maps directly
• Average blended license price: ~$150/seat/year (mix of free tiers at large firms, Teams bundled in M365 at ~$120/seat, and standalone Zoom/Webex at ~$200/seat)

Math walkthrough:

• Knowledge workers: 96 million
• Paid seat penetration: ~65% (some use free tiers; large enterprise is largely paid) = 62 million paid seats
• Price: $150/seat/year
• Market size: 62M × $150 = $9.3 billion

Final answer: ~$9-10 billion annually

Sanity check: Microsoft disclosed Teams revenue bundled in M365 rather than standalone. Zoom reported about $4.7 billion in revenue worldwide in fiscal 2025 (year ended January 2025); add Teams' implicit video conferencing value and Webex, and $9-10 billion for the US market is reasonable. This question is intentionally tricky because Teams bundling makes clean market sizing difficult. The interviewer is testing whether you flag that complexity.

Question 5: What is the US market for HR software (SMB segment)?

Framework choice: Top-down. Define the SMB segment, estimate the number of qualifying companies, and apply average HR software spend per company.

Key assumptions:

• SMB defined as companies with 10-500 employees
• US companies in this range: ~1 million (there are ~6M businesses total; the 10-500 band captures roughly 1 million)
• HR software adoption rate in this segment: ~50% (many SMBs still use spreadsheets or paper)
• Average annual spend on HR software (payroll, benefits admin, ATS combined): ~$3,000/company

Math walkthrough:

• Companies in range: 1 million
• Adoption: 50% → 500,000 paying companies
• ACV: $3,000
• Market size: 500,000 × $3,000 = $1.5 billion

Final answer: ~$1.5 billion (SMB HR software, US only)

Sanity check: Rippling, Gusto, BambooHR, and Zenefits collectively serve several hundred thousand small businesses at sub-$500/month price points, and Gusto alone serves hundreds of thousands. At 500,000 paying companies in total, $1.5 billion feels modestly conservative: the real market is probably somewhat larger once payroll processing fees are included. This is a reasonable place to flag the likely undercount.

Consumer and Retail Market Sizing Questions

Question 6: What is the US market for meal delivery apps (DoorDash, Uber Eats)?

Framework choice: Top-down, frequency-based. Start from US adults who use food delivery, estimate order frequency, and apply average order value.

Key assumptions:

• US adults: ~260 million
• Smartphone owners who have tried food delivery at least once: ~55% = 143 million
• Active users (ordered in the last month): ~40% of that group = 57 million active users
• Average order frequency: ~2x/month
• Average order value (food + delivery fee + tip): ~$35

Math walkthrough:

• Active users: 57 million
• Orders/year: 2/month × 12 = 24 orders/user/year
• Total orders: 57M × 24 = 1.37 billion orders/year
• Revenue per order to platform: ~$8 (take rate of ~22% on $35 order)
• Platform revenue: 1.37B × $8 = $10.9 billion

Final answer: ~$10-11 billion (platform revenue, not GMV)

Sanity check: DoorDash reported ~$10.7 billion in revenue in 2024 (a figure that includes some non-US revenue). Uber Eats adds several billion dollars more, and Uber does not break out its US delivery revenue, so the US platform market is plausibly $13-15 billion in total. Our $10.9 billion looks modestly low, most likely from undercounting active users or order frequency. A good candidate notes this gap and explains that DoorDash alone validates the order of magnitude.

Question 7: What is the US market for premium dog food?

Framework choice: Top-down. Start from US dog owners, segment by willingness to pay for premium, and apply annual spend.

Key assumptions:

• US households: ~130 million
• Households with at least one dog: ~38% = 49 million dog-owning households
• Dogs: average 1.5 dogs per dog household = ~74 million dogs
• Premium food segment (defined as spending 2x+ the mass-market price): ~30% of dog owners
• Annual premium food spend per dog: ~$400/year (roughly $33/month: premium brands like Ollie or The Farmer's Dog start at $3-5/meal)

Math walkthrough:

• Dogs in premium segment: 74M × 30% = 22 million dogs
• Annual spend: $400/dog
• Market size: 22M × $400 = $8.8 billion

Final answer: ~$8-9 billion annually

Sanity check: The American Pet Products Association reports about $65.8 billion of US spending on pet food and treats in 2024. Assume dogs account for roughly half of that, about $30 billion, and that premium and super-premium lines are 25-30% of dog food by value. That gives roughly $8-9 billion, consistent with our estimate.

Take the pet-market estimate into a separate entry decision: assess which customer segments a pet-care business should pursue.

Use market sizing in a pet-care entry decision

Consumer ServicesHard

Start Pet Care Market Entry

Question 8: What is the US market for gym memberships?

Framework choice: Top-down. Start from US adults, filter by exercise behavior, and apply penetration and average membership price.

Key assumptions:

• US adults (18-70): ~200 million
• Adults who exercise regularly enough to consider gym membership: ~40% = 80 million
• Of those, who choose a gym vs home/outdoor workout: ~45% = 36 million gym members
• Average annual membership: mix of budget gyms ($240/year) and premium gyms ($1,200/year). Blended average ~$500/year.

Math walkthrough:

• Active gym members: 36 million
• Annual membership cost: $500
• Market size: 36M × $500 = $18 billion

Final answer: ~$18 billion annually

Sanity check: The Health & Fitness Association (formerly IHRSA) publishes industry data, but figures vary widely with whether boutique studios (SoulCycle, Pure Barre, Orangetheory) are included, so treat any single number as approximate. Boutique studios are plausibly a meaningful extra segment that our $18B leaves out. Our $18B covers traditional gyms and underweights boutique: a sharp candidate notes this gap and offers to add a boutique segment.

Question 9: What is the US market for used cars?

Framework choice: Bottom-up, transaction-based. Estimate the number of used car transactions per year and multiply by average transaction price.

Key assumptions:

• US licensed drivers: ~230 million
• Average vehicle ownership duration before selling: ~5 years
• Implies ~46 million vehicles change hands annually (230M ÷ 5 = 46M, though not every driver is a seller: adjust down to ~38 million for drivers without a vehicle to sell)
• Average used car transaction price: ~$24,000 blended (an assumption: dealer retail listing prices average about $25,500 per Cox Automotive, but private-party sales skew toward older, cheaper cars and pull the blend below that)

Math walkthrough:

• Used car transactions/year: ~38 million
• Average transaction price: $24,000
• Market size: 38M × $24,000 = $912 billion

Final answer: ~$0.9 trillion annually

Sanity check: Cox Automotive puts the average used-vehicle retail listing price at about $25,500 in 2024 and US used-vehicle sales at roughly 36-40 million units a year. At the full listing price that is $0.92-1.0 trillion; because private-party and older-car sales sell below dealer listings, our blended $24,000 gives a slightly lower ~$0.9 trillion, which agrees. A first pass at $28,000 would have given $1.06 trillion, above that range. A candidate who spots that and adjusts earns judgment points.

Healthcare Market Sizing Questions

Question 10: What is the US market for telehealth?

Framework choice: Top-down. Start from insured US adults, estimate telehealth adoption, and apply revenue per visit.

Key assumptions:

• US adults: ~260 million
• Adults with health insurance (private or public): ~88% = 229 million
• Adults who used telehealth at least once in the past year: ~35% of insured adults post-pandemic = 80 million
• Average telehealth visits per user per year: ~4 (primary care, mental health, urgent care)
• Average revenue per visit to the telehealth provider: ~$60 (reimbursement rate for a telehealth visit, below in-person)

Math walkthrough:

• Active telehealth users: 80 million
• Visits/year per user: 4
• Total visits: 320 million
• Revenue per visit: $60
• Market size: 320M × $60 = $19.2 billion

Final answer: ~$19-20 billion annually

Sanity check: McKinsey estimated in 2020 that up to $250 billion of US healthcare spend could be virtualized; that is a ceiling on spend that could move online, not today's revenue. Published estimates of actual telehealth revenue vary widely with the definition (visits only, or platforms and software too), so treat our $19B as an order-of-magnitude result. The most useful check is the mix: mental health telehealth (Teladoc, BetterHelp) typically earns well above the $60 average per visit, so our average may be low. A good candidate flags this sub-segment.

Question 11: What is the US market for fertility treatments?

Framework choice: Top-down. Start from women of childbearing age, filter by those experiencing infertility, and apply treatment cost.

Key assumptions:

• US women aged 20-44: ~52 million
• Share experiencing infertility (clinically defined as 12 months of trying without conception): ~12-15%; use 13% = 6.8 million women
• Share who seek medical treatment: ~40% = 2.7 million
• Share who pursue IVF (the highest-cost treatment): ~25% of those in treatment = 675,000 cycles/year (first pass; revised below)
• Average IVF cycle cost: ~$15,000 per cycle (includes monitoring, retrieval, transfer; excludes medications)
• Remaining 75% pursue lower-cost treatments (IUI, ovulation induction) at ~$1,500/year average

Math walkthrough:

• IVF market: 675,000 cycles × $15,000 = $10.1 billion
• Other treatments: 2.7M × 75% × $1,500 = $3.0 billion
• Total market: $13.1 billion

Final answer: ~$10 billion annually, after the check below corrects the IVF volume. The first pass gave $13.1 billion.

Sanity check: The CDC reported about 435,000 assisted reproductive technology cycles in 2022 (mostly IVF, on about 250,000 patients). We assumed 675,000, which is about 55% too many. Cycle volume is the best-anchored number in the problem, so fix the assumption rather than the answer. 435,000 cycles is 16% of the 2.7 million women in treatment, not 25%. Recalculating with the other inputs unchanged: IVF 435,000 × $15,000 = $6.5B; other treatments 2.7M × 84% × $1,500 = $3.4B; total about $9.9B, so $10B. A candidate who proactively questions their IVF assumption and adjusts is demonstrating exactly the judgment interviewers reward.

Question 12: What is the US market for diabetes management devices?

Framework choice: Top-down. Start from US diabetics, segment by type, and apply device spend per patient.

Key assumptions:

• US adults with diabetes: ~38 million (approximately 11.5% of the US population: this is a commonly cited CDC figure worth knowing)
• Type 1 vs Type 2 split: ~5-10% Type 1 (more device-intensive) = 2.5 million; ~90% Type 2 = 35.5 million
• Device spend varies by management intensity:
  • Type 1 patients: ~$2,500/year in devices (CGM sensors, insulin pumps, supplies)
  • Intensively managed Type 2 patients (~10%, mainly insulin users on a CGM or pump): ~$1,500/year
  • Other Type 2 patients (~90%): ~$150/year (many test rarely, if at all, so average meter and strip spend is low)

Math walkthrough:

• Type 1: 2.5M × $2,500 = $6.25 billion
• Intensive Type 2: 35.5M × 10% × $1,500 = $5.3 billion
• Other Type 2: 35.5M × 90% × $150 = $4.8 billion
• Total: $6.25B + $5.3B + $4.8B = $16.4 billion

Final answer: ~$16 billion annually

Sanity check: A first draft with 30% of Type 2 patients on intensive management at $1,500 and $300 for everyone else gave about $29.7 billion, which felt large. Cross-check: Dexcom reported about $4 billion in 2024 revenue and Abbott's diabetes care business (mostly FreeStyle Libre) about $6.4 billion, so global CGM is roughly $10 billion. If the US is about half to 60% of that (an assumption), US CGM is $5-6 billion. Add insulin pumps and consumable supplies (assume roughly $3 billion) and glucometers, strips and lancets (assume $4-5 billion) and the US device market lands around $12-15 billion. The first draft was about twice that, so we cut the intensive share and the meter spend. $16 billion is slightly high but defensible. The interviewer will likely find the follow-up more interesting: "what's driving growth in this market," which is CGM adoption among Type 2.

Transportation Market Sizing Questions

Question 13: What is the US market for rideshare (Uber and Lyft)?

Framework choice: Bottom-up, transaction-based. Estimate daily rides and multiply by average fare.

Key assumptions:

• US population living in metro areas (rideshare is primarily urban): ~65% of 340M = 221 million
• Adult rideshare users (have app, used in past year): ~25% = 55 million users
• Average trips per user per month: 3 (some heavy users at 20+/month, most at 1-2; weighted average ~3)
• Average fare per trip: ~$18

Math walkthrough:

• Annual trips: 55M users × 3 trips/month × 12 months = 1.98 billion trips
• Revenue per trip (platform gross revenue): $18
• Gross market size (GMV): 1.98B × $18 = $35.6 billion

Final answer: ~$36 billion GMV annually; platform revenue (take rate ~25%) = ~$8.9 billion

Sanity check: Lyft reported 828 million rides and $16.1 billion of gross bookings in 2024, nearly all in North America, so our 1.98 billion trips implies Uber carries about 1.15 billion US trips, which is plausible. Uber does not break out the US, but its global Mobility gross bookings were $22.8 billion in the fourth quarter of 2024 alone; if the US is about a third of that (an assumption), Uber adds roughly $30 billion, for a combined US total near $45 billion. Our $36 billion is modestly conservative, probably because we underestimated heavy users (business travelers, urban non-car-owners). Close enough for a case interview estimate.

Question 14: What is the US market for electric vehicle charging infrastructure?

Framework choice: Top-down, user-based. Start from EVs on the road, split owners by how much they rely on public charging, then apply sessions and revenue per session.

Key assumptions:

• US EVs on the road: ~4 million (as of 2025)
• Share using primarily public charging vs home charging: ~20% rely heavily on public (apartment dwellers, long-distance travelers) = 800,000 heavy public users
• Public charging sessions per heavy user per month: ~6 sessions (once or twice a week for the most active users)
• Occasional public charger users (80% of EV owners, 1 session/month): 3.2M × 1 = 3.2 million sessions/month
• Revenue per session: ~$12 (mix of Level 2 at ~$6-8 and DC fast charging at $15-25)

Math walkthrough:

• Heavy user sessions/month: 800,000 × 6 = 4.8 million
• Occasional user sessions/month: 3.2 million
• Total sessions/month: 8 million
• Annual sessions: 8M × 12 = 96 million
• Revenue per session: $12
• Market size: 96M × $12 = $1.15 billion

Final answer: ~$1.1-1.2 billion annually

Sanity check: This is a rapidly growing market. EVgo reported about $257 million of revenue in 2024 and Blink about $126 million (much of it hardware sales), and Tesla's Supercharger network, the largest, does not report charging revenue separately. Reported charging-network revenue is therefore well below our $1.1 billion, so our estimate looks ahead of current reality, though it is reasonable as a 2025-2026 forward view given EV adoption growth. A good candidate notes this is a supply-constrained market: charging infrastructure buildout is lagging EV adoption.

Question 15: What is the US market for domestic airline travel?

Framework choice: Bottom-up, passenger-based. Estimate domestic passenger boardings and multiply by the average fare per boarding.

Key assumptions:

• US domestic passenger boardings per year: ~850 million (the Bureau of Transportation Statistics counts about 70 million domestic passengers in a typical month, roughly 850 million a year)
• First-pass average fare per boarding: ~$250 (blended between short-haul budget fares ~$150 and longer routes ~$350)

Math walkthrough:

• Annual domestic boardings: 850 million
• Average fare per boarding: $250
• First-pass market size: 850M × $250 = $212 billion

Final answer: ~$125 billion a year (domestic passenger fares only), after the sanity check below lowers the fare per boarding to about $145. The first pass at $250 gave $212 billion.

Sanity check: Delta ($61.6 billion) and United ($57.1 billion) alone reported about $119 billion of total revenue in 2024, and the four largest US airlines together well over $170 billion, across domestic and international flying, cargo and loyalty income. If domestic passenger fares are roughly 55-65% of that (an assumption), the big four earn about $100-130 billion from domestic fares, and smaller carriers add a little more. Our first pass of $212 billion is far too high, and the cause is the fare. The Bureau of Transportation Statistics reports an average domestic itinerary fare of about $384 in 2024, a mix of one-way and round-trip tickets. Assuming a typical itinerary is about half a round trip, that is roughly $190 per one-way trip. A one-way trip is often more than one boarding because of connections; assuming about 1.3 boardings per one-way trip, that is about $145 per boarding. Recalculating: 850M × $145 = $123 billion, so ~$125 billion, inside the cross-check range. A candidate who arrives at $212B, notices it feels large, and adjusts the fare assumption downward is demonstrating strong sanity-check instincts.

B2B and Enterprise Market Sizing Questions

Question 16: What is the US market for corporate legal services?

Framework choice: Top-down. Start from large corporations and work through their average annual legal spend.

Key assumptions:

• Fortune 500 companies: 500 firms with extensive in-house and outside counsel relationships
• Mid-size companies ($50M-$1B revenue): ~20,000 firms
• Small businesses with legal needs ($5M-$50M revenue): ~200,000 firms
• Annual outside counsel spend by segment:
  • Fortune 500: ~$100M/year average outside legal = $50 billion total (very large companies run many matters at once: litigation, deals, regulatory work)
  • Mid-size: ~$750K/year = $15 billion total
  • Small business: ~$40K/year = $8 billion total

Math walkthrough:

• Fortune 500: 500 × $100M = $50.0 billion
• Mid-size: 20,000 × $750K = $15.0 billion
• Small business: 200,000 × $40K = $8.0 billion
• Total: $73 billion

Final answer: ~$75 billion (US corporate outside-counsel spend)

Sanity check: Market-research firms put total US legal services at roughly $300-400 billion depending on the source and scope (Grand View Research says about $396 billion; others report $305-317 billion). That total includes individuals, government and in-house work, so corporate outside counsel is only part of it. If corporate clients are about 20-30% of the total (an assumption), the segment is $60-120 billion, and some industry trackers put business-law revenue near $190 billion. On that second benchmark our $75 billion is probably well below the real figure; both benchmarks are assumptions to test. A first pass with $30M for the Fortune 500, $500K for mid-size firms and $20K for small firms gave only $29 billion, under 10% of the total, which is too small for the segment that pays for most large-firm work, so we raised the per-company spend. A sharp candidate says the estimate is probably well below the real figure and names the usual suspects: companies outside these three tiers and spend by large private companies.

Question 17: What is the US market for management consulting?

Framework choice: Top-down. Estimate the total number of consulting engagements, weighted by firm tier and project size.

Key assumptions:

• Fortune 1000 companies: 1,000 firms; virtually all use external consultants
• Average annual consulting spend among Fortune 1000: ~$80M/year (the large caps run strategy, operations and transformation programs across many business units at once, so the average is pulled up by a long tail of very large spenders)
• Mid-size corporations ($100M-$1B revenue): ~25,000 companies; average annual spend ~$500K/year
• Government sector consulting (federal, state, local): roughly 20% of total private sector spend

Math walkthrough:

• Fortune 1000: 1,000 × $80M = $80 billion
• Mid-size: 25,000 × $500K = $12.5 billion
• Subtotal private sector: $92.5 billion
• Add government (20% of private): $18.5 billion
• Total: $111 billion

Final answer: ~$110 billion (US management consulting, strategy and operations)

Sanity check: Published estimates of the US management consulting market range widely with scope: some market-research estimates are on the order of $110-130 billion (treat these as rough, unverified figures), while IBISWorld, which uses a broad industry code that includes IT and other advisory work, reports a figure above $400 billion. Our ~$110 billion matches the narrower, strategy-and-operations definition. A first pass with a $15M Fortune 1000 average and $200K for mid-size firms gave only $24 billion, about a fifth of the lowest published figure, so we raised the per-company spend. A great candidate clarifies this scope question upfront.

Question 18: What is the US market for employee benefits administration software?

Framework choice: Bottom-up. Start from mid-to-large companies that outsource benefits administration, apply headcount, and price per employee.

Key assumptions:

• US companies with 50+ employees: ~200,000 (using the NAICS count of employers)
• Adoption of dedicated benefits admin software: ~70% = 140,000 companies
• Average employees per company (across the 50-10,000 range): ~250 employees
• Price per employee per month (PEPM model): ~$5-8/month; use $6

Math walkthrough:

• Total employees covered: 140,000 companies × 250 employees = 35 million employees
• Annual revenue per employee: $6/month × 12 = $72/year
• Market size: 35M × $72 = $2.52 billion

Final answer: ~$2.5 billion annually

Sanity check: Benefits administration vendors such as Benefitfocus (before its 2023 acquisition), Businessolver and bswift are each generally understood to earn revenue in the low hundreds of millions of dollars (an estimate, not a checked figure). Add the benefits modules inside ADP and Workday and a $2.5 billion market would need roughly ten vendors of that size. That is at the high end of plausible, so a good candidate says the estimate may be generous and that $6 per employee per month is the assumption to test first.

The Unusual Ones

These two questions are designed to unsettle you. The methodology is identical to any other market sizing question: the interviewer is simply testing whether unusual framing disrupts your structure.

Question 19: How many golf balls fit inside a standard school bus?

Framework choice: Volume estimation. Calculate the interior volume of the bus, the volume of one golf ball, and apply a packing efficiency factor.

Key assumptions:

• School bus interior dimensions: length ~6.5m, width ~2m, height ~1.8m (about 70% of that space is usable once seats and wheel wells are removed)
• Golf ball diameter: 4.3 cm (standard golf ball is 42.7mm; round up to 4.5cm for ease)
• Golf ball volume: V = (4/3) × π × r³ = (4/3) × 3.14 × (2.25cm)³ ≈ 47.7 cm³; round to ~48 cm³
• Packing efficiency (spheres in a box, random packing): ~64% of volume is actually occupied by spheres

Math walkthrough:

• Bus interior volume: 6.5m × 2m × 1.8m = 23.4 cubic meters = 23,400,000 cm³
• Usable space (remove seats, wheel wells): reduce by 30% → ~16,400,000 cm³
• Apply packing efficiency (64%): 16,400,000 × 0.64 = 10,496,000 cm³ filled by golf balls
• Golf balls: 10,496,000 ÷ 48 = 218,667 golf balls

Final answer: ~220,000 golf balls

Sanity check: Cross-check per cubic meter: 1,000,000 cm³ × 64% ÷ 48 cm³ is about 13,000 balls per cubic meter, so ~16 cubic meters of usable space holds about 210,000 balls, in line with the main estimate. A range of 150,000 to 250,000 covers reasonable seat and packing assumptions, and it is what interviewers accept. What they're watching: did you state your formula, make explicit assumptions about packing efficiency, and show your work step by step?

Question 20: How many piano tuners are there in Chicago?

Framework choice: Bottom-up, capacity-based (this is the classic Fermi estimation question, popularly attributed to physicist Enrico Fermi). Start from demand (pianos in Chicago), calculate maintenance frequency, and divide by tuner capacity.

Key assumptions:

• Chicago population: ~2.7 million people; ~1 million households
• Households with a piano: ~2% = 20,000 pianos (piano ownership has declined significantly; this is reasonable for a large city)
• Institutions (schools, churches, concert halls, bars, restaurants): ~3,000 pianos in Chicago (rough estimate)
• Total pianos: ~23,000
• Pianos tuned once per year on average: 23,000 tunings/year
• A piano tuner works 8 hours/day, 5 days/week, 50 weeks/year = 2,000 hours/year
• Time to tune one piano, including travel: ~2 hours
• Tunings per tuner per year: 2,000 ÷ 2 = 1,000 tunings/year

Math walkthrough:

• Total tunings needed per year: 23,000
• Tunings per tuner per year: 1,000
• Number of tuners needed: 23,000 ÷ 1,000 = 23 tuners

Final answer: ~20-30 piano tuners in Chicago

Sanity check: Professional directory listings are one way to check: count the tuners listed for Chicago and see whether a few dozen is the right order of magnitude. Textbook versions of this problem often land near 100 because they assume far higher piano ownership than today. Our lower figure reflects the decline in piano ownership we assumed. The methodology is more important than the final number. Arriving at an estimate within the right order of magnitude with pure logic is what earns full marks.

Common Market Sizing Mistakes (and How to Fix Them)

These five mistakes appear repeatedly across candidate performance. Each one has a pattern fix.

Mistake 1: Jumping straight to numbers without writing out the formula

What it looks like: "OK so there are 340 million people in the US, and maybe 40% of them drink coffee, so that's 136 million, and maybe they spend $5 a day: wait, that seems too high, let me try $3..."

Why it costs you: The interviewer can't follow your logic. You're solving an algebra problem without showing the equation first.

Fix: Write the formula first, out loud: "My formula is going to be: total US adults × percentage who are regular coffee shop visitors × average visits per month × average spend per visit. Let me build each component." Then calculate.

Mistake 2: Asking the interviewer for data

What it looks like: "Do you know what percentage of households own a dog?" or "Can you tell me what the average price is?"

Why it costs you: Interviewers deliberately leave data gaps. The question is testing your ability to build defensible assumptions, not recall facts. Asking signals you don't know how to operate without a data room.

Fix: Make an assumption and state your reasoning: "I don't know the exact figure, but based on what I know about US pet ownership rates, I'll estimate 38% of households own a dog: I'm happy to revisit this if you want to push that up or down."

Mistake 3: Unrealistic penetration rates

What it looks like: "80% of companies will adopt this new software" or "60% of people over 65 use streaming services."

Why it costs you: Experienced interviewers have mental benchmarks for adoption curves. Claiming 80% penetration for almost anything (except basic utilities) is a red flag that signals poor market intuition.

Fix: Default to conservative penetration rates and be explicit about your rationale. Early-market adoption: 5-15%. Mature consumer product with mass appeal: 30-50%. Ubiquitous infrastructure (email, smartphones, running water): 80-95%. Use the product lifecycle to anchor your estimate.

Once the assumption is defensible, practise applying the percentage accurately so arithmetic does not undermine the estimate.

Mistake 4: Skipping the sanity check

What it looks like: Arriving at an answer and stating it without any verification: "So the US market for gym memberships is $18 billion."

Why it costs you: You leave the interviewer no view of whether you believe your number. Sanity checks are also the place where interviewers give partial credit: if your math had an error but your sanity check caught it, you've demonstrated the judgment that matters most.

Fix: Always close with: "Does this feel right? Let me cross-check: [Competitor X] does roughly [Y] in revenue and commands [Z]% market share, which implies a total market of [W]. That's in the same range as my estimate, so I'm comfortable with [$X] as my headline answer."

Mistake 5: Over-complicating the segmentation tree

What it looks like: Splitting US adults into 7 demographic sub-groups before applying a purchase frequency matrix with 4 product categories, resulting in 28 cells of arithmetic.

Why it costs you: You lose the interviewer, make calculation errors, and spend 12 minutes on a question that should take 5. More granularity does not mean more rigor in market sizing.

Fix: The best market sizing formulas have 3-5 variables. If you need more, you're splitting segments that shouldn't be split. Ask yourself: "Does this segmentation change the final answer by more than 20%?" If no, collapse the segments.

Spotting the mistake in someone else's answer is one skill. Catching it in your own answer is harder. The framework-choice drill above explains the setup; use the market sizing question tool to practise a complete estimate and review your assumptions, arithmetic and sanity check.

How to Practice Market Sizing

Reading worked solutions is a starting point, but the skill is built through timed, evaluated repetition. Here is the sequence that moves candidates from "reads solutions" to "delivers them confidently":

Week 1: Build pattern recognition. Work through 10 questions in writing, without a timer. For each question, write out the formula before calculating, and always end with a sanity check. The goal is to internalize the top-down/bottom-up decision and learn the formula template.

Week 2: Add time pressure. Set a 5-minute timer per question. You should be able to complete a full market sizing answer, including the formula, calculation, and sanity check, in under 5 minutes in writing before you can do it under 6 minutes verbally.

Week 3: Practice out loud. Narrate your solution to an empty room, then to a practice partner. The cognitive load of speaking while calculating is significant: it requires its own practice. Record yourself once and review: were you pausing too long between steps? Were you narrating each assumption as you made it?

Say the calculation aloud on fresh numbers now, so you can hear whether your arithmetic still holds together under time pressure.

Week 4: Get evaluated. Human feedback is irreplaceable for structure points, but it's expensive and hard to schedule at 11pm. AI-assisted practice can fill the gaps, not to replace human feedback, but to give you more at-bats with instant scoring on framework quality.

Prep plans for McKinsey, BCG, Bain, Deloitte, Accenture, EY-Parthenon, Kearney, Oliver Wyman, Roland Berger, L.E.K., OC&C, Strategy&, Simon-Kucher, Arthur D. Little, Capital One and your target firm. Choose your firm in the plan builder.

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1

Learn the format

17 steps

• Learn how to caseGuided learning · 35 min
• The Case Interview MethodLesson · 8 min
• Profitability & Turnaround: AfrihealSkill drill · 5 min
• Profitability & Turnaround: Berty BankSkill drill · 5 min

For a more structured practice progression, see case interview practice. If your current weakness is the mental math side, case interview math mental shortcuts covers the calculation techniques that let you move faster without a calculator. And for the adjacent skill of abstract brain teasers, consulting brain teasers covers the non-market-sizing estimation questions that occasionally appear at MBB.