Market Sizing: Step-by-Step Guide with Examples (2026)

Market sizing cases ask you to estimate a market's total revenue, units, or customers using top-down or bottom-up approaches with explicit assumptions. They appear in roughly 25% of McKinsey, BCG, and Bain first-round interviews and in nearly every market entry case, according to IGotAnOffer. Interviewers score your reasoning structure more heavily than the exact number; your target is to land within one order of magnitude. This guide covers a repeatable 5-step method, 6 fully worked examples across B2C, B2B, and service markets, a reference data table for 5 countries, and the one step most candidates skip: stating what the number means for the client's strategy.

TL;DR — What you need to know

• Frequency: Market sizing shows up in 1 of 4 first-round MBB cases and nearly every market entry case (Management Consulted).
• Method: 5 steps. Clarify, choose top-down vs bottom-up, segment, calculate with round numbers, sanity-check and state implications.
• Time budget: A clean answer takes 3 to 6 minutes total. Spend 60 to 90 seconds on structure before any math.
• Top anchors to memorize: US population 340M (2024 Census Bureau), 130M households, $27T GDP. These three unlock most US sizing questions.
• Score multiplier: Always end with implications. "So what does this mean for the client?" turns a number into a recommendation.

What is market sizing?

Market sizing is a case interview question type where you estimate the total size of a market (revenue, units, or customers) using logical segmentation and stated assumptions, without research databases. It tests structure and communication, not memorization. Per PrepLounge, the three golden rules are answer-first structure, sanity check, and strategic implications.

Why does market sizing show up in every interview round?

The answer drives strategic decisions: enter this market, price at this point, target this revenue. McKinsey, BCG, and Bain use these questions to evaluate how you decompose ambiguity. The exact number matters less than clear MECE reasoning.

Per Management Consulted, it appears in ~1 in 4 first-round cases. Bain dedicates an entire first-round session to estimation in many countries; McKinsey often uses standalone sizing as a warm-up. We break down MBB pay vs Tier 2 in the 2026 Consulting Salary Report. For wider context, see the case interview frameworks complete guide.

Interviewers score 3 dimensions: structure (logical tree before numbers), judgment (assumptions tied to reference points), and communication (narrate as you go).

What are TAM, SAM, and SOM?

Markets break down from total opportunity to realistic target. Interviewers often want a specific slice.

• TAM (Total Addressable Market): Total demand at 100% share.
• SAM (Serviceable Available Market): The portion you can reach by geography, channel, and product fit.
• SOM (Share of Market): The portion you realistically capture given competition.

When the interviewer says "size this market," ask whether they want TAM or something narrower. The same logic feeds market entry framework cases, where TAM is the numerator for share calculations.

What numbers should I memorize before the interview?

Memorize these approximate figures so you can anchor estimates. Sourced from World Bank Open Data and US Census Bureau 2024 estimates.

How does the 5-step method work?

Use the same mental sequence every time. The fifth step (implications) turns a number into a recommendation, which is what consultants actually do.

Framework

Market Sizing Method

1. 01

   1. Clarify

   Define market, geography, units, time period

2. 02

   2. Choose approach

   Top-down or bottom-up. State and justify

3. 03

   3. Segment

   Build the tree. State assumptions per branch

4. 04

   4. Calculate

   Round numbers. Narrate as you go

5. 05

   5. Sanity-check + implications

   Benchmark, range, then so-what

This method aligns with how you would structure any case interview question. The difference: market sizing is self-contained because you generate your own data through assumptions.

Step 1: How do I clarify the question?

Lock down 3 things before any math:

• What are we sizing? Revenue, units, or customers produce different numbers.
• Geography and time. "US, last 12 months" is specific. "Global market" is not.
• Exclusions? B2B vs B2C, certain channels, certain segments.

Skipping clarification is the most common market sizing mistake (see how to practice case interviews). One or two sharp questions take 30 seconds and save you from solving the wrong problem.

Step 2: How do I choose top-down vs bottom-up?

• Top-down (demand-side): Start from GDP or population, apply filters.
• Bottom-up (supply-side): Start from one store, one user, one transaction, then scale up.

Top-down fits ~80% of consumer questions. Bottom-up wins for B2B, physical locations, and capacity-constrained markets. You can combine both. Cross-checking with the other approach is one of the highest-leverage moves you can make.

Step 3: How do I segment cleanly?

For customer segmentation in more complex sizing questions, break the population into 2 or 3 distinct groups with different usage patterns rather than applying one average. More than 3 segments and you will run out of time or compound arithmetic errors.

Write the formula on paper before any calculation. Each branch should be MECE: mutually exclusive, collectively exhaustive. A well-built tree means the interviewer can point at any node and ask "why that number?" and you have an answer ready.

Step 4: How do I calculate without errors?

• Use round numbers (340M, $75K). Mental math stays fast.
• State each assumption out loud so the interviewer can correct or steer.
• Anchor to reference points ("10% penetration, similar to early adoption curves").

For arithmetic tactics, see mental math for case interviews, case interview math mental shortcuts, and case interview math practice. Sharpen the calculation reflex with our timed math drills before interview week.

Step 5: How do I sanity-check and state implications?

Compare to a known benchmark ("~0.1% of GDP, plausible for this category"). State a range. Per IGotAnOffer's playbook: if you catch the mistake first, you stay alive; if the interviewer catches it first, you do not.

Then state implications:

• "At $30B, large enough to enter, but our client captures under 1% so differentiation is critical."
• "At 50K units per year, niche. Weigh whether margins justify the investment."
• "At ~$2B, 5% share in 3 years = $100M and ~200 enterprise reps at typical SaaS productivity."

Every number ties back to a recommendation. See case interview synthesis.

How would I size US coffee shop revenue (bottom-up)?

Clarify: US revenue of coffee shops (chains and independents) in the last 12 months. Revenue only.

Approach: Bottom-up. Number of shops is more reliable than counting drinkers.

Tree: Revenue = shops x average revenue per shop.

Estimate:

• Shops: US population ~340M. One shop per 3,000 people = ~110K shops. Round to 100K.
• Revenue per shop: Starbucks does ~$1.5 to $2M; independents far less. Assume $300K average. 100K x $300K = $30B.

Sanity-check: US food-away-from-home spending ~$1T per year (USDA ERS). Coffee shops at 3% of that ($30B) feels right. National Coffee Association reports 66% of Americans drink coffee.

Implications: "Mature, competitive market. For a new entrant, the play is not growing the pie but capturing share through differentiation (specialty drinks, convenience, subscription)."

Range: $25 to $35B.

How do I show sensitivity?

Vary one assumption to stress-test. +20% shops: ~$40B. -20% revenue per shop: ~$24B. Then: "My base is $30B. Range: $24 to $40B." The same analysis appears in profitability framework cases.

How would I size the US diaper market (bottom-up, consumer goods)?

Clarify: Disposable diapers used in the US per year.

Tree: Diapers = children in diapers x diapers per day x 365.

Estimate:

• Children in diapers: US births ~3.6M per year. Diapers from birth to age ~2.5 = 3.6M x 2.5 = 9M children.
• Diapers per day: Newborns ~10, toddlers ~5. Average ~7.
• Total: 9M x 7 x 365 = ~23B diapers per year.

Sanity-check: US diaper market ~$10 to $12B per year (Statista). At $0.30 to $0.40 per diaper, that implies 25 to 40B. Our 23B is on the low end; order of magnitude is right.

Implications: "High-volume business where small per-unit cost efficiencies create significant margin. A new entrant should think distribution scale and supply chain, not differentiation."

Range: 20 to 28B diapers.

How would I size US enterprise SaaS licenses (top-down, B2B)?

Clarify: Enterprise SaaS licenses (businesses with 500+ employees) sold in the US per year.

Estimate (top-down):

• Enterprises (500+ employees): ~20,000 per US Census SUSB.
• Average employees per enterprise: ~5,000 (skewed distribution).
• Total enterprise employees: 20,000 x 5,000 = 100M.
• SaaS licenses per employee: ~8 paid tools average (range 5 to 15).
• Total: 100M x 8 = 800M licenses per year.

Sanity-check: US enterprise SaaS market is ~$150 to $200B (Gartner). At $150 to $250 per license, 800M implies $120 to $200B. Aligns.

Implications: "Volume is enormous but the market is concentrated (Microsoft, Salesforce, SAP). A new entrant should target a specific workflow gap, not try to be the ninth tool on every desk."

Range: 600M to 1B licenses.

How would I size daily rideshare trips in Paris (top-down)?

Clarify: Daily rideshare trips in Paris (city proper, ~2.2M), all providers combined.

Estimate (top-down):

• Addressable users: 2.2M x 50% (ages 20 to 50) x 30% rideshare penetration = ~330K registered users.
• Daily activity: 5% ride on any given day = ~16,500 daily users.
• Trips per user per day: 1.5 (there and back, or one way).
• Total: 16,500 x 1.5 = ~25,000 trips per day.

Bottom-up cross-check: ~3,000 active drivers x ~9 trips per shift = ~27,000 trips per day. Consistent.

Sanity-check: Paris Metro carries ~4M trips/day. Rideshare at under 1% of metro volume is plausible.

Implications: "At 25K trips per day with average fare ~$15, the market is small in volume but high value. Competing on price against Uber requires massive subsidy. A premium or niche segment (airport, corporate) is a better angle."

Range: 20 to 35K trips per day.

How would I size US fitness industry revenue (top-down from GDP)?

This shows a pure top-down approach starting from GDP.

Clarify: Total US fitness revenue (gyms, studios, training, apps) in the last 12 months.

Estimate:

• US GDP: ~$27T. Consumer spending ~68% = ~$18T.
• Health and wellness spending: ~5% of consumer spending = ~$900B.
• Fitness as share of wellness: ~12% = ~$108B.
• Narrow to fitness services (excluding home equipment, ~30%): ~$75B.

Bottom-up cross-check: ~65M Americans have gym memberships (Health & Fitness Association). Average ~$550/year. Memberships alone: $36B. Add training, boutique studios ($150 to $200/month), and apps for ~$70 to $80B.

Sanity-check: Statista puts gym revenue at $35 to $40B; broader fitness services $80B+. Our $75B is in range.

Implications: "At $75B, large but fragmented. No single chain controls more than 10%. The opportunity is not more gyms but a platform across the fragmented ecosystem (scheduling, payments, content)."

Range: $65 to $90B.

What are the most common market sizing mistakes?

1. Skipping clarification and sizing the wrong thing or geography. Most frequent error, easiest to avoid.
2. Mumbling assumptions instead of stating them. The interviewer needs to hear each number and its rationale.
3. Overcomplicating with too many segments. Three is usually enough; more than five and you run out of time.
4. Ignoring the sanity check and not comparing to reality.
5. Stopping at the number without interpreting what it means.

For more on structuring case answers, see the market entry framework, growth strategy guide, and case interview examples for practice with different case types.

How do I practice market sizing efficiently?

Reading is step one. Skill comes from timed, evaluated repetition.

• Week 1: 10 questions in writing, no timer. Build pattern recognition on top-down vs bottom-up.
• Week 2: Add a 5-minute timer. Clean structure under pressure.
• Week 3: Practice out loud. Record yourself once and review.
• Week 4: Get evaluated. Human feedback is irreplaceable for structure points.

Once the method clicks, drill the market sizing questions library: 20 worked examples across tech, retail, healthcare, transportation, and B2B. Learn the method here, then run more reps on our free drill set or grab the free school casebooks. For sequencing, see the consulting interview prep timeline and case interview cheat sheet.

Can I test my market sizing instincts?

Test yourself

1 / 3

Question 1 of 3

You are sizing the US pet food market. You estimate 90M pet-owning households spending $500/year on pet food, giving $45B. US GDP is $27T. What percentage of GDP is your estimate, and is that plausible?

• A0.17% of GDP, plausible for a large consumer category
• B1.7% of GDP, too high for pet food
• C0.017% of GDP, too low for pet food

What sizing prompts should I drill?

Work through these. Your structure matters more than matching the exact number.

How does market sizing connect to other case types?

Market sizing rarely stands alone. It feeds bigger questions:

• Market entry: "Is this market big enough to enter?" See the market entry framework.
• Pricing: "What price captures maximum value?" Needs market size to compute revenue at different points. See pricing strategy cases.
• Growth: "Where should we grow next?" Sizing adjacent markets compares opportunities. See growth strategy cases.
• Profitability: "Why is revenue declining?" Distinguishing market shrinkage from share loss requires sizing. See the profitability framework.

For sequencing, see the consulting interview prep timeline. Firm-specific guides like the McKinsey case interview guide and BCG case interview guide flag where each firm leans hardest on sizing.

How fast should my mental math be?

Aim for under 10 seconds per operation without a calculator. Common ones: multiplying millions and billions (340M x 0.05 = 17M), percentages ("what is 0.2% of $27T?"), and division for sanity checks ("$45B / 340M = ~$132 per person per year"). Build that speed with our timed math drills, and see mental math for case interviews, case interview math mental shortcuts, and case interview math practice for full tactics.

Frequently Asked Questions

What is market sizing in a case interview?

A case question type where you estimate the total size of a market (revenue, units, or customers) using logical segmentation and stated assumptions. Per IGotAnOffer, it appears in ~1 in 4 first-round consulting interviews. Interviewers score structure more heavily than the exact number.

What are the steps in a market sizing method?

Clarify, choose top-down or bottom-up, segment with explicit assumptions, calculate using round numbers, and sanity-check while stating strategic implications. Most candidates skip implications.

Should I use top-down or bottom-up?

Top-down starts from population or GDP and filters down. Bottom-up builds from one unit (one store, one user). Top-down fits consumer markets ~80% of the time. Bottom-up fits B2B, infrastructure, and capacity-constrained markets. Cross-checking impresses interviewers.

How do I make my assumptions defensible?

State each assumption, use round numbers, and tie to reference points. Anchors: US population 340M (Census 2024), GDP ~$27T, households ~130M.

What is a good sanity-check?

Compare to known figures (similar markets, GDP share), check order of magnitude, and ask if the number is plausible for the geography. US consumer spending is 68% of GDP ($18 to $19T), a useful benchmark.

How often does market sizing appear in MBB interviews?

~25% of first-round cases and virtually every market entry case per Management Consulted. McKinsey uses standalone sizing as warm-ups; BCG and Bain embed it. All MBB expect a defensible estimate in 3 to 6 minutes.

How does this guide differ from the market sizing questions article?

This guide teaches the method. The companion market sizing questions article is a 20-question practice bank across tech, retail, healthcare, and B2B. Learn the method here, then drill the bank.

Sources (checked April 28, 2026)

IGotAnOffer market sizing guide, Management Consulted, PrepLounge basics, McKinsey, BCG, Bain, US Census Bureau 2024 population, US Census SUSB, World Bank Open Data, USDA ERS, Statista fitness, Statista diapers, Gartner, Health & Fitness Association, National Coffee Association.

Related guides

• Market sizing questions: 20 practice examples
• Mental math for case interviews
• Case interview math practice
• Profitability framework
• How to practice case interviews
• Case interview examples