Market Sizing Questions: 20 Free Examples with Solutions

Why market sizing questions matter in case interviews

How to answer market sizing questions: the 5-step framework

10 market sizing questions with full solutions

Each example shows the problem, the approach (how to structure and assume), and the answer (the math and the sanity check). Work through them with a timer before you read the solution.

1. Example 1

   How many tennis balls fit inside a Boeing 737?

   Approach

   Volume problem. Approximate the 737 cabin as a cylinder 30m long with a 3m diameter, so volume is roughly pi times 1.5 squared times 30, or about 210 cubic meters. A tennis ball has a volume of about 150 cubic centimeters. Apply a 60 percent packing efficiency to account for gaps and seats.

   Answer

   210 cubic meters is 210 million cubic centimeters. Divide by 150 and apply the 60 percent factor: (210,000,000 / 150) times 0.6 equals about 840,000 tennis balls. Round to roughly 800,000 to 900,000.

2. Example 2

   Estimate the annual US market size for ride-sharing services.

   Approach

   Bottom-up usage model. Start with 330M US population, assume 70 percent are adults with smartphones, so 230M potential users. Assume 40 percent use ride-sharing, giving 92M active users. Average 3 rides per month at $18 per ride.

   Answer

   92M users times 3 rides times 12 months equals 3.3B rides. Multiply by $18 per ride: about $60B annually. Sanity check: Uber and Lyft combined US revenue lands in this zone, so the estimate is defensible.

3. Example 3

   How many coffee cups are sold daily in New York City?

   Approach

   Population-based. NYC has roughly 8.5M residents plus 2M daily commuters and tourists, call it 10M relevant people. Assume 60 percent drink coffee daily, averaging 1.5 cups each, with 40 percent of cups bought out rather than made at home.

   Answer

   10M times 0.6 times 1.5 equals 9M cups consumed daily. Apply the 40 percent purchased-out share: 3.6M cups sold per day, or roughly 3 to 4 million.

4. Example 4

   What is the US market size for electric toothbrushes?

   Approach

   Replacement cycle model. 330M people, 80 percent brush teeth regularly, so 265M users. Assume 35 percent use electric toothbrushes, giving 93M electric users. Replacement cycle is 3 years at an average price of $60.

   Answer

   93M users divided by 3 years equals 31M units sold per year. At $60 per unit, the annual US market is about $1.9B. Round to roughly $2B.

5. Example 5

   How many piano tuners work in Chicago?

   Approach

   Classic Fermi problem. Chicago metro has 9M people, average 2.5 per household means 3.6M households. Assume 3 percent own a piano, giving 108,000 pianos. Each piano is tuned once per year. One tuner handles 4 tunings per day, 5 days a week, 50 weeks a year, so 1,000 tunings per tuner annually.

   Answer

   108,000 tunings divided by 1,000 per tuner equals about 100 piano tuners in Chicago. Real-world directory listings hover in this same range, which validates the method.

6. Example 6

   Estimate the global revenue for the streaming music industry.

   Approach

   Top-down subscription model. Global population 8B, assume 50 percent have internet access (4B), and 20 percent pay for streaming music (800M paying subscribers). Average price $8 per month. Add 15 percent uplift for ad-supported revenue.

   Answer

   800M times $8 times 12 equals $77B in subscription revenue. Apply the 15 percent ad uplift: about $88B total, or roughly $85B to $90B globally. Spotify plus Apple plus YouTube Music triangulate to this order of magnitude.

7. Example 7

   How many diapers are used in the US per year?

   Approach

   Demographic model. US births per year: about 3.7M. Assume diapers are used from birth to age 3, so 3 birth cohorts are in diapers: 11M children. Average 6 diapers per day per child.

   Answer

   11M children times 6 diapers times 365 days equals 24B diapers annually. At roughly $0.25 per diaper, the market is about $6B in raw unit value before brand and retail markup.

8. Example 8

   What is the annual market size for gym memberships in the UK?

   Approach

   Penetration model. UK population 67M, assume 80 percent are adults (54M). Gym penetration is roughly 15 percent, giving 8M members. Average monthly price is 40 pounds.

   Answer

   8M members times 40 pounds times 12 months equals about 3.8B pounds annually. Round to roughly 4B pounds. The real UK health and fitness market sits in this band, confirming the model.

9. Example 9

   How many gas stations are there in the United States?

   Approach

   Demand-driven model. 330M people, assume 230M drivers, each filling up 1 time per week (50 fills per year). One gas station services 300 cars per day across 8 pumps, or roughly 100,000 fills per year per station.

   Answer

   230M drivers times 50 fills equals 11.5B fills annually. Divide by 100,000 per station: 115,000 gas stations. The actual figure is roughly 115,000 to 150,000, so this anchors cleanly.

10. Example 10

    Estimate the number of commercial flights per day worldwide.

    Approach

    Capacity model. Assume 25,000 commercial aircraft globally, and each plane flies an average of 4 flights per day when active. Assume 80 percent of the fleet is active on any given day.

    Answer

    25,000 times 0.8 times 4 equals 80,000 flights per day. The ICAO reported figure in normal years is roughly 100,000, so we are within the acceptable half-to-double range. Flag the gap and adjust active-fleet assumption upward if pressed.

10 more market sizing practice questions (solve these yourself)

5 common mistakes candidates make on market sizing questions

Most common mistake we observe in practice sessions: candidates jump into arithmetic before their structure is locked. Fix these five and you will outrank 80 percent of the field.

• Jumping to math before structuring
  Spend the first 60 seconds building your tree and stating assumptions before touching a single number. Interviewers grade the structure, not the speed.
• Using messy, uneven numbers
  Round to clean numbers: 300M for US population, 8B for world, 10 per household. Ugly numbers waste time and introduce arithmetic errors that kill credibility.
• Hiding assumptions in your head
  Say every assumption out loud and write it down. If the interviewer disagrees with an assumption, you can swap it cleanly without redoing the whole tree.
• Skipping the sanity check
  After the answer, compare it to a reference point: a known company's revenue, a country's GDP, a city's population. An unchecked answer reads as careless.
• Freezing when you do not know a base fact
  Make a defensible guess and move on. If you do not know the US population, say 'I will use 300M as a round anchor' and the interviewer will correct you only if it matters.

Market sizing questions: frequently asked

What are market sizing questions in a case interview?

Market sizing questions ask you to estimate the size of a market, population, or quantity from scratch with no data. Interviewers use them to test structured thinking, assumption quality, and mental math under pressure. You are judged on your logic, not the final number.

How do I answer a market sizing question step by step?

Restate the question, clarify the unit and geography, lay out a top-down or bottom-up structure, state each assumption out loud, do the math in round numbers, and sanity-check the answer against a known anchor. Close with a one-line summary of what the number means.

Are market sizing questions and guesstimate questions the same thing?

Practically, yes. Consulting firms say market sizing, tech and product interviews say guesstimate, and brainteaser interviews use both. The structure, framework, and evaluation criteria are identical, so one prep block covers both formats.

How accurate does my answer need to be?

Accuracy matters less than logic. Interviewers accept answers within roughly a half-to-double range of the real number as long as the assumptions are defensible. A clean structure with a wrong mid-number beats a lucky number with no structure.

How much should I practice market sizing before interviews?

Plan for 15 to 20 reps across different industries before your first real case. Mix population-based, usage-based, and revenue-based prompts, and always practice out loud with a timer. Ten minutes per prompt is the right length for live interview pacing.

Related resources

• Market Sizing Framework
  The top-down and bottom-up structures that work across any prompt.
• Market Sizing Step-by-Step
  A walkthrough of a full market sizing answer, timing and all.
• Case Interview Math: Mental Shortcuts
  Fast multiplication, division, and rounding tricks for live cases.
• Case Interview Math Practice
  Timed drills to sharpen your math before interview week.