Market Sizing Questions: An Eleven-Question Drill on Method

Twenty seconds of silence

It can happen in a case interview, or on a Tuesday when your manager turns around: “Roughly how big is the market for dog grooming in this city? Ballpark is fine.” The first twenty seconds decide how it goes. Some people start multiplying. Some people freeze. The ones who do well ask a question first.

This drill has eleven questions on the moves that make an estimate defensible: clarifying the market, choosing a direction, matching units, finding countable buyers, checking the result and presenting it. Every answer comes with the reason. You leave with a score, and with the one move to practice before the next time someone asks.

The idea behind it: a market sizing question is graded on the parts, not the total. Nobody remembers your number. They remember whether they could follow it, and whether you told them where it might break.

Nobody remembers your number. They remember whether they could follow it.

What the question is really testing

Bain describes its case interviews plainly: “We're not looking for a single right answer; we're looking for how you think, structure problems, and build on ideas as the conversation evolves” ¹. Its tips start with “Make sure you understand the question before diving in” and include “Share your reasoning and calculations, even if you’re unsure” ¹.

The same holds after you get the job. An estimate given in a meeting is a hypothesis other people will test. If they can see the parts, they can improve them. If they see only a total, they can only believe it or not.

The five moves

1. Clarify the marketRevenue or units? Which year, which geography, and what counts as the product? One question here can change the answer tenfold.
2. Pick a directionBottom-up when you can picture the buyer and count them; top-down when a credible total exists. Do both if you have time; the market sizing calculator runs them side by side.
3. Write the formula before any numberSay the chain out loud: households × share with a dog × share using a groomer × visits per year × price. The listener can agree with the structure before arguing about any input.
4. Round hard and count zerosUse numbers you can multiply in your head. Most arithmetic errors in these questions are a misplaced zero, not a wrong digit.
5. Check it and name the swingTurn the total back into a rate you can picture, then say which assumption moves the answer most and how you would check it.

Reading your score

The bands are our rubric, not a validated scale. Use them to decide what to practice.

• Below 50: rebuild the method. Practice the first three moves on paper with no arithmetic at all: clarify, choose a direction, write the formula. Ten everyday markets, five minutes each.
• 50 to 79: sound, with gaps. Look at the questions you missed. Units and sanity checks are the usual gaps, and both can be fixed with a habit: write the unit beside every number, and convert every total back into a rate.
• 80 and above: interview-ready method. Move to speed and to real data. Size one market a week from a public source and see how far your assumptions were from the count.

Where the drill stops

The drill tests method, and method is the part an interview can see. Real sizing work adds two things no quiz can. The first is data. For US business buyers you rarely need to guess a count: County Business Patterns covers establishments with paid employees ², and Nonemployer Statistics covers businesses with none ³.

The second is judgment about which market to size. In 2014 Aswath Damodaran sized Uber's market from taxi and limo spending, $100 billion ⁴. Bill Gurley replied that “The past can be a poor guide for the future if the future offering is materially different than the past” ⁵. Both were careful, and their method was not the issue; they disagreed about the starting market. No drill settles that. It is the question to raise out loud whenever the product changes how people buy.

For the full method on a real case, read TAM, SAM, SOM, done honestly. Then come back to the drill in a week. The score matters less than whether your next estimate shows its parts.

Questions

What are market sizing questions?

They ask you to estimate the size of a market, such as annual spending on dog grooming in a city, with no data in front of you. Interviewers and managers use them to see how you structure a problem, choose assumptions and check your own work. The exact number matters far less than whether someone can follow it.

How do you answer a market sizing question in an interview?

Clarify what is being measured, choose top-down or bottom-up, and say the formula before using any number. Round to figures you can multiply in your head, then check the total by turning it into a rate you can picture. End with the number, the assumption it depends on most and how you would check it.

Should I use top-down or bottom-up in a case interview?

Use bottom-up when you can picture the buyer and count them, and top-down when a credible total is available. If time allows, do both: when they land close, you have a check, and when they disagree, the gap tells you which assumption to examine before you commit to an answer.

Where to go next

• TAM, SAM, SOM, Done Honestly
• The Strategy Manager Role: The Work, the Skills, the Path
• TAM/SAM/SOM
• Market Segmentation
• Unit Economics