How much time should you spend in a market sizing question where a interview is 20 minutes long

A real MBB consultant would answer this question "it depends" ;) 

anyway, my 2 cents:

• Structure & clarifying questions: 2-3 minutes
  • Quickly confirm what exactly you’re sizing (e.g., total market, segment, revenue).
  • Ask one or two clarifying questions to show logic and avoid wrong assumptions.
  • Outline your approach verbally—interviewer should see your thought process clearly.
• Math / calculation: 4-6 minutes
  • Work efficiently with round numbers.
  • Break the problem into MECE components (e.g., population × penetration × frequency × price).
  • Talk through each step so the interviewer can follow and catch errors if any.
• Sanity check / insight: 1-2 minutes
  • Compare your result to known benchmarks or intuitive checks (“this is roughly the size of a medium city’s annual spending”).
  • Draw a quick implication: “This suggests the market is attractive / small / niche.”
• Buffer / interviewer interactions: 2-3 minutes
  • Use time to answer their questions or clarify assumptions without rushing.
  • Keep the overall pace steady; if a calculation is taking too long, simplify rather than get stuck.

Key is to:

• Show structure first: Interviewers value how you think over the number itself.
• Be verbal: Say your assumptions, your logic, and what you’re calculating as you go.
• Prioritize speed over perfect detail: A rough, reasonable answer is better than a perfect answer after 10 minutes.
• End with insight: Even in a sizing, tie the number back to the business question briefly.

Rule of thumb: You want roughly half your time on thinking/structuring, half on math, leaving a minute or two for sanity check and commentary.

20 minutes isn't a lot. You'll have to move quickly, but don't lose your structure.

A rough breakdown:

• 2-3 minutes: Listen to the prompt, ask clarifying questions, and set up your structure
• 10-12 minutes: Work through the problem (this is where market sizing or math fits)
• 3-5 minutes: Summarize your findings and give a recommendation

If the case is purely market sizing, you'll spend most of the 10-12 minutes on the math. If it's a broader case with market sizing as one part, keep the sizing to 4-5 minutes max. Don't get stuck there.

What I'd suggest for short cases:

Keep your structure simple. Two or three buckets, not five. You don't have time for complexity.

Round aggressively. Use easy numbers. 47 million becomes 50 million. Interviewers want to see your logic, not perfect math.

Talk while you calculate. Don't go silent. Walk them through your thinking as you go.

If you're running out of time, say so. Something like: "I'm mindful of time, so let me give you a quick estimate and then share my recommendation." That shows awareness.

Since it's three cases back-to-back:

Pace yourself mentally. Stay calm between cases. Each one is a fresh start. Don't let a rough case affect the next one.

Practice doing cases in 20 minutes before the interview. Get used to the speed. If your practice cases take 30-40 minutes, you'll struggle on the day.

This is a fantastic question and one that reveals exactly how those high-pressure, short-format rounds work. A 20-minute case is not designed to test your deep analytical rigor; it is designed to test your ability to structure under pressure and manage time ruthlessly.

Here is the reality of a 20-minute cycle: the interviewer wants to see you hit 3–4 distinct sections (Structure, Sizing/Analysis, Qualitative Strategy, Recommendation). If you spend more than 50% of the time on one component, you automatically fail the pacing test, regardless of how accurate your answer is.

For the market sizing component, you need to cap yourself strictly at 4 to 6 minutes maximum. The key is to sell the interviewer on your mechanism (your logical steps and structure) far more than the actual math. Use simple, easily divisible numbers (e.g., population 300 million, not 330 million) and round liberally. When you finish the structure, briefly ask, "Does this approach make sense? I will use simple estimates to ensure we have time to cover the strategy portion." This shows you are prioritizing the overall case flow, which is what the boutique is testing here.

Think high-level, confident estimates, not deep Excel accuracy. If you hit 6 minutes and still have calculations pending, state your assumptions, declare the final estimate, and quickly transition to the next step.

All the best!

Hi there,

In a 20-minute case, market sizing should be kept very tight. As a rule of thumb, you should aim to spend around 5 minutes total on a market sizing question, sometimes even less. Interviewers at boutique firms are usually testing structure, logic, and communication rather than your ability to do long calculations.

A good breakdown is:

• ~1 minute to clarify the question and lay out a clear structure
• ~3 minutes to run through clean, simple math using round numbers
• ~1 minute to sanity-check the result and state the implication

If you find yourself going deep into detailed calculations or second-guessing assumptions, that’s usually a sign you’re spending too long. It’s better to move quickly, state assumptions confidently, and show you can get to a reasonable answer efficiently.

Since you have three back-to-back 20-minute cases, pacing matters a lot. Interviewers will want to see that you can get to insight fast and then move on, not perfect the math.

Best,
Evelina

Hi there,

20 min for a case is already on the shorter end, so market sizing should be quick and focus should be on demonstrating that you can identify key drivers and structure the approach. Try to allocate ~5 min on it.

hey there :) In a 20 minute case you should spend around 3 to 5 minutes on market sizing max, with clear structure and clean assumptions mattering more than perfect math, especially in a first and only round at a boutique. Happy to help if you want to practice one quickly or sanity check your approach. best, Alessa :)

You have to be efficient in addressing the problem.

Usually, a market sizing question solved in an efficient way does not take more than 5-7 minutes to solve.

If you are taking longer it means that you have not yet systematized how to do market estimates, and are likely doing things the wrong way.

Then you have a few more minutes to answer one or two qualitative questions (a.k.a. "brainstorming question" + a question on trends / testing business sense).

Market estimates is one of my specialties - feel free to reach out if you would like to schedule a session in order to improve this topic.

Most case interviews last about 30 minutes and cover 3-4 questions.

That doesn't mean you spend an equal amount of time on each question, but 8-10 minutes is an average.

So if you have 20-minute interviews, you can expect 2-3 questions in that time.

If one of them is a market sizing, then you're looking at about 10 minutes.

1 minute for thinking and developing the structure.

The rest is you working with the interviewer to generate the assumptions, align them, do the computations, sanity-check the result and interpret it.

If you need help with market sizing, drop me a line. 

Best,
Cristian