Brainteasers and market sizing
By SuperdayReps · Wharton MBA · ex-Greenhill · investment banking interview coaching
7 min read · updated July 2, 2026
Let's set expectations first, because most students get this backwards. Brainteasers are a small minority of what you'll face in an IB interview. When one shows up, it's usually an interviewer with ten minutes to kill, not a make-or-break gate. Do not spend a weekend memorizing riddles off some list. Spend it on the three statements, valuation, and your story. Those are what actually get you the offer.
That said, there's one "brainteaser" you should be ready for every single time: market sizing. It's the estimation question, and it's the one bankers actually like, because it looks like the job. How big is the market for a product? How many of something fit in a space? You will not have a calculator, and there is no right answer sitting in the interviewer's notes. They're watching how you think.
What they're actually grading
Here's the part beginners miss. The number you land on almost doesn't matter. The interviewer wants to see a structured, out-loud reasoning process with assumptions they can follow and poke at. A candidate who says "about 8 million" with no path gets nothing. A candidate who builds to "roughly 8 million" through four clean, stated steps gets the check mark, even if the true figure is 5 million or 12 million.
Market sizing is a communication test wearing a math costume. They're checking whether you can take a fuzzy, open-ended problem, impose a structure on it, and walk someone through your logic without freezing. That's the entire job of a junior banker: make the unknowable tractable and defensible. Land the framework and the number takes care of itself.
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Frequently asked
- How do you approach a market sizing question in an interview?
- Restate the scope, anchor to a big round number like 300 million for US population, segment down with clean percentages, multiply through, then sanity-check the magnitude. Say every assumption out loud as you go. The interviewer is grading your structured reasoning, not the exact figure you land on.
- How important are brainteasers in investment banking interviews?
- They are a small minority of what you face, usually an interviewer killing ten minutes rather than a make-or-break gate. Do not spend a weekend memorizing riddles. The one estimation question worth preparing every time is market sizing, because it looks like the actual job.
- Does the final number matter in a market sizing answer?
- Almost not at all. The interviewer wants a structured, out-loud reasoning process with assumptions they can follow and poke at. A candidate who builds to roughly 8 million through four clean stated steps passes, even if the true figure is 5 or 12 million. Land the framework and the number takes care of itself.
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