Pure brainteasers, the riddle-style questions about manhole covers or golf balls in a plane, are now uncommon in mainstream consulting interviews. Estimation and market-sizing questions are still very much alive, either on their own or embedded in a case. Prepare thoroughly for estimation, and have a calm approach ready in case you meet an occasional logic puzzle, especially at smaller firms or in some finance interviews.
What's the difference between a brainteaser and an estimation question?
- Brainteaser: a logic or lateral-thinking puzzle with a "trick" or a single correct answer, for example "You have two ropes that each burn for an hour unevenly; how do you measure 45 minutes?"
- Estimation / market sizing: a structured approximation of a real-world quantity, for example "How many electric vehicle chargers are needed in Manchester?" or "What is the annual UK market for school uniforms?"
Firms favour estimation because it mirrors real consulting work: making reasonable assumptions with incomplete data and sanity-checking the result.
Are brainteasers still used?
Major consulting firms' public guidance focuses on case (problem-solving) interviews and behavioural or personal experience interviews, not puzzles. Candidate reports suggest pure brainteasers come up occasionally, for example at some boutiques, in quant-leaning finance roles or with individual interviewers. They aren't central to MBB processes. Estimation is a different matter: CaseCoach, for example, reports that market sizing remains common at Bain.
How do you answer an estimation question?
1. Clarify the scope
"Annual UK market for school uniforms: do we mean value in pounds, and should I include both state and independent schools, and secondhand sales?"
2. Choose an approach
Common approaches:
- Top-down: start from a population and narrow down.
- Bottom-up: start from units, such as stores or machines, and multiply up.
- Supply-side: estimate capacity, for example the number of producers × output.
Say which you'll use and why. Where possible, triangulate with a second approach.
3. Build a clean equation
For school uniforms (illustrative assumptions only): number of school-age children × share wearing uniforms × annual spend per child.
- Suppose roughly 10 million school-age children in the UK (an assumption to state, and to check against Office for National Statistics data before publishing any real figure).
- Suppose most wear uniform. Assume 90%, so 9 million.
- Assume an annual spend of £150 per child, with secondary uniforms costing more than primary.
- 9 million × £150 = £1.35 billion.
4. Sense-check
"That's about £1.35bn. Per household with school-age children, that feels plausible. If I segmented primary and secondary, the secondary share would dominate because blazers and branded items cost more."
5. Say what drives the answer
"The answer is most sensitive to spend per child, so if this matters for a decision, I'd validate that with retailer data."
What makes an estimation answer strong rather than weak?
- Segmentation where it matters: splitting by age group, region or customer type when behaviour differs.
- Round, explicit assumptions: "I'll assume 25 million households", stated, not hidden.
- Speed: most standalone estimations should take around five minutes, not fifteen.
- Judgement at the end: what the number means for the client's question.
How should you handle an actual brainteaser?
If you get a genuine puzzle:
- Restate it to be sure you understand the constraints.
- Think aloud. Interviewers usually care about your process, not the answer.
- Try small cases or extreme values to find patterns.
- Stay calm if stuck. Say what you've ruled out and why, and ask whether you can take a hint.
Don't spend weeks memorising puzzle collections. A few hours of exposure is plenty for consulting.
Do finance interviews use brainteasers?
Some finance interviews, particularly for trading or quantitative roles, use probability or logic questions more often than consulting does. Investment banking and private equity interviews more typically focus on technicals (accounting, valuation, LBOs) and deal discussion. Mental arithmetic shows up in paper LBOs (see our guide to the PE case study vs paper LBO).
How should you practise estimation?
- Do one estimation a day, taking a quantity from daily life (coffee cups sold at a station, rail season tickets, parcel deliveries).
- Practise both top-down and bottom-up, and compare.
- Practise inside full cases, because market sizing often appears as a step in a market-entry case. An AI interviewer such as caize.ai can throw sizing questions at you mid-case and push back on weak assumptions.
FAQ
Will McKinsey, BCG or Bain ask me a brainteaser?
It's unlikely to be a core part of the process. Expect estimation and case questions instead.
Do I need to know real statistics for market sizing?
A handful of anchors helps (UK population around 69 million, number of households, typical working days). Reasonable, clearly stated assumptions matter more than precision.
What if my estimate is far off the "real" number?
Interviewers mainly assess structure, assumptions and sense-checking. A logical answer that is somewhat off is far better than a lucky guess.
Should I always use two approaches?
If time allows, a quick second approach to triangulate is impressive. It isn't always necessary.
