Guesstimates: a 5-step method to size any market

Guesstimates scare people because there's no "right" answer. That's exactly the point: interviewers grade how you think, not the final number.

Here's a 5-step method that works for almost any market-sizing question.

1. Clarify what you're sizing

"Cups of chai sold in Bengaluru per day" — does that include chai made at home, or only sold by vendors and cafés? Ask. It changes the answer by 10×.

2. Pick an approach out loud

• Top-down: start from population and narrow down.
• Bottom-up: start from supply — number of vendors × cups each sells.

Saying which one you'll use (and why) is half the marks.

3. Break it into a simple equation

Cups sold per day = People × % who buy chai outside × cups per person per day

Write it down. This is your driver tree.

4. Use round, defendable numbers

• Bengaluru population: ~1.3 crore
• Share who buy chai outside on a given day: ~30%
• Cups each: ~1.5

1.3 crore × 30% × 1.5 ≈ 58 lakh cups a day.

Round aggressively. Nobody wants to watch you multiply 13,217,486 by 0.297.

5. Sanity-check and sense-check

Cross-check bottom-up: ~25,000 chai stalls × ~200 cups a day ≈ 50 lakh. Close to our top-down number — good sign. If they're wildly different, say so and explain which you trust more.

What interviewers listen for

• A clear structure before any numbers
• Assumptions stated out loud ("I'll assume…")
• Sensible rounding
• A sanity check at the end

Practise it

Try three guesstimates a week and your speed will double within a month. Case Flex has guesstimate drills with an AI interviewer that challenges your assumptions in real time.