Size any market without flinching
Market-sizing questions reward structure over arithmetic: clean segmentation, a deliberate top-down or bottom-up path, and a number you can sanity-check out loud. Here's the method, worked end to end.
The skill is structure, not the number
Interviewers don't expect you to be right to the decimal — they want to watch you reason your way to a defensible estimate.
Segment first, estimate second
Strong sizing starts by breaking the population into clean, meaningful segments — by age, geography, or usage — so each assumption sits on a number you can defend.
Top-down vs bottom-up
Top-down narrows a big population down with filters; bottom-up builds up from a unit. Knowing which to reach for — and when to cross-check with the other — is the skill.
Sanity-check everything
A market-sizing answer is only as good as its plausibility. Round to clean numbers, state every assumption out loud, and gut-check the final figure before you commit to it.
How many EVs will sell in Germany next year?
“Estimate the number of new electric vehicles that will be sold in Germany in the coming year. Walk me through it.”
How a strong candidate opens
“I'll go top-down: start from Germany's annual new-car sales, then apply the EV share of those sales. I'll size total new-car volume first, then estimate what fraction is electric, and sanity-check the result against the EV adoption trend.”
The worked numbers
- Population: ~84M people in Germany.
- Cars per person: ~1 car per 2 people → ~42M cars on the road.
- Replacement cycle: a car lasts ~15 years → ~42M ÷ 15 ≈ 2.8M new cars sold a year.
- EV share: assume ~20% of new sales are electric next year.
- Estimate: 2.8M × 20% ≈ 560,000 EVs.
Sanity check
~0.5M EVs against ~2.8M total cars is roughly one in five — consistent with where adoption is heading. The number holds.
Reps until the method is reflex
Rehearsa listens to your spoken structure, not just your final figure — and flags the moment an assumption goes unstated.
- Default to a clear approach choice up front: 'I'll go top-down here because we have a known population to filter.'
- Segment out loud so the interviewer can follow — and catch a wrong assumption before it compounds through the whole estimate.
- Round aggressively to numbers you can multiply in your head, and flag where you rounded so accuracy is a choice, not an accident.
- Cross-check the result against a second angle or a known benchmark — 'that's roughly X% of all cars sold, which feels right.'
- Narrate your structure before you compute, so a shaky number never sinks an otherwise strong, well-reasoned answer.
Top-down and bottom-up, side by side
For each sizing prompt, Rehearsa shows how the same estimate looks built from the top down and from the bottom up — so you learn to pick the cleaner path instinctively and use the other as your cross-check.
108
sizing prompts
2
paths per estimate
< 5m
to a defensible number
100%
graded on structure
Stop practicing. Start rehearsing.
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