Product Manager Interview (+ AI PM) · Estimation & Market Sizing · Lesson 1 of 2
Structured market sizing
'How many X are there / sold in the US?' tests structured reasoning under uncertainty. The interviewer does not care about the exact number — they grade whether you decompose the problem, state assumptions, and do clean arithmetic out loud.
- Clarify scope: US or global? Per year or right now? A specific segment? Pin the definition first.
- Pick an approach: top-down (start from population, filter down) or bottom-up (units × frequency × price). Choose one and say why.
- Build the equation: write the chain of factors before plugging numbers (e.g. population × % who own × replacement rate).
- State assumptions with round numbers: US population ~340M, ~130M households — use anchors that are easy to compute with.
- Compute step by step, then sanity-check the result against something you know.
Narrate every assumptionSay the number and where it comes from: 'US population is about 330 million, assume ~4 phones replaced per person per decade…'. A transparent, checkable assumption beats a precise-looking number pulled from nowhere.
Round numbers are a featureTreat estimation like mental Fermi arithmetic: use 340M not 341,145,670. Clean anchors keep the math fast and let the interviewer follow your logic without a calculator.
◆ Lock it in
- Estimation grades decomposition + stated assumptions + clean math, not the exact number.
- Pick top-down or bottom-up explicitly and build the equation before plugging numbers.
- Use round anchors (e.g. ~340M US population) and narrate every assumption.
Feynman drill — say it out loudWalk through estimating the number of coffee cups sold daily in the US using a bottom-up chain.
Step 1 rate your confidence · Step 2 pick your answer
In a market-sizing question, what are interviewers primarily evaluating?
Step 1 — how sure are you?