Intuition
Market sizing feels impossible — "how many tennis balls are sold in the UK each year?" — until you realize nobody expects you to know. They expect you to build. It's like estimating how many jellybeans are in a jar: you don't count them, you figure out the volume of the jar, the volume of a bean, and divide. The magic is the method, not the memory.
A good estimate is just a chain of reasonable assumptions, each one said out loud so the interviewer can nod along.
Framework
- Choose top-down or bottom-up. Top-down starts from a big population and filters down. Bottom-up builds from a unit (one store, one customer) and scales up. Pick whichever has a cleaner anchor.
- Segment when it changes the rate. Split the population only where it affects the number (e.g. coffee drinkers vs non-drinkers).
- Use round, defensible numbers. US population ≈ 330M. Round so the math stays clean — precision is fake comfort here.
- Sanity-check the answer. Convert it to a per-person or per-day figure and ask "does that feel real?"
Worked Example
"How many smartphones are sold in the US per year?" Top-down: ~330M people → ~80% own a smartphone → ~265M phones in use → people replace roughly every 3 years → ~265M / 3 ≈ 88M phones per year. Sanity check: that's about one phone per four people per year, which feels right for a mature market. Every step is a number the interviewer can challenge — and you can defend.
Pitfalls
- Diving into arithmetic before laying out the path.
- Over-segmenting (splitting by age when age doesn't change the answer) and drowning in steps.
- Reporting a wild number without the sanity check — order-of-magnitude errors are the most common fail.