How to answer a market sizing question
Pick a structure, state your assumptions out loud, do the arithmetic in round numbers, and sanity-check the result. Interviewers care about the logic and the explicit assumptions, not the exact final number. This calculator handles the math so you can focus on the structure.
Top-down vs. bottom-up
Top-down starts from a big number, a population or total market, and narrows it with percentages. Bottom-up starts from a single unit, one store, one customer, and multiplies up. Use whichever has data you can reason about, and mention the other as a cross-check.
Worked example
How many coffees are sold per day in a city of 1M? Start from 1, 000, 000 people × 60% coffee drinkers × 1.5 cups/day = 900, 000 cups/day. Each factor is an assumption you can defend or adjust, exactly what the calculator above lets you do.