Market sizing interview questions

A market sizing question asks how many customers, purchases or dollars a defined market represents. The useful answer is not an unsupported number: it is a clear model another person can inspect. Fermi estimation gives you a way to build that model from explicit, approximate assumptions.

Define the unit before doing arithmetic

Ask which geography, customers and time period count. Annual revenue, daily purchases and the number of products currently owned are different quantities. A stock, such as bicycles owned, is not an annual flow, such as new bicycles sold. Replacement frequency can connect them, but it must be an explicit assumption.

Choose a model that fits the question

• Demand: population × fraction buying × purchases per buyer per year.
• Revenue: annual purchases × average price per purchase.
• Capacity cross-check: outlets × purchases per outlet per day × operating days.

Segment only when it changes an important assumption. For example, commuters and occasional riders might buy at different rates. More segments do not automatically make a model better; avoid counting the same customer twice.

Worked example: annual bicycle sales

Hypothetical practice inputs, not market statistics: imagine a city with 1 million residents. Assume 40% own a bicycle and replace it every 8 years. Estimate replacement sales only; exclude first-time purchases, visitors and second bicycles.

1. Installed base: 1,000,000 people × 0.4 bicycles per person = 400,000 bicycles.
2. Replacement flow: 400,000 bicycles ÷ 8 years = 50,000 bicycles per year.
3. At an assumed average price of $300, replacement revenue is about $15 million per year.

The ownership rate and replacement interval need editorial or interview discussion, not false precision. Keeping ownership fixed, a 5–10 year replacement interval gives 40,000–80,000 replacement sales, or $12–24 million annually at the assumed price. That is a sensitivity range for this model, not a statistical confidence interval or the total bicycle market.

Finish with a defensible answer

Say the unit, your central estimate, the biggest assumption and what you excluded: “About 50,000 replacement bicycles per year, mainly depending on the replacement interval; this excludes new owners.” Check whether a capacity model gives a compatible order of magnitude. If not, revisit the assumptions rather than averaging incompatible answers.

Practice prompts

• How many takeaway coffees do commuters buy in a city each working day?
• What is the annual replacement market for office chairs in a town?
• How many bicycle repair visits could local workshops handle each year?

Try each with a defined unit, a short model and a range. Review the worked Fermi examples, or practice today's puzzle.

Method references

The Berkeley Consulting sample case illustrates confirming market parameters and segmenting demand. CaseInterview's market-sizing guidance discusses explicit assumptions and choosing an appropriate level of detail. These support the approach; the bicycle numbers above are invented practice inputs.