Market sizing questions: method, 10 examples and answers

Market sizing questions ask you to estimate the size of a market, or any number you could not know offhand, using logic and round numbers: how many smartphones are sold in the US each year, how much one coffee shop takes. They are also called guesstimates or estimation questions. The way to answer is always the same: write a formula, fill it with stated assumptions, do the maths cleanly and test the result. Below are the method, ten worked examples and a cheat sheet of anchor numbers.

They come up as standalone questions, often early in a first-round interview, and inside full cases, where 'is this market big enough to enter?' turns into a sizing question halfway through. Either way, nobody is grading the final number. A sound approach that lands 30% off beats a lucky guess with no logic behind it.

There are two ways into any market. Top-down starts from a big number you know, usually population or households, and narrows it. Bottom-up starts from one unit you can picture, a shop, a machine, a worker, and scales it up.

Size whichever side is the bottleneck. MRI scans are limited by machines, not by how many patients could use one, so size the supply. Chocolate is limited by appetite, so size the demand. When you have time, run the other side as a cross-check: two methods that land within about 25% of each other give you a confident range.

1. Clarify with two or three questions, not ten. Which product exactly? Which country? Annual? Units or revenue, and at retail or wholesale prices?
2. Choose the approach whose inputs you can estimate with most confidence, and say why in one sentence.
3. Segment only where a number in your formula really differs: children and adults for nappies, businesses by size for laptops. Splitting by star sign adds work and no accuracy.
4. Estimate with friendly numbers: 340m, not 337.5m; a third, not 31%. Write each input with its unit and a one-line reason.
5. Check the answer against something you can picture, then close with a range, the assumption that drives it and the so-what.

Each answer below is one defensible estimate, not the true figure. The assumptions are the kind you would state in the room; if your interviewer offers different numbers, use theirs. Cover the last two columns and try each one out loud before you read the working.

Notice the patterns. Examples 1, 6 and 10 turn a stock of owners into a yearly flow of purchases by dividing by lifespan. Examples 2 and 7 multiply capacity by how busy it is. Examples 3, 4 and 9 size demand, then divide by what one supplier can handle. Once you see these four or five shapes, most new questions become a variation on one of them.

A sanity check compares your answer with something you already know. It takes ten seconds and catches most dropped zeros before the interviewer does. Use one or two of these on every estimate:

• Per person or per household: does the implied spend or usage a year feel right in daily life?
• Per outlet: does each shop or station serve a believable number of customers an hour?
• Against the economy: one consumer product cannot be a large slice of household spending.
• Against the client: if the client's revenue is bigger than your whole market, one of them is wrong.
• A second method: size it from the other side and see whether the two land within about 25%.

If a check fails, find the assumption that is off and fix that. Do not just shave the total until it looks plausible. Practise spotting bad numbers with the sanity checks drills, starting with this one:

• Numbers before structure. Opening with 'so, there are 340m people...' builds a chain nobody can follow or fix. Write the formula first, as you would an issue tree.
• Skipping scope. Sizing all glasses when the question was prescription glasses, or units when the client wanted revenue.
• Stock versus flow. Counting everyone who owns a product as a buyer this year.
• Mixing time units. Daily customers with annual prices, or monthly users with weekly visits.
• False precision. 337.5m people and 47% penetration add effort, not accuracy.
• Over-segmenting. Six segments with invented splits multiply the guesswork.
• No so-what. Ending on a number instead of what it means for the client.

These are rough, round figures for starting points and sanity checks. Say 'roughly' when you use them, and if the interviewer gives you a different number, use theirs.

• Daily to annual: × 365, or × 360 when you want easy maths.
• Weekly to annual: × 50 for working weeks, × 52 for all weeks.
• Owners to annual purchases: ÷ the product's lifespan in years.
• Thousand × thousand = million; thousand × million = billion.

Do sizing questions out loud, against a timer: about a minute to clarify and structure, three to five minutes of estimates and maths, and a minute to check and conclude. The top-down sizing and bottom-up sizing drills give you the inputs, so you can focus on structure and arithmetic. The market sizing lesson goes deeper on segmentation and triangulation, and the maths underneath is in our guide to case interview math.

Then try sizing inside a full case, where the number feeds a decision. The featured ready-meal case is the gentlest start; the coffee case below asks you to size the opportunity and then decide how to enter.

BainMeridian Coffee enters IndiaMarket Entry · Medium

Meridian Coffee enters India

Market sizing inside a market entry case: the number has to lead to a decision.

Interviewer

Meridian Coffee operates 410 stores across the United States and has never operated outside North America. The board is considering entering India and has asked us whether they should, and if so, how. Where would you like to start?

Start the caseGuided, step by step