Market sizing questions: why your final number matters less than you think

“How big is the market for dog grooming in this country?” Most candidates hear that and panic about the answer. Is it £200m? £2bn? They don't know and they assume the interviewer does.

Often the interviewer doesn't know either. Even when they have a figure in mind, it's usually a rough one. A market sizing question is not a quiz with a right answer. It's a short, contained test of how you reason when you don't have the data, which is most of what a junior consultant does all day.

What the interviewer is actually listening for

There are four things being judged and the final number is only loosely part of one of them.

1. 01A clear approach before any maths. Do you say how you'll break the problem down and does the breakdown make sense?
2. 02Sensible assumptions, said out loud. Not perfect ones. Ones you can defend in a sentence.
3. 03Clean arithmetic. Units kept straight, zeros not lost, rounding done on purpose.
4. 04A sanity check and a so-what. Does the answer pass a smell test and what would you do with it?

Notice that a candidate can land within 20% of the “real” figure and still do badly, because they guessed a top-down number with no visible logic. Another can be out by half and do well, because every step was reasonable and they caught the weakest assumption themselves.

A worked example

Here is how a good answer to the dog grooming question might sound, sized as annual spend in a country of 60 million people. The assumptions are deliberately round.

The structure is the whole answer. Population to households to dogs to groomed dogs to spend. Each step is one assumption and each assumption is something a person could argue with. That's the point: you've made your thinking easy to inspect.

The sanity check matters more than it looks. Dividing back to about £19 per household a year, across all households including the three quarters with no dog, feels plausible. If it had come out at £400 per household you'd know a zero had gone wrong somewhere.

The follow-up is where it's really decided

Strong interviewers rarely stop at the number. They push on an assumption to see what you do under pressure.

That last line is what separates a good answer from a fine one. The candidate didn't defend the old number or apologise for it. They redid the sum quickly and then said which driver matters most. That's the instinct consulting firms are hiring for.

Choosing your assumptions

You'll never know the exact share of households with a dog. Nobody expects you to. What they expect is that your numbers are in a believable range and that you pick round ones so the maths stays easy. 25% is better than 27% because you can multiply by it in your head.

If you're unsure, say why you chose the figure. “I'll assume a quarter of households have a dog. It feels common but far from universal.” One sentence like that turns a guess into a judgement.

It's also fine to segment when one average would hide something. Small dogs and large dogs get groomed at different prices. If you think it changes the answer, split them. If it doesn't, say you've considered it and move on. Knowing when not to add detail is part of the skill.

Finish with what the number means

A number alone is half an answer. Close by tying it back to whatever the client cares about. “So the market is roughly £450m to £500m. If our client wants £20m of revenue, they'd need around 4% share, which is achievable for a chain with national coverage.” Now the estimate has done a job.

Get the structure, the assumptions and the so-what right and the final number tends to look after itself. Get them wrong and even a lucky number won't save the answer.