Is there a "right answer" to a market sizing question?

No. The interviewer doesn't expect you to be exact. They're assessing your process: structure, logic, assumptions, and sense-checking. A number within 20-30% of reality, with clear reasoning, is a strong answer.

Should I use top-down or bottom-up?

Both. Use whichever feels more natural for the question, but validate with the other approach. If they diverge, investigate why and reconcile. This demonstrates analytical rigour.

How much time should I spend on a market sizing question?

In a case interview, 5-10 minutes. Outline your approach (1 min), make key assumptions (2-3 min), calculate (2-3 min), sense-check (1-2 min). Then move on. Don't get bogged down.

What if I don't know a key assumption (e.g., average spend per customer)?

Estimate reasonably and state your assumption: "I don't know the exact spend, but I'll assume £30/year because it's comparable to [related product]." The interviewer will accept reasonable estimates if you explain your logic.

How do I sanity-check my answer?

Compare to: (1) Total addressable spending (what fraction of UK consumer spending is this?), (2) Known competitor revenues (does this market size support the revenue of major players?), (3) Supply-side metrics (how many stores, how many employees, how many transactions).

What if my top-down and bottom-up estimates differ by 50%?

That's normal for rough estimates. Investigate the gap: what assumptions are different? Is penetration lower than you thought? Is average spend higher? This investigation process is more valuable than the number itself.

Technical Interview Guide

Market Sizing Questions Interview Questions & Walkthrough

Step-by-step explanation, real interview questions, model answers at different levels, and AI-scored practice.

Quick answer

Market sizing questions ask "how big is a market?" and are tested in consulting case interviews to assess analytical thinking. Use a top-down approach (start with total addressable market, narrow down) or bottom-up approach (build from customer cohorts). Always structure your approach, state assumptions clearly, and stress-test them.

Step 1

What are market sizing questions and why consultants ask them

Market sizing questions appear in McKinsey, BCG, and Bain case interviews. They test analytical thinking, not maths — you won't be expected to recite exact numbers. Instead, interviewers assess: (1) Can you structure an ambiguous problem? (2) Do you make reasonable assumptions? (3) Can you sense-check your answer? (4) Are you systematic?

The goal is not the final number — it's to see how you think. A consultant might be asked "What's the UK market for coffee?" Not because the firm needs a precise answer, but because answering well demonstrates frameworks, logical reasoning, and the ability to break down complex problems.

There are two approaches: Top-Down (start with total addressable market, narrow down to the segment you care about) and Bottom-Up (start with customer cohorts, aggregate up). The best answers combine both and reconcile them. If they diverge significantly, investigate why.

Step 2

Top-down approach: TAM, SAM, SOM

TAM = Total Addressable Market (the total market if you captured everyone). SAM = Serviceable Addressable Market (the subset you can realistically reach). SOM = Serviceable Obtainable Market (what you expect to capture in 3-5 years). Start with TAM, then narrow.

Example: "What's the UK market for instant coffee?" Start with UK population: 67 million. What fraction drinks coffee? Maybe 40% = 27 million coffee drinkers. What fraction buys instant coffee (vs ground beans, pods)? Maybe 30% = 8 million potential customers. Average annual spend: perhaps £20 per person (4 jars at £5). TAM = 8 million × £20 = £160 million.

Sense-check: there are roughly 8,000 supermarkets in the UK. If each sells an average of £20,000 of instant coffee per year, that's £160 million. This aligns with our top-down estimate, so we're in the right ballpark. TAM is roughly £150-200m.

Step 3

Bottom-up approach: customer cohorts and spend

Start by segmenting customers and estimating cohort size and spend. Example: UK households = 27 million. Proportion that buys instant coffee regularly: maybe 25% = 6.75 million households. Average annual spend: £15-25. Bottom-up TAM = 6.75 million × £20 = £135 million.

Alternative bottom-up: by geography. UK has 4 regions (assume: Scotland, Wales, Northern Ireland, England). England has 56 million people. Assume 50% of England has access to supermarkets and buys instant coffee. Each buys £20/year. England TAM = 28 million × £20 = £560m. That's too high — reveals an assumption error (maybe not 50% buy regularly).

Adjust: If 30% of England regularly buys instant coffee, England TAM = 16.8 million × £20 = £336m. Add Scotland, Wales, Northern Ireland (smaller populations). Total UK TAM ≈ £400m. This diverges from the top-down estimate (£160m). Investigate: maybe average spend is lower (£10 instead of £20), or penetration is lower (20% instead of 30%). Reconcile the two approaches.

Step 4

Key tactics: structure, assumptions, and sense-checks

Structure matters most. State your approach before diving into numbers: "I'll start top-down: UK population → coffee drinkers → instant coffee buyers → annual spend. Then I'll validate with a bottom-up approach by household. Finally, I'll sense-check against supply (number of stores, volume per store)."

Assumptions should be explicit and testable. Don't hide behind vague numbers. Say: "I'm assuming 40% of the UK drinks coffee regularly" or "I'm assuming instant coffee costs an average of £20 per household per year." The interviewer will challenge weak assumptions — that's the point.

Sense-checks are critical. If your calculation says the UK instant coffee market is £5 billion, that's nonsense — larger than the total tea market. A quick sanity check catches errors. "Does £160 million sound reasonable? That's £6 per capita for the whole UK, or £20 per regular coffee drinker. That seems about right."

Step 5

Common market sizing questions and frameworks

Consumer product: "How big is the market for X in the UK?" Use population × penetration × average spend. E.g., "Market for shampoo: 67m people × 80% buy shampoo × £30/year = £1.6bn. Reasonable?" Data point: global shampoo market is ~£80bn, UK is ~1% of global, so £800m is reasonable.

B2B market: "How many companies would need solution X?" Estimate the addressable customer base (by industry, company size) and estimate annual spend per customer. E.g., "Sales software: maybe 100,000 UK SMEs, £5,000/year spend each = £500m. Reasonable?"

Transportation/logistics: "How big is the market for delivery services in the UK?" Start with e-commerce (online sales), estimate the fraction that require delivery, and estimate per-delivery cost. Or: population × average deliveries per person per month × cost per delivery.

Questions

Market Sizing Questions interview questions

1How big is the UK market for coffee?
2What is the market size for instant coffee in the UK?
3How many bottles of water are sold in London annually?
4What is the total addressable market for project management software in the UK?
5How many shopping bags does a supermarket use per day?
6Market size for furniture delivery services in the UK.
7How big is the market for cybersecurity software for SMEs in Europe?
8What is the market size for fitness trackers in the UK?
9How many meals are delivered per day in London?
10Market size for video conferencing software post-COVID.
11How big is the market for sustainable packaging in the UK?
12Walk me through your approach to sizing the market for an AI-powered HR tool.
13How would you estimate the market for freelance services in the UK?
14What is the total addressable market for logistics software in Europe?

Model answers

Example answers at different levels

Click a level to see the expected answer depth.

Question

How big is the market for coffee in the UK?

Answer

I'd start by thinking about the UK population, about 67 million people. Maybe 40% drink coffee regularly, so that's 27 million people. Each person probably spends maybe £100-150 per year on coffee (either at home or at a café). That's 27 million × £120 = £3.2 billion. I'd sanity-check this: major coffee chains like Starbucks and Costa have thousands of stores, and there's also home consumption. £3.2 billion sounds reasonable for the total UK market.

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Follow-ups

Common follow-up questions

What if instant coffee consumption increases 10% — how does that change TAM?

How would you segment the instant coffee market by customer type (retail, foodservice, etc.)?

What assumptions did you make that are most uncertain?

How would you estimate the market for instant coffee in Europe?

If you were entering the instant coffee market, what would be the SAM vs SOM?

How would you validate your estimate against actual company revenues?

What would change your estimate the most — price, volume, or penetration?

Avoid

Common mistakes on market sizing questions questions

Forgetting to segment the market. Coffee, instant coffee, instant coffee in retail, instant coffee premium brands — these are different markets with different sizes. Be precise about what you're sizing.

Using inconsistent assumptions. If you say 40% of people drink coffee, and then calculate spend as if 50% do, you'll be off by 25%. Track your assumptions and keep them consistent.

Not sanity-checking. If your answer is £50 billion for a £3 billion total UK consumer market, something is wrong. Sense-check against GDP, total consumer spending, known competitor revenues.

Ignoring geographic or demographic variation. London spends more on coffee than rural Wales. If you're sizing by region, adjust for differences.

Not distinguishing TAM from actual market. TAM is the theoretical maximum. The actual market (SAM) is much smaller. Be clear about which you're calculating.

Using round numbers without justification. "Maybe 50%" for a percentage, "£100 per person per year" — justify these assumptions with reasoning.

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