You are 20 minutes into a case interview. The interviewer says: "Estimate the market for electric scooters in Berlin." Your mind goes blank. You know you should structure something, but the numbers won't come. This is the moment that separates candidates who get offers from those who don't. Market sizing case interview questions are not about knowing the answer — they are about showing a repeatable method under pressure. The course Strategic Consulting — MBB-Level Preparation teaches exactly that method, and in this article, we'll break down how to answer market sizing questions so you never freeze again.
Why market sizing matters more than you think
Consulting firms use market sizing to test structured thinking, business intuition, and mental math. Unlike a profitability case, there's no data to analyze — you have to build the estimate from scratch. Interviewers watch how you decompose the problem, justify assumptions, and sanity-check your result. A candidate who says "about 500 million" without showing work fails. One who walks through a MECE issue tree, states assumptions clearly, and arrives at a defensible number passes — even if the number is off by 30%.
The core idea: market sizing is a structured estimation problem, not a trivia question. You need a method you can apply to any market, any product, any geography. More on the topic: “Product Management”.
The 4-step method that works every time
Here's the framework we teach: clarify the metric, build a MECE issue tree, assign defensible assumptions, and sanity-check the final number.
Step 1: Clarify the metric
Before you calculate anything, ask: what exactly are we sizing? Revenue or units? Annual or monthly? New sales or installed base? For a scooter market in Berlin, do we mean the number of scooters sold per year, the revenue from scooter sales, or the number of rides taken? Each definition leads to a different calculation. Clarifying shows you understand the business context.
Step 2: Build a MECE issue tree
MECE means mutually exclusive, collectively exhaustive. Break the market into drivers that don't overlap and cover everything. For scooter sales in Berlin:
- Population of Berlin → number of potential riders → adoption rate → replacement cycle.
Or: number of households → % that own a scooter → average scooters per household → annual replacement rate.
Both work. The key is to pick a tree that uses numbers you can estimate. Avoid trees with more than three levels — you'll get lost in arithmetic.
Step 3: Assign defensible assumptions
This is where most candidates fail. They pull numbers from thin air. Instead, anchor each assumption to something you know or can estimate from general knowledge. For Berlin:
- Population: ~3.7 million (you can round to 4 million).
- Adults: ~80% → 3.2 million.
- % who use scooters regularly: maybe 5%? That's 160,000 users.
- Average scooter lifespan: 3 years → annual replacement ≈ 53,000 units.
Each number is a guess, but it's a reasoned guess. State it as "I'll assume X% because..." and move on. Interviewers care more about the logic than the precision.
Step 4: Sanity-check against a known anchor
Your final number should pass a smell test. If you estimate 53,000 scooter sales per year in Berlin, does that make sense? Compare to bike sales (maybe 100,000 per year in a city that size) — scooters are newer, so a smaller number is plausible. If your calculation gave 5 million, you'd know something is off.
A fully worked example with real arithmetic
Let's size the market for coffee shops in London — annual revenue.
Step 1: Metric — annual revenue from coffee sales in London.
Step 2: MECE tree — Revenue = number of coffee shops × average cups sold per shop per day × average price per cup × 365.
Step 3: Assumptions
- Population of London: ~9 million.
- Number of coffee shops: estimate 1 per 2,000 people → 4,500 shops. (Reasonable? London has many cafes; 4,500 seems plausible.)
- Cups per shop per day: 200 (a busy shop might sell 300, a quiet one 100; 200 is a middle ground).
- Average price per cup: £3.
Step 4: Calculation
Revenue = 4,500 × 200 × £3 × 365 = 4,500 × 200 = 900,000 cups/day; × £3 = £2.7 million/day; × 365 ≈ £985 million per year.
Sanity check: London's coffee market is often cited as over £1 billion — our estimate is in the right ballpark. Good.
Now, what if you had used a different tree: number of coffee drinkers × cups per week × price? That would also work, but you'd need to estimate the number of coffee drinkers. The first tree is simpler because it uses shop density, which is easier to guess.
The 3 mistakes interviewers penalize most
- Unclear metric — starting to calculate without defining what you're measuring. Interviewers see this as sloppy thinking.
- Unjustified assumptions — saying "I'll assume 10%" without explaining why. Even a simple reason ("because urban adoption of new mobility tends to be in the single digits") is better than silence.
- No sanity check — delivering a number that's off by an order of magnitude without noticing. Always compare to something you know.
How to rehearse under time pressure
You can't learn this by reading. You need to practice with a timer. Start with 10 minutes per case, then reduce to 5. Use a structured approach: write down the tree, fill in assumptions, calculate, check. Record yourself and listen for filler words or hesitation. More on the topic: “Prompts That Read a P&L Like a CFO”.
For a deeper dive into structuring case interviews, see our guide on System Design Interview: A Step-by-Step Preparation Plan for FAANG and Top-Tier Companies — while it's for tech, the principle of structured decomposition applies similarly. More on the topic: “System Design Interview”.
Self-study vs. structured course: which fits you?
Self-study is free but slow. You can find case books and YouTube videos, but you won't get feedback on your logic. You might practice 20 cases and still make the same mistake. A structured course gives you a proven method, worked examples, and a way to check your answers. The trade-off: courses cost money, but they save time and reduce the risk of ingraining bad habits.
If you're preparing alone, focus on building a feedback loop: record yourself, compare your approach to sample solutions, and ask a friend to critique your assumptions. If you want a guided path with exercises and a mock interview, consider Strategic Consulting — MBB-Level Preparation — it covers market sizing and other case types with hands-on practice.
Who this is for (and who it isn't)
This method is for anyone targeting MBB or other top consulting firms, or for business analysts who need to size markets quickly. It's not for people who want a quick hack — market sizing requires practice and comfort with ambiguity. If you're not willing to put in 20-30 hours of deliberate practice, even the best course won't help.
Ready to stop freezing?
The difference between a blank stare and a confident answer is a repeatable method. Practice the 4-step approach on random markets: the number of gyms in Paris, the market for pet food in Tokyo. The more you do, the more natural it becomes. For a structured path with worked examples and feedback, the course Strategic Consulting — MBB-Level Preparation walks through exactly these calculations and gives you a framework to apply under pressure. Start with one case today — clarity comes from repetition.