The Market Sizing Case Interview: A Complete Guide

Market sizing is a core consulting skill because clients ask "how big is this opportunity?" before almost every strategic decision. Knowing how to structure a consulting case gives you the scaffolding to organize your sizing before a single number is calculated.

Think of market sizing like dead reckoning on a ship before GPS existed. A navigator who didn't know the exact position of the destination could still plot a course by using known speed, known heading, and elapsed time. The answer was approximate, but it was disciplined and defensible. Consultants do the same thing with markets: they don't have perfect data, so they build a chain of defensible assumptions that leads to an estimate they can commit to.

Interviewers want to see whether you can navigate this way. The skill is not arithmetic. It is the ability to decompose a large unknown into smaller knowns, chain them together, and know when to sanity-check the result.

Every market sizing problem can be attacked from two directions, and knowing which to use doubles your credibility.

Top-down sizing starts with a large known number and carves it down. Total US population, multiply by the fraction that fits your customer profile, multiply by average spend per customer per year. You start with the widest circle and shrink it.

Bottom-up sizing starts with the supply side. How many providers are in the market, what is their average revenue, multiply through. You build the market from its component parts.

Use top-down when customer data is more reliable. Use bottom-up when supply data is more reliable. When both are available, use both and triangulate. A gap between the two estimates tells you where your assumptions are weakest.

When you get a market sizing question, resist the urge to jump straight to math. Take 30 seconds, write down your structure, and narrate it before you calculate.

Say: "I'll size this bottom-up using the supply side, then sanity-check with a top-down approach. I'll start by estimating the number of providers, then average revenue per provider." Then execute that plan.

Structure before numbers is not a courtesy. It is how you avoid getting lost mid-calculation and having to restart. The candidate who narrates a clear structure and then executes it, even with a small arithmetic error, outperforms the candidate who dives into numbers immediately and loses the thread.

Practice this framework on a real case: the Zoom COVID-2020 case on BoardroomIQ forces you to think through explosive demand scaling, which is exactly the kind of market sizing challenge where your assumptions about adoption rates and user behavior determine whether your estimate is in the ballpark.

Every assumption you make is an invitation for the interviewer to push back. That is not a threat. It is the format.

When you state an assumption, give it a number, a rationale, and a range. "I'll assume average American household spends $150 per year on streaming. That feels right based on typical subscription prices of $12-15 per month for one to two services, and I'd expect the true number is somewhere between $100 and $200." Now the interviewer can correct you, push you to the edge of the range, or accept it and move on. Any of those paths keeps the case moving.

The assumption you don't state out loud is the assumption that will kill you. Interviewers cannot push back on what they can't hear.

You calculated a number. Before you say it with confidence, run two sanity checks.

First, does the per-unit math work? If you estimated the US coffee market at $100 billion, that implies the average American spends $300 per year on coffee. Does that feel right? At roughly $5 per visit and assuming three visits per week for half the population, you get $390 per year per coffee drinker. Close enough to be defensible. If your implied per-unit number is wildly off, your aggregate estimate is wrong.

Second, can you find a proxy? If you know the US restaurant industry is about $900 billion, and you think coffee shops are maybe 5% of that, that implies $45 billion. Your bottom-up estimate and your proxy should land in the same order of magnitude. When sizing a growing market, it also helps to anchor on a CAGR — knowing whether the market is growing at 3% or 30% per year changes how you project current size from historical reference points.

Exercise 1: Daily sizing warmup. Every morning, pick one object in your environment and size the market. The US ballpoint pen market. The NYC taxi market. Do it in under 5 minutes and write down your three key assumptions. This builds the muscle memory for decomposition before you need it under pressure.

Exercise 2: The sense-check drill. Find published market size data for 10 industries. Size each independently using first principles, then compare to the published number. When you're off by more than 3x, diagnose which assumption broke. The diagnosis is more valuable than being right.

Exercise 3: Peer pushback sessions. Present your sizing out loud to a partner. Ask them to challenge two of your assumptions. Practice defending them with logical rationale, not just restating the number louder. The ability to defend under pressure is the actual interview skill.

The best way to practice market sizing is under realistic pressure, with a case that fights back.