Market sizing is a staple in almost every strategy consulting interview, and it's increasingly popping up in M&A, Private Equity, and Transaction Services interviews too.

Most candidates know the drill by heart: clarify, structure, calculate, conclude. Yet, so many still fall short. Why? Because the methodology is only half the battle. What really sets you apart is a reflex that almost nobody clearly explains.

What is market sizing?

Market sizing—also known as a guesstimate—involves estimating the size of a market with little to no data available.

How many restaurants are there in Paris? What is the size of the lipstick market in the US? What is the annual revenue of a McDonald's on the Champs-Élysées? These types of questions don't have a single right numerical answer, but they do require sound reasoning.

Interviewers never actually care if you know the exact number. They are evaluating your ability to break down a complex problem, make logical assumptions, and piece them back together to reach a sensible order of magnitude.

The mistake 90% of candidates make

Most candidates automatically default to a demand-side approach without stopping to consider if it actually fits the question.

However, every market sizing problem hinges on a limiting factor—the bottleneck that actually constrains the market's size. And that factor isn't always demand.

Take two seemingly similar questions:

• How many Pepsis are consumed in the US each year? Here, supply is unlimited—any supermarket sells it. The limiting factor is consumption, which means it's driven by demand.

• How many visas are issued by an embassy in a year? Here, even if thousands of people apply, the number of visas issued depends entirely on the embassy's processing capacity. The limiting factor is supply.

Candidates who automatically use a demand-side approach without asking this question miss half the picture. Identifying the limiting factor before choosing your approach is precisely the reflex that separates top-tier candidates from the rest.

The two approaches you must master for market sizing

The Demand-Side Approach

This approach starts with the population or number of households, then applies a percentage of target customers, purchasing frequency, and average price. It works in most cases, but only when there are no constraints on accessing the product or service.

The Supply-Side Approach

This approach starts with the number of suppliers, then factors in production capacity and utilization rates. It is essential whenever capacity constraints exist—such as available seats, staffing limits, or fixed infrastructure.

Some cases, like estimating the number of restaurants in a city, require blending both approaches—for example, starting with customer demand to determine a realistic number of establishments based on their seating capacity.

Market Sizing: Top-Down vs. Bottom-Up, your next big decision

Once you have chosen your approach, you need to decide on the direction of your logic.

Top-down starts with a known macro figure, like the total population, and refines it step-by-step. It is quick but can sometimes lack precision.

Bottom-up starts with individual behavior and scales up to a global estimate. It is more demanding, but it is often more rigorous and carries more weight in an interview.

The two approaches can also complement each other: you can quickly cross-check a bottom-up result with a top-down calculation to make sure both align on the same order of magnitude.

How to structure your market sizing analysis

Once you have chosen your approach, solving the problem always follows the same four-step logic.

Clarify the question before doing any math. Volume or value? What geography? What timeframe? What product scope? These questions aren't just a formality—they show the interviewer that you know how to frame an ambiguous problem before diving in.

Set up a simple macro equation. For example: Market Size = Units Sold × Average Price. This equation serves as the roadmap for your entire analysis.

Break down each variable into justified assumptions. This is the longest and most revealing step. For every number you propose, you must be able to explain where it comes from—whether it's personal observation, a benchmark from a similar industry, or logical deduction.

• Always round your numbers to make mental math easier;

• Use powers of 10 instead of dealing with endless zeros;

• Talk through every assumption out loud, even if it's an estimate—your logic matters far more than exact precision.

Calculate, then sanity-check your result. A raw number means nothing without context. Dividing the result by the total population, for example, is a great way to spot a sanity-check error before presenting it to the interviewer.

A concrete example to illustrate market sizing

Let’s look at this question: What is the size of the chewing gum market in France, by value?

Let's start with a rounded French population of 60 million, divided into four equal age groups of 15 million each. Let's assume that under-20s consume the most chewing gum, followed by 20-40 year olds, then very light consumption for 40-60 year olds, and almost none beyond that.

By estimating an average consumption of 2 packs per week for the youngest group and 0.5 packs for working adults, then multiplying by a weighted average price across premium, mid-range, and discount channels, we get a market size in the range of several hundred million euros per year.

The exact number doesn't matter. What counts is that every assumption is stated out loud and backed up by credible observations—like your own habits or those of people around you.

The most common market sizing mistakes

Certain mistakes crop up constantly and can completely derail an otherwise solid performance.

• Diving into calculations without structuring them first, which makes you look like you're winging it;

• Thinking in silence, which prevents the interviewer from following—or course-correcting—your reasoning as you go;

• Making unrealistic assumptions, like putting the French population at 100 million, which shows a lack of general business knowledge;

• Aiming for excessive precision, such as calculating down to multiple decimals, when the exercise is all about finding an order of magnitude;

• Delivering a raw number without context, which misses a key opportunity to show you can take a step back and analyze your own work.

Key market sizing takeaways before your interview

Market sizing isn't a mental math test. It’s a test of judgment—specifically, your ability to choose the right approach before you even start calculating.

Before practicing dozens of cases, get into the habit of asking yourself one simple question for every prompt: What actually limits this market—supply or demand?

That reflex, much more than memorizing formulas, is what will make you stand out to an interviewer who has already heard the exact same approach hundreds of times.