18.1 Why Firms Ask Market-Sizing Questions
Market sizing combines several skills in a compact exercise: structuring, numerical reasoning, business judgment, assumption setting, and communication.
You may be asked to estimate the annual market for coffee in a city, the number of electric-vehicle chargers needed in a country, or the revenue opportunity for a new software product.
What is being tested: Can you turn an unfamiliar question into a logical calculation using limited information?
18.2 Top-Down Estimation
A top-down approach starts with a large population or market and narrows it to the relevant group.
For example, to estimate the annual market for running shoes, you might begin with the country’s population, estimate the percentage who run regularly, estimate pairs purchased per runner per year, and multiply by average price.
Use top-down: When population-level information is relatively easy to estimate and the market can be narrowed through logical segmentation.
18.3 Bottom-Up Estimation
A bottom-up approach begins with smaller economic units and builds toward the total market.
For example, to estimate restaurant revenue in a city, you might estimate the number of restaurants, average customers per restaurant per day, average spending per customer, and operating days per year.
Use bottom-up: When you can estimate the number and economics of individual outlets, customers, transactions, or assets reasonably well.
18.4 Population-Based Approaches
Population-based sizing starts with total population and narrows it based on relevant characteristics.
A simple structure might be:
Market Size = Population × Relevant Population Share × Usage × Price
Useful segmentation variables include age, income, employment, geography, and behavior.
Warning: Do not create unnecessary segments. Segment only when different groups have meaningfully different likelihoods of buying or using the product.
18.5 Household-Based Approaches
Some products are purchased at the household rather than individual level. Examples include refrigerators, home internet, insurance policies, and household cleaning products.
In these cases, beginning with households may be more accurate than starting with population.
Typical structure: Households × Penetration × Purchases per Household × Average Price.
Always ask whether the natural purchasing unit is a person, household, company, store, or another entity.
18.6 Customer-Based Approaches
Customer-based sizing is useful when you can estimate the number of potential customers and their average spending.
For a B2B software product, for example:
Market Size = Number of Target Companies × Adoption Rate × Annual Revenue per Customer
You might segment target companies by size if large companies are much more likely to purchase or pay significantly more.
Focus: Define the addressable customer clearly before estimating adoption.
18.7 Usage-Based Approaches
Usage-based sizing starts with how frequently a product or service is consumed.
For example, to estimate the annual coffee-shop market:
Consumers × Visits per Consumer per Week × Average Spend × 52 Weeks
This approach works well for recurring purchases such as food, transportation, entertainment, and subscription-related usage.
Key question: What behavior directly generates revenue?
18.8 Supply-Side Approaches
Sometimes it is easier to estimate a market from the supply side.
For example, you might estimate hotel revenue using the number of hotels, rooms per hotel, occupancy rate, and average nightly rate.
Supply-side structure: Number of Suppliers × Capacity × Utilization × Price.
A supply-side estimate can also provide a useful cross-check against a demand-side calculation.
18.9 Choosing Defensible Assumptions
Market sizing almost always requires assumptions. A good assumption is reasonable, clearly stated, and easy to defend.
Instead of saying, “I’ll assume 37% of people buy the product,” explain the logic behind your estimate. You might say, “I’ll assume roughly one-third of adults use the service, based on the fact that it appears to be a mainstream but not universal product.”
Best practice: Prefer simple, round assumptions unless additional precision is justified.
18.10 Segmenting Populations Intelligently
Segmentation improves an estimate when groups behave differently.
For example, teenagers, working-age adults, and retirees may have very different usage patterns for ride-sharing services.
However, excessive segmentation creates complexity without improving accuracy.
Practical rule: Segment when the difference in behavior is large enough to materially change the answer.
18.11 Avoiding False Precision
Market sizing is an estimation exercise. Using highly precise assumptions can make an answer appear less credible rather than more credible.
An assumption of 30% adoption is usually more appropriate than 31.7% unless the interviewer has provided exact data.
Remember: Precision in arithmetic does not compensate for uncertainty in the assumptions.
18.12 Sanity Checks
After reaching an estimate, test whether it is plausible.
Useful checks include:
- Per-person check: What does the result imply per person or household?
- Revenue check: Does the implied market seem reasonable relative to known industries?
- Capacity check: Could suppliers realistically serve this level of demand?
- Alternative approach: Would a different sizing method produce a similar order of magnitude?
If your answer implies every resident buys ten refrigerators per year, something is clearly wrong.
18.13 Communicating Uncertainty
Your estimate will not be exact, and you should not pretend otherwise.
State which assumptions have the greatest impact on the result. If appropriate, explain how changing one important assumption would affect the estimate.
Example: “The result is approximately $500 million annually, with the largest uncertainty coming from my assumption about purchase frequency.”
This demonstrates judgment without undermining your conclusion.
18.14 Standalone Market-Sizing Questions Versus Market Sizing Within a Case
A standalone market-sizing question may be primarily about demonstrating your estimation process. In a full case, market sizing usually serves a larger decision.
If a client is considering market entry, for example, the important question is not simply whether the market is worth $2 billion. You may also need to estimate the realistic share the client could capture and whether that revenue would justify the required investment.
Final principle: Market sizing is not about guessing the exact market number. Define the market carefully, choose a logical approach, make transparent assumptions, keep the arithmetic manageable, and test whether your answer makes sense. Most importantly, when sizing is part of a broader case, connect the result back to the client’s decision.
