Market Sizing for Founders

Market sizing is the exercise most founders do worst and most investors judge hardest. The standard approach, start with a huge TAM, narrow to SAM, narrow again to SOM, produces numbers that are simultaneously too large to be useful and too small to be impressive. The exercise becomes theatre: numbers designed to impress rather than inform.

This essay proposes a different framework. One that starts from the bottom, builds upward, and produces numbers that actually guide strategy.

Core Insight

The fundamental problem with top-down market sizing is that it treats markets as containers to be measured. Markets are not containers. Markets are groups of people who will pay for a specific solution to a specific problem. The size of the market depends on the specificity of the solution, which means the market size changes as the product evolves.

This has a crucial implication: there is no fixed market size for a startup. The market you can address today is different from the market you can address in two years, which is different from the market you can address in five. A useful market sizing exercise must capture this dynamism.

The framework has four components: entry market, expansion mixes, practical output, and common pitfalls.

Entry Market

The entry market is the market you can address right now, with the product you have today, selling to the customers you can reach with your current resources. It is always smaller than founders want it to be. That is the point.

To size the entry market, answer four questions:

1. Who has the problem? Not “who might have the problem” or “who could benefit from our solution.” Who has the problem right now, acutely enough that they are actively looking for a solution or spending money on workarounds? This is your buyer. Be specific: industry, company size, job title, geography.

2. How many of them are there? Count them. Not estimate. Count. If your buyer is “VP of Operations at European logistics companies with 500+ employees,” go to LinkedIn, apply the filters, and count. The number will be smaller than you hoped. That is correct. A market of 2,000 potential buyers is a real market if each buyer pays $50K/year.

3. What will they pay? Not what you want to charge. What they will pay. This requires evidence: conversations with potential buyers, analysis of existing spending on the problem, comparison with alternative solutions. Price anchoring against alternatives is more reliable than theoretical willingness-to-pay analysis.

4. What is your conversion rate? Of the 2,000 potential buyers, how many will you actually close in the first two years? Be honest. If you have no brand, no references, and a product that is still being built, a 2-5% conversion rate is realistic for the first year. Your entry market is not 2,000 x $50K = $100M. It is 2,000 x 3% x $50K = $3M. That is a useful number. It tells you whether you can build a viable business in year one.

The entry market answers the survival question: is there enough revenue here to keep the company alive while we build toward the expansion?

Expansion Mixes

The entry market funds the company. The expansion markets define its trajectory. Expansion happens along three axes, and each axis requires different capabilities:

1. Persona expansion. Selling to different roles within the same type of company. You started with VP of Operations. Can you sell to CFOs? To COOs? Each persona has different problems, different buying criteria, and different budgets. Persona expansion is the easiest axis because you already understand the industry.

2. Industry expansion. Applying your solution to adjacent industries. You built for logistics. Does the same problem exist in manufacturing? In retail? In healthcare? Industry expansion requires adapting the product, not just the sales pitch. Each industry has domain-specific requirements that must be encoded in the product.

3. Product expansion. Building additional products for your existing customers. You solved shipment exceptions. Can you solve demand forecasting? Route optimisation? Fleet management? Product expansion is the hardest axis because it requires building genuinely new capabilities, not just adapting existing ones.

Each expansion mix has a different risk profile, time horizon, and capital requirement. The market sizing exercise should model each axis independently and sequence them based on difficulty and capital availability.

Modelling Expansion

For each expansion axis, estimate:

• Time to enter: How many months before you can credibly sell in this expanded market?
• Incremental investment: What engineering, sales, and operational investment does the expansion require?
• Incremental TAM: How many additional buyers does this axis unlock, and at what price?
• Conversion advantage: Does your entry market success give you a conversion rate advantage in the expanded market (references, case studies, brand)?

The total addressable market is the sum of all expansion axes at full maturity. But the path to that total is not a straight line. It is a sequence of expansions, each funded by the previous one.

Practical Output

A useful market sizing exercise produces three numbers and one narrative:

1. Entry market (Year 1-2). The revenue you can generate with your current product, selling to your current buyer, in your current geography. Typically $1-10M for a B2B startup.

2. Near-term expansion (Year 3-4). The revenue you can generate after one persona or industry expansion. Typically 3-5x the entry market.

3. Full potential (Year 5+). The revenue at full expansion across all axes. This is your TAM, but it is built bottom-up from concrete expansion paths, not top-down from industry reports.

4. The narrative. The story that connects these three numbers. How does the entry market success enable the first expansion? How does the first expansion enable the second? This narrative is more important than the numbers because it demonstrates that the founder understands the path, not just the destination.

Common Pitfalls

Five mistakes that undermine market sizing exercises:

1. Confusing TAM with SAM with entry market. The TAM is the total revenue opportunity at full expansion. The SAM is the subset you could serve with your current capabilities. The entry market is what you can actually sell in the next 18 months. Investors want to see all three, but they judge you on the entry market’s realism, not the TAM’s size.

2. Using analyst reports as gospel. Gartner says the market is $47B. Great. But your entry market is not $47B. Your entry market is the tiny slice of that $47B where your specific solution solves a specific problem for a specific buyer. Analyst reports measure categories, not markets.

3. Ignoring willingness to pay. A large market where nobody will pay is not a market. It is a wish. Validate willingness to pay with actual conversations, not surveys. Ask potential buyers what they currently spend on the problem. If they spend nothing, they may not value a solution enough to pay for one.

4. Linear scaling assumptions. “We have 10 customers paying $50K. There are 10,000 potential customers. Therefore our market is $500M.” This ignores that the first 10 customers were the easiest to close. Each subsequent cohort is harder, not easier. The 10,001st customer is the one who has the weakest version of the problem and the most alternatives.

5. Ignoring competition dynamics. Your market sizing assumes you capture a share of the market. But competitors are also sizing the same market and planning to capture it. A realistic market sizing includes a competitive scenario: who else is going after this market, what share will they take, and what is your defensible position?

---

Market sizing is not a presentation slide. It is a strategic exercise. Done well, it reveals whether the entry market can sustain the company, whether the expansion path is credible, and whether the full potential justifies the risk. Done badly, it produces large numbers that impress no one and guide nothing.

Start with the entry market. Count the buyers. Validate the price. Model the expansion. Tell the story. That is all market sizing needs to be.