Market sizing is one of the most misunderstood parts of a fundraise. Teams treat it as a marketing exercise, picking the largest plausible number from a Gartner or IDC report and putting it on a slide. Investors have seen this move so many times it has become noise. The number that matters is not the total addressable market. It is the serviceable addressable market your specific go-to-market can reach in the next 24 months, built from the ground up with defensible assumptions. A $4B TAM figure pulled from an industry report is decoration. A $180M SAM derived from 12,000 companies matching your ICP, each spending an average of $15,000 per year on solutions in your category, is a business case.

The Problem with Top-Down Sizing

Top-down sizing starts with a macro market estimate from a third-party research firm and applies a percentage to arrive at your share of that market. It is fast, it gives you a big number, and it is almost always wrong in ways that matter. The macro estimates are usually built on different product definitions, different customer segments, and different geographic scopes than your actual business. When an investor asks how did you get to $400M SAM, and the answer is we took the $4B market and assumed 10% share, the conversation is over. There is no analytical content to probe. The method also systematically overstates near-term opportunity because it ignores the friction of actually reaching customers: distribution costs, sales cycle length, churn dynamics, and competitive displacement rates.

Building a Credible Bottom-Up Model

Bottom-up sizing forces you to do the work that makes the number credible, and every assumption becomes visible, debatable, and refinable.

Start with your ICP definition: the specific type of company or person that buys your product, defined by firmographic or demographic criteria narrow enough to count. Then size the population: how many entities matching that profile exist in your target geography. Then apply a conversion assumption: what percentage of that population has the problem you solve acutely enough to pay for a solution. Multiply by your expected contract value, adjusted for churn. That is your realistic SAM. The power of this method is not that the number is larger. It usually is not. The power is that every assumption is visible, debatable, and refinable. Investors can challenge your conversion rate or your ACV, and you can defend them with evidence from your pilot customers.

How RECON Builds Your Sizing Model

RECON builds bottom-up market sizing models from your ICP parameters and live market data, so you are not estimating population counts from scratch. The platform pulls firmographic data, industry classifications, and revenue band distributions to give you a defensible population estimate in minutes. It also flags the sensitivity drivers in your model: which assumptions, if wrong by 20%, would most change the output. That kind of sensitivity analysis is what separates a sophisticated market sizing presentation from a static slide. Investors at the Series A level expect founders to know exactly which levers matter and have a point of view on their current values.

Combining Both Methods Strategically

The most effective market sizing presentations combine both methods strategically. Lead with a credible bottom-up SAM built from your ICP, then situate it within a top-down TAM frame to show the expansion opportunity over a longer horizon. The bottom-up number earns trust. The top-down number earns ambition. The combination tells a story: here is what we can realistically capture in 36 months, and here is the ceiling of what this market can become if we execute the category vision. Be explicit about the assumptions connecting the two numbers. Growth path, expansion segments, and product extension roadmap should all map logically from SAM to TAM.

Sources and further reading: Sequoia Capital Market Sizing internal framework (publicly discussed) | CB Insights Global Venture Report 2024 | IBISWorld Industry Research methodology | Statista Digital Market Outlook methodology notes | McKinsey Global Institute Measuring the Addressable Market working paper