Blog · 24 Feb 2026

How to size your market without guessing

Market sizing has a bad reputation among founders, and it has earned it. Most of the numbers in most decks are reverse-engineered to look impressive. Investors know this, which is why a credible market size is now a differentiator rather than a formality.

Done properly, sizing is not a slide. It is a test of whether the business can ever be large enough to justify the years you are about to spend on it.

The three numbers, plainly

• TAM is everyone who has the problem, if you had no constraints at all.
• SAM is the portion you could serve with your current model, geography and channel.
• SOM is what you can realistically win in three to five years given your team and capital.

The mistake is treating these as three sizes of the same circle. They are three different questions: is the problem widespread, can we reach the people who have it, and can we beat the people already trying.

Build it from the bottom, every time

Top-down sizing starts with a big industry report and applies a percentage. It is fast, and it is worthless, because the percentage is invented.

Bottom-up sizing starts with a unit you can count. The formula is always some version of:

number of potential customers, multiplied by what each pays, multiplied by how often they pay

Suppose you sell an inventory tool to independent pharmacies in Egypt. You need three inputs: roughly how many independent pharmacies exist, what share of them are plausibly reachable through your channel, and what a pharmacy would pay per month. Each of those is a number you can research, defend, and be wrong about in a specific way. That is what makes it credible.

Sanity-check against reality

Once you have a bottom-up figure, test it two ways. First, compare it against the revenue of the largest existing player in the space. If your SAM is smaller than a competitor's actual revenue, your inputs are wrong. Second, ask what would have to be true for the number to be right. If the answer requires every pharmacy in the country to adopt within two years, adjust.

The number nobody asks for but everybody should

More useful than TAM, at seed stage, is the answer to this: how much revenue can you get from the next fifty customers you could name? Not a segment. Named accounts. Founders who can answer that have a plan. Founders who can only answer TAM have a spreadsheet.

When the market is genuinely small

Sometimes honest sizing reveals a market that cannot support a venture-scale outcome. This is useful information, not a failure. It means one of three things: the business is a strong, profitable, founder-owned company that should not raise venture money, or the wedge is right but the eventual market is adjacent, or the idea needs rethinking. All three are better discovered now than after two years.