TAM/SAM/SOM Market Sizing

What it is

A way of estimating how big a market is in three nested layers. Total addressable market (TAM) is the revenue available if every possible customer for this kind of solution bought it. Serviceable addressable market (SAM) is the part of TAM your product can actually serve today, given its features, geography, price point and channels. Serviceable obtainable market (SOM) is the share of SAM you can realistically win in a set period, usually three years, given your competition and go-to-market capacity. No single person invented the approach; it grew out of venture capital and start-up practice, and Steve Blank and Bob Dorf's The Startup Owner's Manual is one of the best-documented versions. The layers matter because each answers a different question: TAM tells leadership whether the opportunity is worth pursuing at all, SAM tells PMM which segments to target, and SOM sets a realistic revenue goal. The most reliable method builds the numbers from the bottom up (number of target customers multiplied by what each would pay) rather than taking a percentage of a large analyst figure.

When to use it

• Choosing between segments in STP targeting, where each candidate segment needs a size estimate before you compare attractiveness.
• Building an investment case for a new product, a new vertical or international expansion, where leadership or investors will ask how big the prize is.
• Setting annual or three-year revenue goals for a product line, so targets rest on the number of reachable customers rather than last year's growth plus a percentage.
• A board deck or fundraising round quotes a top-down market figure that nobody in the business can explain or defend.
• Comparing growth options (for example, in an Ansoff Matrix or BCG Growth-Share Matrix review), where each option needs a sized market so risk and reward can be compared.

How to run it

1. Define the customer and the problem precisely. Write one sentence naming who buys, what problem the product solves, and what counts as a sale. "Mid-sized UK accountancy firms buying practice management software" can be sized; "SMBs that need productivity" cannot.
2. Build TAM from the bottom up. Count every organisation (or person) that has the problem, using public business registers, industry associations, LinkedIn company counts or census data. Multiply by the annual contract value a typical customer would pay. Record every source.
3. Cross-check with a top-down estimate. Find an analyst or industry figure for the category and compare it with your bottom-up TAM. A gap of more than about two times means one of your assumptions is wrong; investigate before moving on.
4. Narrow to SAM. Remove customers you cannot serve today: the wrong geography, the wrong size, missing integrations, required compliance you lack, or a channel you do not have. Each cut should be a named, sourced filter.
5. Estimate SOM. Apply a realistic share of SAM for the next three years based on evidence: your current win rate, sales capacity, marketing reach, and how many competitors split the market. A new entrant claiming more than a low single-digit share of SAM in three years needs strong evidence.
6. Document assumptions and ranges. Present a low, central and high case, with the two or three assumptions that move the number most. Leadership needs to see which assumptions to test.
7. Feed the numbers into decisions. Use SAM by segment in STP targeting, SOM in revenue planning and goal setting, and TAM in investment cases.
8. Refresh when inputs change. Update the model when pricing changes, when the product opens a new segment, or at least once a year in planning.

Cadence & ownership

PMM commonly runs market sizing at early and growth-stage companies, because it holds the segmentation, pricing and competitive inputs. At a scaled company, a strategy, finance or market research team often owns the official model, with PMM supplying segment definitions and validating assumptions against customer evidence; finance usually signs off any figure used in an investor or board document. A full model takes one to three weeks the first time. Refresh it annually during planning, and whenever pricing, target segments or geography change.