# TAM SAM SOM: market size in three layers, with a worked example

> TAM SAM SOM in plain words: the three layers of market size, how to calculate each with a worked example, and when the acronym actually helps.

- Source: https://productovi.com/blog/tam-sam-som
- By: Sergey Bruh
- Published: 2026-10-02
- Topics: Market size, Niche, Valuation

**TAM SAM SOM is a three-layer way to size a market. TAM is the total market for what you do, SAM is the part of it you can realistically serve, and SOM is the share of that you can realistically win in the next few years.** The three letters shrink in order, like a funnel: TAM is the ocean, SAM is the part of the ocean your boat can reach, and SOM is what you can actually catch. Investors ask for it to check that the ceiling is high enough, and founders use it to check that the plan adds up.

## The three layers in plain words

- **TAM — total addressable market.** Everyone who could spend money on the problem you solve, in the widest sense. For a pet-food shop, that's the whole pet-food market, every cat and dog owner in the country.
- **SAM — serviceable addressable market.** The part of TAM you can actually serve, given what your product is and where it sells. For an online-only pet-food shop, that's online pet-food buyers in the country — not every owner, because you can't reach the ones who only buy in a physical store.
- **SOM — serviceable obtainable market.** The share of SAM you can realistically win in a set period, usually two or three years. This is the number that matters for planning, because it's the only one you can actually act on.

Notice the direction: each layer is a narrowing of the one before. TAM is a claim about the world; SOM is a promise about you.

## A worked example: CatChow

CatChow is an online shop for high-quality cat food, starting in one country. Here is how its founder builds the three layers from the top down.

First, TAM. The country spends about 4 billion dollars a year on cat food. That's the total addressable market: every euro, dollar or pound anyone spends feeding a cat.

Second, SAM. CatChow only sells online and only premium food, so it can't serve everyone in TAM. Roughly 20% of cat-food spending happens online, and within that, maybe half buys premium. That gives a serviceable addressable market of about 400 million dollars a year: 4 billion times 20% times 50%.

Third, SOM. A new shop can't win the whole online premium segment in three years. If CatChow reaches about 5% of that segment, its serviceable obtainable market is about 20 million dollars a year. That's the number the business plan should be built on.

The chain is the whole lesson: **4 billion → 400 million → 20 million.** Each step takes a share of the previous number, and each step is a checkable assumption.

![CatChow's market narrows from 4 billion dollars of cat-food spending (TAM) to a 400 million dollar online premium segment (SAM) and a 20 million dollar share it can win in three years (SOM)](/blog/tam-sam-som/diagram.png)

## How to calculate each layer

- **TAM** usually comes top-down from a market report: total spending, total units, total number of buyers. It's the easiest to find and the least useful on its own.
- **SAM** comes from filtering TAM by your actual constraints: geography, channel (online vs store), price point, customer type. Each filter is a percentage, and each percentage is an assumption you can test.
- **SOM** comes from your own capacity and competition: how fast can you realistically grow, and what share of the segment would that make? It's a judgement call, not a lookup.

The [TAM/SAM/SOM worksheet](/tools/tam-sam-som-calculator) does the multiplication for you — enter the three percentages and the total, and it shows the layers and how sensitive the final number is to each assumption.

If you'd rather work from the bottom up — counting the customers who could actually buy and multiplying by your price — that's the other half of market sizing, and it's covered in [Bottom-up market sizing](/blog/bottom-up-market-sizing). The two approaches are best used together: TAM/SAM/SOM gives you the ceiling, bottom-up gives you the floor.

## When TAM SAM SOM helps, and when it's just a slide

The acronym earns its place in three situations:

1. **Checking the ceiling.** If your SOM, even optimistically, is far smaller than the business you want to build, that's a signal to change the niche or the plan before you spend a year on it.
2. **Comparing niches.** Run the same three layers for two candidate markets and the differences become visible and comparable, instead of a gut feeling.
3. **Talking to investors.** Investors want to see a big TAM and a believable SAM/SOM path, because it tells them how much room there is to grow and whether you understand who you can actually reach.

Where it turns into theatre is when the numbers are pulled from a report and multiplied without ever being tested. A giant TAM with a SOM that's "1% of a huge market" is a story, not a plan. The only layer that matters for this year is SOM, so that's the one worth the most scrutiny.

## TAM SAM SOM vs bottom-up market sizing

|                | TAM SAM SOM                               | Bottom-up                                             |
| -------------- | ----------------------------------------- | ----------------------------------------------------- |
| Direction      | Top-down: start from the whole market     | Bottom-up: start from one real customer               |
| First question | "How big is the ocean?"                   | "How many customers like this one can I find?"        |
| Strong at      | Ceiling, comparing niches, investor story | A plan you can act on next quarter                    |
| Weak at        | Untested percentages can flatter you      | Understates the upside if you miss adjacent customers |

Use both. TAM SAM SOM keeps you honest about the ceiling; bottom-up keeps you honest about the first steps. The [free lesson on bottom-up market sizing](/courses/bootstrapped-product/the-market-bottom-up/theory) walks through the customer-by-customer version.

## Key takeaways

- **TAM SAM SOM = total, serviceable, obtainable.** Three shrinking layers: the whole market, the part you can serve, the part you can win.
- Each step is a narrowing: TAM is a claim about the world, SOM is a promise about you.
- The worked example collapses to **4 billion → 400 million → 20 million** dollars for one shop.
- TAM comes from reports, SAM from filtering by your constraints, SOM from your own capacity and competition.
- The only layer that matters for planning is SOM; a big TAM with an untested "1% share" is theatre, not a plan.
- Pair it with bottom-up sizing: TAM/SAM/SOM gives the ceiling, bottom-up gives the floor.

## FAQ

### What does TAM SAM SOM stand for?

Total addressable market, serviceable addressable market, and serviceable obtainable market. They describe the whole market, the part you can actually serve, and the share you can realistically win.

### How do I calculate SAM?

Filter TAM by your real constraints, each as a percentage: geography, channel (online vs in-store), price point, and customer type. Multiply them through. Every percentage is an assumption, so note it and test it rather than trusting it.

### What is a good SOM percentage?

There's no universal number. A believable SOM is a small share of SAM that still reflects how fast you can actually grow in two or three years — often single-digit percent for an early product. If your plan needs 40% of SAM to work, the plan is the problem.

### What's the difference between TAM SAM SOM and bottom-up sizing?

TAM SAM SOM works top-down from the whole market, while bottom-up sizing counts the customers who could actually buy and multiplies by price. They complement each other: TAM SAM SOM sets the ceiling, bottom-up sets the floor. See [Bottom-up market sizing](/blog/bottom-up-market-sizing).

## Related lessons

- [The market, bottom up: a market size example](https://productovi.com/courses/bootstrapped-product/the-market-bottom-up.md)
- [Ideal customer profile: narrow or widen the niche](https://productovi.com/courses/bootstrapped-product/narrow-or-widen-the-niche.md)
- [Market expansion: another city or another kind of shop?](https://productovi.com/courses/bootstrapped-product/another-city-or-another-kind-of-shop.md)
- [Money milestones for a startup](https://productovi.com/courses/bootstrapped-product/money-milestones.md)
- [How much is the business worth?](https://productovi.com/courses/bootstrapped-product/what-the-business-is-worth.md)