Validation guides
TAM, SAM, SOM for AI Startups: A Bottom-Up Sizing Example
Most market size slides start with a large number from an industry report. A bottom-up estimate starts with buyers you can count. Here is the difference, worked through with a real public count and every assumption shown.
TAM is the total annual revenue if every possible buyer bought your product. SAM is the part you can actually serve with your product, location and channel. SOM is the part you can realistically win in the next few years. Calculate them bottom-up: count the buyers, multiply by what each would pay a year, then narrow with assumptions you write down.
A common question is how to size a TAM without making the number up. The honest answer is that some assumptions are unavoidable. What you can avoid is hiding them.
Key takeaways
- TAM, SAM and SOM are three nested estimates: everyone who could buy, everyone you can serve, and the share you can win.
- Bottom-up beats top-down for an early startup: count buyers and multiply by price.
- Start from a public count where one exists, such as government employment or business statistics.
- Write every assumption next to the number, so a reader can change it.
- Investors and buyers care about SOM most. It is the only one that says what you will actually sell.
What do TAM, SAM and SOM mean?
They are three nested estimates of annual revenue: the total addressable market, the serviceable available market, and the serviceable obtainable market.
| Term | Stands for | The question it answers |
|---|---|---|
| TAM | Total addressable market | If every possible buyer bought, what would they spend a year? |
| SAM | Serviceable available market | How much of that can your product, location and channel actually serve? |
| SOM | Serviceable obtainable market | How much can you realistically win in the next few years? |
Each is a subset of the one above it. A pitch that shows only TAM tells a reader very little; a pitch that shows how you got from TAM to SOM tells them how you think.
Should you size the market top-down or bottom-up?
Bottom-up, whenever you can count the buyers. Top-down figures from industry reports are useful as a sanity check, but they rarely say whether your first customers exist.
Top-down starts from a published total ("the dental software market is worth X") and takes a percentage. It is fast, but the source's definition of the market is rarely the same as yours, and the percentage is usually a guess.
Bottom-up starts from the number of buyers and what each would pay. It is slower, but every input can be checked and argued with, which is exactly what you want from a number you are going to make decisions on.
What does a bottom-up TAM, SAM and SOM calculation look like?
Here is one for a hypothetical AI tool that writes clinical notes for general dentists, sold at an assumed $100 per dentist per month. Only the dentist count is data; everything else is a labelled assumption.
Step 1, count the buyers. The US Bureau of Labor Statistics estimates that offices of dentists employed 109,280 general dentists in May 2023 (BLS). That is our buyer count.
Step 2, TAM. 109,280 dentists × $100 a month × 12 months = $131,136,000 a year.
Step 3, SAM. Assume the product only works with the practice management systems you integrate with at launch, and that those cover 40% of practices. $131,136,000 × 40% = $52,454,400 a year.
Step 4, SOM. Assume that with one founder selling and one channel, you could win 2% of SAM within three years. $52,454,400 × 2% = $1,049,088 a year.
| Input | Value | Type |
|---|---|---|
| General dentists in offices of dentists | 109,280 | Data (BLS, May 2023) |
| Price per dentist | $100 a month | Assumption |
| Share of practices you can integrate with | 40% | Assumption |
| Share of SAM won in three years | 2% | Assumption |
The outputs matter less than the table. Anyone who disagrees with an assumption can change it and see the effect, and your first sales conversations will tell you whether $100 a month holds.
What are the most common TAM, SAM, SOM mistakes?
Using a TAM that includes buyers you could never serve, picking a SOM percentage with no reasoning, and presenting the numbers without the assumptions behind them.
- The borrowed TAM. A figure for "global healthcare software" is not the market for a dental notes tool.
- The 1% SOM. "If we get just 1% of the market" is not a plan. Tie SOM to how many deals your channel and team can actually close.
- Hidden assumptions. If a reader cannot see the price or the percentages, they cannot trust the result.
- Counting users instead of buyers. In B2B, the unit is often the company or the practice, not the individual user.
- Confusing people with establishments. Our example counts dentists, not practices. If you price per practice, you need a count of practices instead.
How can you calculate TAM, SAM and SOM quickly?
Use a structured calculator that asks for each input separately, so the assumptions stay visible. NELL's is free.
The free TAM, SAM, SOM calculator walks through the same steps as the example above. For a full report on a specific idea, DeepValidate adds sourced market sizing from live research, alongside named competitors and demand signals, and links every figure to where it came from.
For sizing a niche B2B market by accounts rather than people, see our guide to market sizing for a niche B2B startup.
Frequently asked questions
What is the difference between TAM, SAM and SOM?
TAM is the total annual spend if every possible buyer bought. SAM is the part your product, location and channel can serve. SOM is the part you can realistically win in the next few years.
How do you size a TAM without making the number up?
Start from a count you can cite, such as a government statistic on how many businesses or professionals exist, multiply by a stated price, and write every assumption next to the number so a reader can check or change it.
Is a bigger TAM always better for a startup?
No. A very large TAM with no clear path to the first customers is weaker than a smaller market you can reach. Investors usually look at how you get from TAM to SOM.
What is a realistic SOM for a startup?
It depends on how many deals your team and channel can close. Build it from sales capacity, for example deals per month times months times price, rather than picking a percentage.
Should TAM be annual or lifetime revenue?
Annual. TAM, SAM and SOM are normally stated as yearly revenue, which makes them comparable with your own revenue targets.
Start where you are
The worked example uses one public count and three assumptions. Yours can be the same.
Sources
The dental notes tool is hypothetical. Only the dentist count is data; the price and percentages are assumptions for illustration, not estimates of any real market.