The short answer
To estimate market size, count the buyers you could realistically serve and multiply by what each would pay per year. That bottom-up figure gives your serviceable market (SAM); a wider version covering everyone with the problem is your total market (TAM); the share you could plausibly win in a few years is your obtainable market (SOM). Bottom-up beats quoting a large industry figure because every number can be checked.
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What TAM, SAM and SOM actually mean
All three are annual revenue figures, not counts of people. Keeping them in money forces you to think about price from the start, which is where most sizing goes wrong.
- TAM, total addressable market — Annual revenue if every possible buyer with this problem bought your product at your price. A ceiling, not a target.
- SAM, serviceable addressable market — The part of TAM you can actually serve with your product, geography, language, channel and business model today.
- SOM, serviceable obtainable market — The part of SAM you could realistically win within a defined period, usually three to five years, given competition and how fast you can sell.
Why bottom-up beats top-down
Top-down sizing starts with a big published number, such as the value of an entire industry, and takes a percentage of it. It is quick and it produces impressive figures, but it hides every assumption. Saying you will capture one per cent of a huge market tells nobody how you will find, reach or convert a single customer.
Bottom-up sizing starts from units you can count: the number of businesses or people who fit your buyer profile, and the price each would pay. Each input can be checked, argued with and improved. If someone challenges your estimate, you can point to the specific number they disagree with.
Bottom-up also tells you something useful about the business itself. If you need a large share of every buyer in your segment to reach a modest revenue goal, the market may be too small or the price too low. A top-down figure will never reveal that.
How to build a bottom-up estimate
- Define the buyer precisely — A type of organisation or person, plus the characteristics that make them a buyer. 'Independent physiotherapy clinics with two to ten practitioners' is countable; 'healthcare' is not.
- Count them — Use public registers, trade association membership figures, industry directories, or government business statistics. Note the source of each count and how confident you are in it.
- Set an annual price per buyer — Use your intended price, ideally tested in conversations, not a hopeful one. Multiply monthly prices by twelve.
- Apply filters for SAM — Remove buyers you cannot serve yet: other countries, other languages, sizes your product does not suit, or segments your channel cannot reach.
- Estimate a realistic share for SOM — Base it on how many customers you can actually acquire per month given your sales approach, not on a round percentage.
A worked illustrative calculation
The table below uses invented numbers for a hypothetical booking and notes tool for small physiotherapy clinics. The counts are placeholders to show the method; in a real estimate each would come from a source you can cite.
| Step | Assumption | Calculation | Result |
|---|---|---|---|
| Buyers with the problem (TAM base) | Say 20,000 small clinics across target region | 20,000 clinics | 20,000 |
| Annual price per clinic | Say 60 a month | 60 x 12 | 720 a year |
| TAM | Every clinic buys at that price | 20,000 x 720 | 14.4 million a year |
| SAM filter | Only English-speaking markets, 2-10 practitioners: say 40% of clinics | 20,000 x 0.4 = 8,000 clinics; x 720 | 5.76 million a year |
| Realistic acquisition | Say 25 new clinics a month for three years, minus churn: about 700 retained | 700 x 720 | about 504,000 a year |
| SOM | Share of SAM won in three years | 700 / 8,000 | about 9% of SAM |
What the example tells you
In this example, the TAM looks large, but the SOM is about half a million a year after three years. That is a perfectly reasonable small business, a decent bootstrapped company, and probably too small for venture investors who need a path to far larger outcomes. Neither interpretation is wrong; they suit different goals.
Notice that SOM was derived from an acquisition rate, not chosen as a percentage. Working from 'how many customers can I win per month' is more honest than picking a share, because it ties the market size to an actual sales plan. If 25 clinics a month seems implausible with your channels, lower it and see what happens to the result.
Also notice how sensitive the result is to price. Doubling the price to 120 a month doubles every revenue figure. Market size and pricing are the same conversation, which is why sizing without a tested price is guesswork.
Finally, use the estimate to set a range rather than a single figure. Recalculate with a pessimistic and an optimistic version of each key input, such as half and double the acquisition rate, and present all three. If even the optimistic SOM does not support the business you want, you have learned something important for the cost of an afternoon, and no amount of growth tactics will change that arithmetic.
Common market sizing mistakes
- Quoting an industry report figure as your market — The size of a whole industry is not the amount anyone will spend on your product.
- Counting people instead of buyers — Everyone who has the problem is not someone with budget and authority to pay for a solution.
- Using an untested price — An optimistic price inflates every layer of the estimate.
- Picking SOM as a round percentage — 'We only need 1%' skips the question of how you would win that 1%.
- Ignoring churn — Customers you win and then lose do not count towards the market you hold.
- Mixing one-off and recurring revenue — Keep everything annual and be explicit about which is which.
- Hiding sources — If you cannot say where a count came from, nobody, including you, should trust it.
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Questions
What is the difference between TAM, SAM and SOM?
TAM is the annual revenue if every possible buyer bought your product. SAM is the part of that you can actually serve today, given your product, geography and channels. SOM is the part of SAM you could realistically win in a set period, typically three to five years. Each is narrower than the last, and SOM is the one that matters for planning.
Is top-down market sizing ever useful?
It can be a rough sanity check. If your bottom-up SAM is larger than a credible published figure for the entire industry, one of your assumptions is probably wrong. But top-down should not be your main estimate, because it says nothing about how many customers you can reach or what they will pay.
How big does a market need to be for a startup?
It depends on your goal. A business aiming to support a small team profitably can thrive in a market where SOM is modest. A venture-funded company usually needs a path to a very large outcome, which requires a much bigger SAM. Decide which you are building first, then judge the market against that goal rather than against an abstract threshold.
Where can I find data to count my buyers?
Common sources include government business registers and statistics, trade association membership numbers, professional licensing bodies, industry directories, and listings on review or marketplace sites. Each has gaps, so note the source and your confidence for every count, and cross-check two sources where you can.
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