How to calculate TAM, SAM and SOM

What TAM, SAM and SOM actually mean

TAM (Total Addressable Market) is the annual revenue available if every possible buyer on earth bought your category of solution from someone. SAM (Serviceable Addressable Market) narrows that to the segment your product and business model can actually serve — the right geography, language, regulation, price point and channel. SOM (Serviceable Obtainable Market) is the slice of the SAM you can realistically win in a defined period, given your capacity, competition and distribution.

The three numbers are not a funnel of optimism. They are a chain of arguments: TAM says the category is worth entering, SAM says you are legally and operationally able to sell into part of it, SOM says here is the revenue you are forecasting and here is why it is achievable.

Bottom-up beats top-down every time

Top-down sizing starts with an analyst report — 'the market is worth $40B, we only need 1%'. Nobody experienced believes it, because 1% is not a plan. Bottom-up sizing starts with a countable unit and multiplies by price.

TAM = (number of potential customers worldwide) × (annual revenue per customer). SAM = (customers you can serve: your regions, segments, compliance envelope) × (your actual price). SOM = SAM × (share you can win through named channels in the period) — where that share is derived from channel capacity, not from a round percentage.

The countable unit is the whole trick. Solo physiotherapy practices. Shopify stores doing over $1M GMV. Registered accountants in Nigeria. Fleet operators with 20–200 vehicles. If you cannot count it from a directory, an association register, a job-posting scrape, a platform app store or a census table, your market size is a guess dressed as arithmetic.

A worked example

Product: scheduling and billing software for independent physiotherapists, priced at $40/month ($480/year).

TAM: roughly 400,000 solo/small physiotherapy practices worldwide × $480 = about $192M/year. That is the category ceiling for this price point — note it is far smaller than the 'healthcare software' figure you would find in a report, and that honesty is the point.

SAM: you launch in English-speaking markets with card payments and no clinical-record regulation in scope — about 90,000 practices × $480 = about $43M/year.

SOM: your only channels are two professional associations (combined reach 12,000 practices), search traffic worth roughly 300 qualified visits/month, and partner referrals from 15 equipment suppliers. Model realistic conversion: 12,000 × 3% = 360 customers, search 3,600 visits/year × 2% = 72 customers, partners 60 customers. Total ≈ 492 customers × $480 = about $236K ARR in year one. That is a SOM you can defend line by line.

The five mistakes that destroy credibility

Using a category report as TAM. 'Global HR tech is $30B' tells a reader nothing about your product, and signals that you have not counted your buyer.

Assuming a percentage of market share. Share is an output of channel capacity and conversion, never an input.

Pricing the TAM at enterprise rates and the SOM at self-serve rates. Keep one price per layer and state it.

Ignoring the serviceable constraint. Regulation, language, payment rails and support hours routinely cut a market by 70% — say so explicitly and you look rigorous, hide it and you look naive.

Sizing revenue you cannot collect. If the buyer needs a procurement cycle you cannot survive, that revenue belongs in TAM, not in SOM.

Sanity checks before you present it

Divide your year-one SOM by your average contract value. If the implied customer count is larger than the total reach of every channel you have named, the number is fiction.

Compare your SOM to the revenue of the closest comparable company at the same age. Being 10× better than a funded competitor's first year needs an explanation.

Recompute the whole chain at half your price and at double it. If the business only works at one specific price, pricing is your biggest unvalidated risk — test it before you build.