TAM SAM SOM · market sizing
TAM SAM SOM, sized in accounts you can name
TAM SAM SOM are three nested measures of a market: the total addressable market, the part you can serve, and the part you can realistically win.
This guide defines each one, shows the top-down and bottom-up methods, and gives you a calculator that starts from real UK business counts.
Reviewed by Dan Keegan, GTM Quest · 24 October 2026 · 5 sources linked below
TAM, SAM, SOMfrom widest to narrowest
- TAMTotal addressable marketEVERYONECOULD EVER BUY
- SAMServiceable addressable marketREACHABLEYOUR PRODUCT · CHANNELS · REGION
- SOMServiceable obtainable marketWINNABLEAFTER COMPETITION · CAPACITY
- ICPNamed accountsTHE LISTWHERE GTM STARTS
UK businesses with 50 to 249 employees, start of 2025 (DBT)38,435
01/ definitions
What is TAM SAM SOM? three nested markets
TAM, the total addressable market, is the total demand for what you sell across every potential customer.
Carta's guide calculates it as total potential customers multiplied by average annual contract value.
SAM, the serviceable addressable market, is the part of the TAM your product, channels and geography can actually reach.
SOM, the serviceable obtainable market, is the part of the SAM you can realistically capture with the resources and competition you face.
Each is a subset of the one before it, which is why they are usually drawn as circles inside circles.
02/ step by step
How to calculate TAM, SAM and SOM the top-down way
Start wide and narrow down, writing every assumption next to its number.
Count everyone who could ever buy
Pick the population first: every UK business of a size, every company in a sector, every hospital trust.
Multiply the count by what one customer would pay a year.
- ✓ Count from an official source where one exists
- ✓ Use your own contract value, not a competitor’s
Remove who you cannot serve
Take out the regions, sizes, languages, integrations and use cases your product or channels do not cover today.
- ✓ Write each exclusion down
- ✓ Keep it as a share of TAM
Estimate what you can win
Apply the share you can realistically win given competitors, sales capacity and how often customers switch suppliers.
- ✓ Check it against your actual win rate
- ✓ Express it in accounts, not only pounds
Turn SOM into names
A SOM of 400 accounts is a list of 400 companies.
Building that list is where market sizing becomes go-to-market.
- ✓ Named accounts
- ✓ Buying group per account
03/ calculator
TAM SAM SOM calculator move the sliders
Your assumptions in, your market out.
Nothing is stored.
The default of 38,435 accounts is the number of UK businesses with 50 to 249 employees at the start of 2025. Department for Business and Trade ↗
Every other input is yours: the shares and the contract value are assumptions to argue about, not benchmarks. Method ↗
Serviceable obtainable market
£5.8m
About 577 accounts a year at £10,000 each, from a serviceable market of 11,531 accounts (£115.3m) inside a total of 38,435 (£384.4m).
The account steps of 8,335 and 38,435 are the Department for Business and Trade's counts of UK businesses with 250 or more employees and with 50 to 249 employees at the start of 2025.
Change them to the population that fits what you sell.
04/ method
Top-down or bottom-up market sizing? use both
The two methods check each other: if they disagree badly, one of your assumptions is wrong.
→ Salesforce's guide puts it plainly: top-down suits a new product with little data; bottom-up tends to be more accurate.
→ Karl Ulrich of Wharton adds the beachhead market: the first segment you focus on inside the serviceable market.
05/ worked example
TAM SAM SOM example a UK B2B software firm
An illustrative firm selling software to mid-sized UK companies.
The business count is official; every percentage and price is an assumption for the example.
The 38,435 comes from the Department for Business and Trade's Business population estimates 2025.
The 30%, the 5% and the £10,000 contract value are illustrative assumptions, not benchmarks.
06/ pitfalls
Where TAM SAM SOM goes wrong four traps
01
A TAM borrowed from a report
A global industry figure says little about the customers you can sell to.
Count your own population.
02
A SOM with no capacity check
Winning 5% of a market means a number of deals a year.
Check your team can run that many.
03
Pounds without accounts
A market in pounds cannot be prospected.
A market in accounts can.
04
Assumptions with no owner
Every share and price should have a name and a date next to it, so it can be challenged.
07/ UK data
Where to count a UK total addressable market pick your buyer
Who do you sell to?
Official
Department for Business and Trade counts
The Business population estimates give UK private sector business counts by employee band each year.
Free
Companies House advanced search
The Companies House advanced search filters the register by nature of business (SIC code), location and status.
Bottom-up
Start from your list
If you already know the accounts, count them, then size up from the customers you have won.
08/ from sizing to selling
From TAM SAM SOM to a GTM plan what we do next
Market sizing answers how big; go-to-market answers who, when and how.
As a London GTM agency, we turn the SOM into an ideal customer profile, a named account list and the signals that show when each account is moving.
For the first step of that, read our guide to the ideal customer profile.
For the wider plan, see GTM strategy.
15 minutes · video or phone
Sized the market? Now reach it.
Book 15 minutes and we will tell you how we would turn your SOM into a named account list.
- 0115 minutes, video or phone
- 02Your market, in accounts
- 03No forms, just a call
Pick a day that suits · live availability
09/ questions
TAM SAM SOM FAQ
Short answers to what people ask about market sizing.
Three nested measures of a market.
TAM, the total addressable market, is everyone who could buy.
SAM, the serviceable addressable market, is the part you can reach.
SOM, the serviceable obtainable market, is the part you can realistically win.
Total addressable market.
Some people say total available market.
Multiply the number of potential customers by what one customer would pay a year.
Count the customers from an official source where you can.
SAM is the part of the market your product and channels can reach.
SOM is the share of that you can realistically win given competition and capacity.
Both.
Top-down is quicker and suits a new product; bottom-up uses your own sales data and tends to be more accurate.
If they disagree badly, revisit your assumptions.
Yes, on this page.
It starts from the Department for Business and Trade count of UK businesses with 50 to 249 employees and lets you set every other assumption.
Taking on new clients
You know the size of the market now go and win it
Fifteen minutes is enough to turn a SOM into a first account list.
11/ sources
Every source on this page
Each figure above links to one of these. All were opened and checked on 24 October 2026.
- Department for Business and Trade, Business population estimates for the UK and regions 2025gov.uk
- Carta, Market size: what is TAM, SAM and SOM?carta.com
- Salesforce, TAM, SAM and SOM made simplesalesforce.com
- Karl T. Ulrich, TAM, SAM, SOM and the beachhead marketktulrich.com
- Companies House, Advanced company searchfind-and-update.company-information.service.gov.uk