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TAM / SAM / SOM sizing

Three nested numbers that turn "this is a big market" into something checkable. The third one is the only one that matters and the one people skip.

The printable canvas and the handoff into Claude Code are part of a paid plan. See what a plan includes. It needs the Claude desktop app on this machine, and your team and the prompt library already installed in that project — we cannot see your disk, so open it there.

Stage
02 Explore
Works at
Corporate, Business
Maturity
An idea → Scaling
Time
Half a day for a defensible first cut

What it is

Three nested market sizes used to move from a headline number to something a business could plan against. Each level narrows the one above it for a stated reason, and the value of the exercise lies almost entirely in whether those reasons are honest.

TAM — Total Addressable Market
Everyone who could buy this category, unconstrained. Usually sourced from published research, and usually the least interesting number on the page.
SAM — Serviceable Available Market
The portion you could reach given your geography, channel, regulatory position and product fit. Each constraint applied should have a reason written beside it.
SOM — Serviceable Obtainable Market
What you could realistically capture in a defined period, given capacity, brand and competition. The only one that should drive a plan.
The narrowing logic
The stated reasons for moving from each level to the next. This is the actual analysis; the three numbers are its output.
The share assumption
What proportion of SAM you assume you win, checked against your share of anything you already compete in. Where optimism enters, almost always.
The mistake people makeTAM is the number that gets quoted and the least useful of the three. A plan built on a large TAM and an unexamined share assumption fails in the same way every time: the market really was that big, and you really did get 0.3% of it.
What it’s forYou need to know whether an opportunity is big enough to be worth the trouble.
What it’s not forYou are working in an established core market you already understand. This is an entry-decision tool.

How you run it

  1. Define the product and the buyer first"The UK tools market" is not a definition. Which products, sold to whom, through which channel, in which geography.
  2. Size the total addressable marketEveryone who could conceivably buy this category if there were no constraints. Cite the source; this number is usually somebody else's.
  3. Narrow to serviceable availableConstrain by geography, channel, regulation, segment and product fit. Each narrowing needs a stated reason, and this is where most of the honesty lives.
  4. Narrow again to obtainableWhat you could realistically win in three years given your capacity, brand and sales capability. Sanity-check against your current share of anything.
  5. Show the arithmeticA sizing whose steps cannot be followed cannot be challenged, and one that cannot be challenged will not be believed.

The prompt

Two ways to run it

Run this tool in your own Claude

The short prompt starts your partner against the library on your disk. The long one carries everything with it and needs nothing installed.

🔒 Prompt locked

Everything above is free — what this tool is, what it is for, what it is not for, and how to run it as a workshop. Three tool prompts a month are free with an account; beyond three, and for the printable canvas, it is the paid part.

See plans — from £19 a month

Your playbook

It lands in the earliest stage this tool suits. Move it on the Playbook page.

You’ll need

  • A precise definition of what you are selling to whom
  • Published market data, or a route to build it bottom-up
  • Honesty about your realistic share

You’ll end up with

  • Three sized numbers with the assumptions behind each
  • The reasoning that narrows one to the next
  • A share assumption you would defend in front of a board

Runs well with

Acquisition screen and synergy thesisAdjacency mapAI opportunity mapAnalysis plan