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Research · Diagnose, Explore

Market sizing — top-down and bottom-up

The standard consulting opener. Its real output is not the number — it is the assumptions underneath it, and the gap between the two methods.

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
01 Diagnose, 02 Explore
Works at
Corporate, Business
Maturity
Any
Time
Half a day of research, an hour to reconcile

What it is

Sizing a market twice by two independent routes and reconciling the difference. The number is not the output — the gap between the two methods is, because it isolates the assumption that the whole estimate rests on.

Boundary
Which customers, products, geography and year. Most sizing disagreements are boundary disagreements that nobody surfaced.
Top-down
Starting from a published category figure and narrowing by share, segment or geography. Fast, and only as good as the source and each narrowing step.
Bottom-up
Units times price times frequency, built from the customer end. Slower, harder to fake, and far more persuasive to anyone who has been sold a market size before.
The reconciliation
Why the two differ. A 2x gap is normal; the assumption driving it is the thing worth testing.
Confidence range
A high and a low rather than a point, with the input that moves it most named.
The mistake people makeBuilding bottom-up while looking at the top-down number guarantees they agree, because you will unconsciously tune the assumptions until they do. Build them independently, ideally by different people.
What it’s forYou need a defensible number for how big something is, and the two methods to disagree with each other so you can see where the uncertainty sits.
What it’s not forYou already operate in the market at scale and know the volumes. Sizing an established core is busywork.

How you run it

  1. Define the boundary before you countWhich customers, which geography, which product definition, which year. Most sizing arguments are boundary arguments in disguise.
  2. Build it top-down from published dataStart from an industry or category figure and narrow with published shares. Cite every step; a number with an uncited step is not defensible.
  3. Build it bottom-up independentlyUnits × price × frequency, from the customer end. Do not look at the top-down figure while you do this or you will anchor to it.
  4. Reconcile and explain the gapA 2x gap is normal and informative. Work out which assumption drives it — that assumption is the thing to test.
  5. State the confidence honestlyGive a range, not a point. Name the one input that would move the answer most.

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 is being sized
  • Any industry data, trade body figures or filings you hold
  • Your own volumes, if you are already in the market

You’ll end up with

  • A top-down number from published sources
  • A bottom-up number built from units × price
  • The reconciliation, and what the gap tells you

Runs well with

Acquisition screen and synergy thesisAdjacency mapAI opportunity mapAnsoff matrix