Reachable Market Sizing
How big is the market we can actually reach? Smaller than the top-down total, and more useful. Build it bottom-up, as who you can reach, times how often they buy, times what each purchase is worth, and write down the reach constraints that stop it growing.
Two numbers, side by side.
- Beside itThe top-down totalEvery buyer of this kind, anywhere: the TAM, SAM and SOM view.
- Layer 1Who you can reachThe buyers you can get in front of, through a route you have.
- Layer 2How often they buyPurchases, renewals or uses in a year.
- Layer 3At what valueWhat each one is worth to you.
- The ceilingReach constraintsWhat stops the reachable number growing, written down.
What each layer needs
TAM, SAM and SOM describe a market from the top down. We keep that view, and put the bottom-up number beside it, because the gap between the two is where most plans go wrong.
The top-down total
The total addressable market, the part you could serve, and the part you could plausibly win. Useful for context, and dangerous as a plan, because nobody reaches a market as a percentage.
A named source for every figure, with its date.
Who you can reach
Count buyers you can get in front of through a route you actually have: a list, a partner, a community, a channel you've tested. A buyer with no route to them doesn't count yet.
The route, named, and how many buyers it reaches.
How often they buy
How many times a year each buyer pays: once, monthly, per use. A product used rarely needs many more buyers to reach the same number.
How often they buy the workaround today.
At what value
What each purchase is worth to you, not the price you hope to charge. Start from what they pay for the workaround now.
A price that has been paid, or the cost of the workaround it replaces.
Reach constraints
Whatever caps the number: the size of the route, the language, a licence you need, the hours in your team's week. Write them down, so the next estimate starts from them rather than forgetting them.
One reachable number, its formula, and the constraints that cap it.
Your reachable market, from your own numbers.
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A booking assistant for independent clinics.
Illustrative example The clinic from Cut It, sized the way we'd size a real one. No figures, because an illustration shouldn't invent them.
Top-down
Every dental clinic in the country, from a named, dated source. Set it aside as context.
Who you can reach
Only the clinics a dental supplier would introduce you to, because that's the one route the team has.
How often, at what value
A monthly subscription, valued at what a clinic now pays a receptionist to phone patients the day before.
The constraint
One supplier, one region, one language. The next estimate starts by removing one of those, not by assuming a percentage of the country.
Where it goes wrong
- Misread
"If we get just a small share"
Why A share of a top-down total isn't a plan. Nobody reaches a percentage; you reach buyers, one route at a time.
- Misread
Counting buyers you can't reach
Why A buyer with no route to them is a hope. Count them once you have the route.
- Misread
Pricing from ambition
Why The value layer starts from what's paid today. A price nobody has paid inflates every layer above it.
In the Journal: Ideate: is the problem real? and Market: can we reach them repeatably?
Published 28 September 2026. A Graylemon adaptation of TAM, SAM and SOM, the standard top-down way of sizing a market.