Define the customer, use case, geography and time period first. Estimate the broad total addressable market, narrow it to the customers your offer can actually serve, then build a reachable-market case from sales capacity, channels, price and realistic customer access. Use more than one method and investigate large differences and explain the gap.
Key points
- A market-size calculation begins with the market definition, not with a large industry number.
- Top-down and bottom-up estimates should be compared and reconciled when their results differ.
- TAM, SAM and the first reachable market answer different planning questions.
- Show the assumptions that move the result most.
Define the market before calculating it
Specify who buys, what they buy, the geography, the use case, the buying unit and the time period. “U.S. small-business software” is still too broad if the offer serves only one industry or company size.
A narrow definition produces a smaller number and a more useful one.
Build a top-down estimate
Start with a reliable industry, population or spending figure and apply filters that match the real market boundary. Document the year and definition for every source.
Top-down sizing is useful for checking scale but can overstate the opportunity when the source category is broader than the offer.
Build a bottom-up estimate
Count the potential buying units that fit the target customer, then multiply by realistic annual spend or units purchased. Where possible, use actual prices, customer data and channel capacity.
Bottom-up sizing makes the assumptions visible and can be easier to test against the operating plan.
Narrow TAM to the serviceable and reachable market
Remove customers the business cannot serve because of geography, regulation, product fit, language, channel or other real constraints. Then estimate what can be reached with current sales and marketing capacity.
Avoid choosing a flat percentage of TAM without a mechanism showing how the company would acquire that share.
Run sensitivity checks before presenting the number
Change the customer count, adoption, price and other major assumptions to see which ones drive the result. Present a range when uncertainty is material.
If top-down and bottom-up methods produce very different answers, treat the gap as a research problem.
Questions people ask before they act
What does TAM mean?
Total Addressable Market is the full revenue or unit opportunity under a defined market scenario if all eligible demand were served.
What does SAM mean?
Serviceable Addressable Market narrows TAM to the portion the current offer and business model can actually serve.
What does SOM mean?
Serviceable Obtainable Market is commonly used for the share a company expects to capture. A credible estimate needs a time period and a mechanism tied to sales capacity, channels, conversion and competition.
Should market size be one number or a range?
A range is often more honest when key assumptions are uncertain. The model should show the base case and the inputs that move it.
Primary data and official references
These sources are useful for checking market size, business activity, economic conditions and the rules that shape a commercial decision.
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