Define the Market Before Calculating Its Size
Market sizing answers three different questions: How large is the overall opportunity? Which part can your business serve? How much could it realistically win? TAM, SAM, and SOM organize those questions, but they do not prove that customers want your product. A spreadsheet can describe a large opportunity even when willingness to pay remains uncertain.
TAM, or total addressable market, is the annual revenue opportunity if every eligible customer bought a solution within your defined market. SAM, or serviceable available market, narrows that opportunity to customers your offering and business model can serve. SOM, or serviceable obtainable market, estimates the portion you could realistically capture within a stated period.
Start with a specific customer, problem, and purchasing unit. For example, a scheduling product might target independent maintenance companies rather than all small businesses. Decide whether the buyer is a company, branch, team, or individual. Mixing those units can inflate customer counts before you calculate revenue.
Build TAM From Customers and Realistic Spending
A useful bottom-up formula is: eligible purchasing units multiplied by realistic annual revenue per unit. For subscriptions, estimate annual spending after likely discounts, not simply the highest advertised plan. For transaction businesses, distinguish total transaction value from the revenue your company would retain. These are different measures and should not share the same market-size label.
Consider a hypothetical software business with 200,000 eligible companies and an assumed annual subscription of $1,200. Its modeled TAM would be $240 million annually. That calculation is transparent, but both inputs need support. Do those companies experience the relevant problem? Can they purchase independently? Is $1,200 consistent with their budgets and the value delivered?
Use broad industry totals as a cross-check rather than your starting conclusion. An industry's total revenue is rarely the addressable market for one tool sold into it. Likewise, existing spending on an alternative is informative but not automatically transferable. Customers may keep that alternative, split their budgets, or decide that switching is not worthwhile.
Narrow SAM With Explicit, Nonoverlapping Filters
SAM should reflect the market your planned offering can actually support. Relevant filters might include geography, language, company size, required integrations, purchasing requirements, and delivery constraints. Apply these filters to the customer population rather than choosing a convenient percentage of TAM. Explain why each excluded group cannot currently be served.
In the hypothetical example, suppose only 25,000 companies meet the product's geographic, operational, and integration requirements. At the same assumed annual price, SAM becomes $30 million. This does not mean those companies are ready to buy. It means they fit the defined serviceable market, subject to validating need and willingness to pay.
Watch for overlapping exclusions. Companies outside your supported geography may also use unsupported systems; subtracting both groups independently can double-count exclusions. Where possible, classify customers using a single dataset or a clearly defined sequence of filters. Also revisit pricing: if your serviceable segment consists mainly of smaller companies, the TAM-wide spending assumption may not fit.
Estimate SOM From Acquisition and Retention
SOM is not a token slice of a large market. It should connect to a time horizon and an operating plan. Estimate how many suitable buyers you can reach, how many become qualified opportunities, how many purchase, and how many remain customers. Account for sales cycles, onboarding capacity, competitive alternatives, and the resources required to deliver the product.
Suppose the business models 120 new customers in year one, 180 in year two, and 240 in year three. Assuming 80% of each earlier cohort remains after each subsequent year, approximately 461 customers would be active at the end of year three. At $1,200 annually, that represents roughly $553,000 in annualized subscription revenue, or about 1.8% of the illustrative SAM.
That is an assumption-driven scenario, not a forecast established by the arithmetic. It also describes year-end annualized revenue, not revenue collected throughout year three. To test feasibility, work backward: if 20% of qualified opportunities convert, acquiring 240 customers requires 1,200 qualified opportunities. Can your channels, sales team, and onboarding process support that volume?
Make Uncertainty Visible and Update the Model
Build conservative, base, and upside cases around the assumptions that matter most: eligible customer count, achievable price, conversion, and retention. Keep the scenarios internally consistent. Higher prices may reduce conversion, while expansion into new segments may increase support costs. Combining every optimistic assumption usually produces a less useful estimate.
Maintain an assumption log showing each input, its basis, and your confidence level. Separate observed customer behavior from estimates and untested beliefs. Interviews can clarify problems; paid pilots, completed purchases, and renewals provide stronger evidence of willingness to pay. None automatically establishes demand across an entire segment.
Use market sizing to guide decisions, not decorate a pitch. A credible model identifies which customers to pursue, what must be validated next, and which constraints limit growth. Update it as pricing tests, sales results, and retention patterns replace assumptions with evidence.
