Sizing a market in a week means estimating total, serviceable and obtainable demand (TAM, SAM and SOM) from two directions, top-down from published totals and bottom-up from customers and prices, and then reconciling the two. A tight definition and written assumptions matter more than precision: the goal is a defensible range, not one perfect number.
A market size usually feeds a decision: whether to enter, how much to invest, or how to frame an investment case. A number that can't be traced to its assumptions is hard to defend when a board member asks where it came from, so the work is as much documentation as arithmetic.
What do TAM, SAM and SOM mean?
TAM is the total demand for a product category, SAM is the part you can serve with your offer and channels, and SOM is the share you can realistically win in a set period. Each one narrows the one before it.
- Total addressable market (TAM): everyone who could buy this kind of product, measured in annual revenue or units.
- Serviceable addressable market (SAM): the part of the TAM your product, geography and channels can actually reach.
- Serviceable obtainable market (SOM): the portion of the SAM you can expect to capture, given competitors, sales capacity and time.
Here is an illustration with made-up numbers. Say 40,000 firms fit your customer definition and each would spend about $10,000 a year, so the bottom-up TAM is $400 million. Your product and channels reach 2 in 5 of those firms, so the SAM is $160 million. If you expect to win 1 in 20 of those within three years, the SOM is $8 million.
Keep units consistent across all three. Mixing customer counts with revenue, or one year with another, is a common reason sizes don't add up.
How should you plan the week?
Give each day one job, and don't start calculating until the market is defined. A simple plan looks like this:
- Day 1: define the market. Write down the product, the customer, the geography, the year and the unit of measure, and decide what is out of scope.
- Day 2: gather inputs. Collect published market totals, industry reports, public company filings, customer counts and price points, noting the source and date of each.
- Day 3: build top-down. Start from a published total and narrow it by segment, geography and fit.
- Day 4: build bottom-up. Multiply the number of potential customers by their likely annual spend.
- Day 5: reconcile, test and write up. Compare the two estimates, run sensitivities on the biggest assumptions, and document the range.
Each step produces something reviewable, so if the definition changes on day four you know which calculations to redo.
You don't need to build the workbook from scratch. The free TAM SAM SOM market sizing template is an Excel model with a bottom-up and a top-down build, a reconciliation, low and high cases and built-in checks. Replace its example numbers with your own.
How do top-down and bottom-up estimates differ?
Top-down starts with the whole market and narrows it; bottom-up starts with customers and builds up. Each catches the other's blind spots.
A top-down estimate is quick and anchored in published figures, but those figures often define the market differently from you. A bottom-up estimate reflects how customers actually buy, but it depends on assumptions about customer counts and spend that can be optimistic.
When the two land close together, confidence rises. When they don't, the gap usually points to a definitional mismatch or to one assumption that needs better evidence.
What makes a market size credible?
Credibility comes from traceable sources, explicit assumptions and an honest range, which reviewers trust more than a precise-looking single figure.
- Cite every input with its source and date, and flag figures that rest on a single weak source.
- State each assumption in plain words next to the number it drives.
- Show a sensitivity table for the two or three assumptions that move the result most.
- Ground the SOM in competition: name the main rivals and say why you can win share from them, rather than picking an arbitrary share.
What mistakes should you avoid?
Many sizing errors come from scope and units rather than arithmetic. Watch for these:
- Counting the same revenue twice across overlapping segments.
- Treating the TAM as the opportunity and skipping the SAM and SOM.
- Using a published total for a different year or region without adjusting it.
- Hiding the assumption that drives most of the result.
Key takeaways
- A market size is a defensible range built from written assumptions, not a single perfect number.
- Define the product, customer, geography, year and unit before collecting any data.
- Build both top-down and bottom-up estimates, and use the gap between them to find weak assumptions.
- Cite every input and show which assumptions move the result most.
How Forward Deployed helps
Ask for Market Sizing (TAM / SAM / SOM) in your AI assistant: give the market, geography, segment and year, see the price, approve it, and get a Word sizing memo with top-down and bottom-up estimates and cited sources, plus an Excel model with the TAM, SAM and SOM build-up and a sensitivity table. It uses public sources, and low-confidence figures are flagged. Industry and Market Landscape adds the market's structure and key players. Both are listed under CMO, CPO and insights; to scope a larger question, talk to us.
Frequently asked questions
Can you size a market without paid research databases?
Yes. Published industry totals, public company filings, government statistics and pricing pages are often enough for a first, defensible range. The trade-off is that some figures are older or less specific, so the estimate should flag low-confidence inputs and show how the result changes if they turn out to be wrong.
How precise does a market size need to be?
Precise enough to support the decision it feeds. An entry decision usually needs the right order of magnitude and a clear view of the assumptions, not a figure to the nearest dollar. Presenting a range with the drivers behind it is more useful, and more honest, than a single number.
What is the difference between SAM and SOM?
SAM is the part of the total market that your product and channels can reach. SOM is the share of that serviceable market you can realistically win in a given period, after competitors, sales capacity and adoption speed. It can't be larger than the SAM.