Market Sizing Template: TAM SAM SOM Worksheet
A market sizing template turns your market size into an auditable worksheet instead of a single guessed number. It has input rows for customer count and annual price, filter rows that narrow TAM to SAM, and a capture-rate row that lands SOM — plus an assumption log recording where every figure came from.
Quick Answer: A market sizing template is a TAM/SAM/SOM worksheet. TAM = customer count × annual price per customer (ACV). SAM = TAM after filtering out what you can't serve (geography, product fit, regulation). SOM = SAM × the share you can realistically capture soon. Source every input, and log the assumption behind it in its own column.
A single guessed market number dies in the first stage-gate review; a worksheet survives it. When you take a new venture to an investment committee — or defend a portfolio bet at a quarterly review — the panel doesn't want your headline figure. They want to pull one thread and watch whether the estimate unravels. This template hands them threads to pull and holds together when they do. It's the worksheet an Edmired go/no-go review expects to see: copy the rows into a spreadsheet and fill one cell at a time.
The Market Sizing Template: Input Rows and Assumption Log
The template is one worksheet with three stacked blocks — TAM inputs, SAM filters, and SOM constraints — and an assumption-log column beside them that records the source of every number. Build it bottom-up: start from a customer count and a price you can defend, never from a headline market figure you can't.
Each row below is a cell to fill. The first three columns tell you what to enter and where to find it; the last shows an illustrative entry so the shape is clear. Every number in that final column is invented for the example only.
| Field (row) | What to enter | Where to source it | Illustrative entry (hypothetical) |
|---|---|---|---|
| Customer unit | The single entity you send an invoice to | ICP work, early sales calls, customer interviews | "One mid-sized logistics firm" |
| Segment | A countable sub-group with its own price | Split by company size, vertical, or plan tier | "Fleets of 200–1,000 vehicles" |
| Customer count | Entities matching the unit, per segment | Census/statistical agencies, trade associations, licensing bodies, business registries, platform directories, paid databases | 12,000 firms |
| Annual price (ACV) | Annual contract value one account pays | Your pricing page, closed deals, signed LOIs, competitor pricing | $18,000 per firm/year |
| Segment TAM | Customer count × ACV | Calculated | $216M |
| TAM | Sum of every segment TAM | Calculated | Sum of segments |
| SAM filter(s) | Each reason you can't serve part of TAM | Product scope, roadmap, geography, compliance limits | "One country; cloud-connected fleets only" |
| SAM | TAM remaining after every filter | Calculated | ~50% of TAM |
| SOM constraint | The near-term share you can realistically win | Funnel capacity, historical win rate, sales coverage | "4% over 3 years" |
| SOM | SAM × capture rate | Calculated | ~4% of SAM |
| Assumption log | The source or logic behind each cell | You — one line per input | "Count: 2024 trade-association directory" |
Takeaway: The assumption-log column is what turns this from a guess into a template. Any cell without an entry there is a placeholder, not a number — and a reviewer will find it before you do.
Field-by-Field: What Belongs in Each Row
Each row does exactly one job, and two of them carry the whole estimate: the customer unit and the price. Get those right and the rest is arithmetic; leave them fuzzy and every downstream cell inherits the error.
The customer unit is whatever you invoice. Not the end user, not the parent company — the entity that signs the contract and pays the annual fee. For a per-seat tool that might be a company; for a marketplace, a merchant; for a prosumer app, one person. You count and price this unit, so define it once and hold it constant.
Segment before you count. "Small businesses" is not countable; "independent clinics with two to ten practitioners" is. Tight segments are easier to source from a real registry and harder for a reviewer to wave away — and they let you price each slice on its own, which most markets demand.
The customer count comes from a registry, not a round number. Pull counts from census and statistical agencies, trade associations, licensing boards, business registries, and platform directories — and count only the addressable subset that fits your unit, not the whole category. A sourced count anchors any credible startup market sizing exercise; a "let's say a million" is the tell of a made-up market.
ACV is an annual number, always. Revenue per customer means annual contract value — what one account pays across twelve months. If you bill monthly, multiply by twelve before it enters the sheet. Dropping a monthly price into an annual TAM understates the market twelvefold, an error that is embarrassingly common on early business cases.
Every SAM filter needs a stated reason. SAM is not TAM times a comfortable percentage. It is TAM minus the parts you genuinely cannot serve — the wrong geography, an unsupported platform, a segment compliance has ruled out. Name each filter in the sheet so the cut is a decision, not a fudge.
SOM is a capture rate you can defend from your funnel. SOM is the share of SAM you can realistically win in the near term, derived from pipeline capacity, historical win rates, and how many reps or channels you actually have. It describes how you start, not a ceiling — so ground it in your own conversion data, not optimism.
A Filled Market Sizing Example, End to End (Hypothetical)
Here is the worksheet filled from top to bottom. Every figure below is invented to demonstrate the method — none of these are researched market numbers, and you should never quote a teaching figure like this as a real market size.
Picture a hypothetical fleet-maintenance SaaS sold to logistics firms, billed per firm per year and priced by fleet size. Building TAM bottom-up, segment by segment, the illustrative inputs stack up like this:
| Segment (illustrative) | Firms (assumed) | ACV (assumed) | Segment TAM |
|---|---|---|---|
| Small fleets (50–200 vehicles) | 40,000 | $6,000 | $240M |
| Mid fleets (201–1,000 vehicles) | 12,000 | $18,000 | $216M |
| Large fleets (1,000+ vehicles) | 3,000 | $60,000 | $180M |
| Total (illustrative) | 55,000 | — | $636M |
Add the three segment totals and the illustrative TAM is $636 million — a made-up teaching figure, not a claim about any real market. Notice what segmenting reveals: 3,000 large fleets carry nearly as much value as 40,000 small ones, an insight a single blended price would have buried. This is the bottom-up structure in miniature; the full bottom-up TAM build walks each sourcing step.
Narrowing the Hypothetical TAM to SAM and SOM
TAM is the ceiling; SAM and SOM narrow it to what you can serve and then win. The numbers stay invented — only the method carries over.
SAM removes what you can't serve. Suppose the product only works with cloud-connected telematics and you launch in one country. Cutting firms outside that geography and on incompatible hardware might leave roughly half the market — an illustrative SAM near $318 million. Each cut is a named filter, never a percentage chosen because it felt about right.
SOM is the slice you can win soon. Based on funnel capacity and win rate, you might capture 4% of that SAM over three years — an illustrative SOM around $13 million. That figure describes your near-term path through named channels at a stated conversion rate. For the complete three-layer narrowing, see how to calculate TAM, SAM, and SOM.
Adapting the Template for B2B, B2C, and Marketplaces
The worksheet's structure never changes across business models — only two rows do: the customer unit and what "annual price" means. Swap those, keep everything else, and the same template sizes any market.
Here is how the two variable rows resolve for the three most common models:
| Model | Customer unit to count | Where to count them | "Annual price" means |
|---|---|---|---|
| B2B SaaS | The account you invoice | Registries, trade associations, paid company databases | Annual contract value (ACV) |
| B2C / prosumer | One paying individual | Census demographics, platform user counts, survey panels | Price × annual purchase frequency |
| Marketplace | One participant on the side you monetize | Platform directories, competitor disclosures, category proxies | Take rate × annual GMV per participant |
Takeaway: For a marketplace, count only the side you actually monetize — usually sellers — and never add both sides together, or you double-count the same transaction. Once the unit and price rows are set correctly, the TAM, SAM, and SOM math below them runs identically for all three models.
Key Takeaways
- A market sizing template is a TAM/SAM/SOM worksheet, not a number. Its value is that every input sits in a labeled cell a reviewer can inspect, challenge, and trace to a source.
- Build bottom-up: customer count × annual price. Start from countable units and a defensible ACV rather than shaving a headline market figure with arbitrary percentages.
- Fix the customer unit first. Both the count and the price depend on it, so a fuzzy unit — the invoiced entity, not the end user — corrupts the whole sheet at once.
- ACV is always annual. Slip a monthly price into an annual TAM and you understate the market by twelve times, the most common template error.
- SAM and SOM are cuts with stated reasons, not comfortable percentages. Each SAM filter names something you can't serve; SOM is a capture rate drawn from your own funnel, not optimism.
- The assumption log is the point. A cell with no source behind it is a placeholder — log where every figure came from and the final number defends itself.
- The same template fits B2B, B2C, and marketplaces. Only the customer-unit and price rows change; swap those two and the TAM/SAM/SOM structure carries over unchanged.
Frequently Asked Questions
What Should a Market Sizing Template Include?
A market sizing template should include a defined customer unit, a customer count per segment, an annual price (ACV) per segment, and calculated TAM, SAM, and SOM rows. Critically, it also needs an assumption-log column recording the source behind every input. Without that log, the worksheet is just a tidier guess that a reviewer can still dismantle.
How Do I Calculate TAM, SAM, and SOM in the Template?
Multiply customer count by annual price per customer to get TAM, ideally segment by segment and summed. Apply named filters — geography, product fit, regulation — to narrow TAM to SAM. Then multiply SAM by the share you can realistically capture, based on funnel capacity and win rate, to get SOM. Each step is one row of arithmetic over sourced inputs.
What Goes in the Template When I Have No Market Data?
Fill the cells with clearly-labeled proxies: borrow customer counts from comparable public companies, use competitor pricing for ACV, and cross-check against analyst estimates. Mark each as an assumption in the log and give a range rather than a false-precision point figure. A sourced estimate you can defend beats a confident number you invented — and you refine it as real data arrives.