Market Sizing SOP

A market sizing SOP answers "how much money could this idea make if it works?", and an honest version takes about a week of desk research and checks. Most founders copy a giant number from an industry report and call it their TAM. Investors skip that slide. This SOP builds the estimate bottom-up, from a count of real customers and what they already spend, and uses the report numbers only as a cross-check.

What is the market sizing SOP and when should you use it?

Market sizing estimates three numbers:

• TAM (Total Addressable Market): yearly revenue if every possible customer in the world bought your type of product.
• SAM (Serviceable Addressable Market): the part of TAM you can serve with your product, language, regions and channels.
• SOM (Serviceable Obtainable Market): the part of SAM you can realistically win in the next three years.

Use this SOP:

• Before you raise money, because every investor deck needs a market slide that survives questions.
• When choosing between two customer segments, to see which one supports a business.
• When a persona is ready and you can count the people who match it.

For choosing a country or city to launch in, see geographic market validation. For entering a second market after launch, see market expansion testing.

What do you need before you start?

• One customer profile with filters you can count: company size, industry code, household type, job title.
• A price or spend estimate from interviews or an online survey: what customers pay today for the problem.
• Access to public statistics: Eurostat for the EU, the US Census Bureau and Bureau of Labor Statistics for the US, and national business registers such as Companies House in the UK.
• A spreadsheet with one tab per number and a "source" column next to every input.

How do you run it, step by step?

1. Define the market in one sentence

Write who buys, what they buy and where: "Independent dental clinics in Germany, France and the Netherlands paying for appointment software." A vague definition such as "healthcare software" produces a meaningless TAM.

2. Count the customers bottom-up

Find the number of businesses or people who match your profile from official statistics, registers, trade associations or LinkedIn search counts. Write the source and date next to each figure. If no source fits, build a Fermi estimate (a chain of reasonable guesses, each one written down) and mark it as weaker evidence.

3. Set the annual spend per customer

Use what customers already spend on the problem: current tools, freelancers or staff hours valued at a wage. Interview and survey answers beat guesses. Multiply customers by annual spend for your bottom-up TAM in the regions you defined.

4. Narrow to SAM

Remove customers you cannot serve in the next three years: wrong language, regulation you cannot meet, segments your product does not fit, sizes your channels cannot reach. Each cut gets a one-line reason. What remains is SAM.

5. Estimate SOM from channel capacity

Take the channels you can afford, such as content, a sales person or partnerships, and estimate customers won per month from each. Multiply over 36 months and cap it at a believable share of SAM. A SOM built from channel maths beats "we will win 1%", because it shows how you will get there.

6. Cross-check top-down

Find one or two published figures for the wider market from Statista, analyst reports or listed competitors' annual reports, and narrow them to your segment. If top-down and bottom-up differ by more than a factor of 3, one of your inputs is wrong. Find it.

7. Stress-test the three riskiest inputs

Change customer count, spend and win rate by 30% up and down. Note which input moves SOM most. That input is the next thing to validate with real customers.

Template you can copy

MARKET SIZING SHEET

Tab 1: Definition
Who buys: ___________  What: ___________  Where: ___________

Tab 2: Bottom-up TAM
Input                      | Value   | Source + date      | Evidence (strong/weak)
Number of matching customers |       |                    |
Annual spend per customer  |         |                    |
TAM = customers x spend    |         |                    |

Tab 3: SAM cuts
Cut (reason)               | Customers removed | Remaining
Language / region          |                   |
Regulation                 |                   |
Segment fit                |                   |
SAM = remaining x spend    |                   |

Tab 4: SOM (36 months)
Channel | Customers won per month | Months | Total customers
SOM = total customers x spend      | % of SAM: ____

Tab 5: Top-down cross-check
Published market figure | Source | Narrowed to segment | Gap vs bottom-up (x)

Tab 6: Sensitivity
Input | -30% SOM | Base SOM | +30% SOM

Worked example: software for independent dental clinics

This is an illustrative case with invented numbers. A founder wants to sell appointment and recall software to independent dental clinics in three EU countries.

From national registers and dental associations she counts about 52,000 clinics, of which roughly 30,000 are independent. Interviews with 15 clinic owners show they pay €80 to €200 a month for scheduling tools, so she uses €120 a month, or €1,440 a year. Bottom-up TAM: about €43 million a year.

She cuts clinics using hospital-group software and those in regions where she cannot offer local-language support, leaving 18,000 clinics: SAM about €26 million. Her channels (one sales person, dental trade fairs and a referral offer) produce about 25 clinics a month after a ramp-up, or roughly 700 clinics in 36 months: SOM about €1 million a year, 4% of SAM.

A published estimate for the dental practice software market in those countries, narrowed to independents, lands at €60 million. The gap is 1.4x, inside her factor-of-3 rule. Her sensitivity test shows price moves SOM most, so her next step is a pricing sensitivity test.

What should you measure, and when do you stop?

Stop when both estimates agree within a factor of 3 and you know which input matters most. More research rarely changes a decision after that point; testing the riskiest input with customers does.

Which mistakes should you avoid?

• Quoting a global industry figure as your TAM. "The global wellness market is worth trillions" tells an investor nothing about your business.
• The 1% fallacy. "If we get 1% of the market…" ignores how you will reach those customers. Build SOM from channels.
• Using made-up prices. Spend must come from what customers pay today.
• Hiding your sources. Every number needs a link and a date, or it will not survive a due diligence call.
• Sizing too broad a market. A precise small SAM beats a vague big TAM, and it is easier to defend.

How can AI speed this up?

AI assistants are good at finding which statistics office or register publishes the count you need, and at checking your spreadsheet formulas. Ask one to list possible data sources for your exact customer filter, then open each source yourself. Never paste an AI-generated market figure into your model without the original source, since AI tools produce confident numbers with no basis. The free market size calculator gives you a quick TAM, SAM and SOM structure to start from.

Where does this SOP fit in your validation plan?

Market sizing closes the Customer Discovery stage and feeds every money conversation after it.

1. Get counts and filters from your persona.
2. Get spend from interviews and an online survey.
3. Check that customers will pay with problem monetization assessment.
4. Feed SAM and SOM into financial model validation.

See the full Customer Discovery category. The F/MS investor playbook module explains what investors look for in a market slide, and the pitch deck tool helps you place the numbers in your deck.

FAQ

What is the difference between TAM, SAM and SOM?

TAM is all possible revenue for your type of product, SAM is the part you can serve with your product and channels, and SOM is what you can realistically win in about three years.

Should I use top-down or bottom-up market sizing?

Both. Build bottom-up from customer counts and spend, then cross-check top-down with published figures. Investors trust the bottom-up number more.

How big does my market need to be for venture capital?

Big enough that your three-year SOM supports the revenue a venture investor expects, and that SAM leaves room to grow far past it. Ask each investor about their own threshold; it varies by fund.

Where can I find free market size data?

Eurostat, the US Census Bureau, the Bureau of Labor Statistics, national business registers, trade associations and the annual reports of listed competitors.