What Is Market Sizing?
Market sizing estimates the revenue opportunity in a given market. Learn TAM, SAM, and SOM and how to size your market credibly.
Key Takeaways
- TAM = total addressable market (the full global opportunity if you had 100% market share)
- SAM = serviceable addressable market (the portion of TAM you can realistically reach)
- SOM = serviceable obtainable market (the portion of SAM you can realistically capture in the near term)
- Bottom-up sizing from real customer data is more credible than top-down from industry reports
Why market sizing matters
Market sizing tells you and your investors how big the prize is if your business succeeds. A business pursuing a £50 million market has a fundamentally different growth ceiling than one pursuing a £5 billion market — and requires different funding, hiring, and ambition to match. Sizing your market correctly also helps you identify whether your current target segment is the right beachhead or whether you need to expand or pivot.
TAM, SAM, and SOM
The standard market sizing framework uses three nested concepts. Total Addressable Market (TAM) is the total global revenue opportunity if your product had 100% market share — useful for framing the scale of the opportunity but not a realistic operating target. Serviceable Addressable Market (SAM) is the portion of TAM that your product, in its current form, could realistically serve given geographic, language, and distribution constraints. Serviceable Obtainable Market (SOM) is the share of SAM you can realistically capture in the next 3-5 years, given your resources and competition.
Top-down vs bottom-up sizing
Top-down sizing starts with an industry report (the global CRM market is worth $80 billion) and works down by applying percentage assumptions (we can capture 0.1% of the UK market). It produces impressive-sounding numbers and is easy to manipulate. Bottom-up sizing starts with real units of demand — there are 50,000 potential customer businesses in the UK, our average contract value is £12,000 per year, so our SAM is £600 million. Bottom-up sizing is harder to produce but far more credible because every assumption is defensible.
Common mistakes
The most common market sizing mistakes are: using TAM as if it were SOM (presenting a £50 billion market when you can realistically access £20 million of it); ignoring competition (your SOM is the market share you take from incumbents, not the market as if it were empty); and using third-party report numbers uncritically without checking methodology. Sophisticated investors see through inflated market sizes immediately — it signals a lack of rigour in the whole business case.
How to size your market practically
Start with your ideal customer profile. Count how many of those customers exist in your target geography — use LinkedIn, Companies House, industry directories, or trade association membership data. Multiply by your planned average contract value or average order value. That is your SAM. Then apply a realistic market penetration rate over your planning horizon — 1-5% in year 3 is typical for a new entrant — to get your SOM. Build the assumptions into a simple model so you can stress-test them.