How to calculate TAM, SAM and SOM (with worked examples)
Total, Serviceable, and Obtainable market sizes explained with worked examples, plus the bottom-up method investors trust and a step-by-step template you can copy.
TAM, SAM and SOM are the three numbers every investor asks about and most founders get wrong. Done well they show you've thought clearly about who you can realistically sell to. Done poorly they make the rest of the plan unbelievable.
This guide explains the difference between top-down and bottom-up sizing, shows the bottom-up method investors actually trust, and walks through a worked example you can adapt to your own business.
Definitions, in plain English
TAM (Total Addressable Market): the total revenue opportunity if you sold to every possible customer worldwide. Think of it as the size of the ocean.
SAM (Serviceable Addressable Market): the portion of TAM your business model and geography could realistically reach. The size of the bay you actually fish in.
SOM (Serviceable Obtainable Market): the realistic share you can capture in the next 3–5 years given your team, capital and channels. The size of the catch.
Funded plans usually show: TAM in the billions, SAM in the hundreds of millions, SOM in the tens of millions for an early-stage company. Wildly out-of-proportion numbers (e.g. $400B TAM, $10k SOM) signal the founder doesn't understand the framework.
Top-down vs bottom-up — and why bottom-up wins
Top-down sizing starts with a published industry number ('the global cybersecurity market is $250B by 2027') and chips away with percentages. It's fast and almost always wrong, because it assumes the published number maps to what you actually sell.
Bottom-up sizing starts with one customer ('our average customer pays $4,800/year') and multiplies by a realistic count ('there are 120,000 US dental practices'). It produces smaller, defensible, more useful numbers. Every credible investor prefers bottom-up.
Step by step
- 01
Define your ideal customer in one sentence
Before any math, name the buyer. 'A US-based dental practice owner with 1–3 locations and ≥3 chairs per location.' Vagueness here corrupts every downstream number.
- 02
Calculate annual revenue per customer (ARPC)
Sum of subscription + services + transaction fees, annualised. Use the price you actually charge or plan to charge, not aspirational pricing. For a freemium model, only count paid users in the average.
- 03
Find the SOM customer count
How many of your ideal customers can you realistically reach in the next 3 years given your channels? Be conservative — 1–5% of SAM is typical for an early-stage company with two acquisition channels working.
- 04
Compute SOM = ARPC × SOM customer count
This is the most important number on the page. It should be a believable revenue ramp, not a fantasy. If SOM × current win rate doesn't fund the next round, your plan has a problem.
- 05
Compute SAM by expanding the customer pool to your full reachable geography and segment
Same ARPC, but now multiply by every customer in your buyable footprint regardless of capacity to acquire. This is your medium-term ceiling.
- 06
Compute TAM by going global and including adjacent segments
Same ARPC, multiplied by everyone in the world who fits the broad profile. Cite a credible source for the worldwide customer count (a government statistics body, a research firm like IBISWorld, or a recognised industry report).
- 07
Sense-check by triangulating against a public proxy
Find one public company in your space and divide their revenue by their market share. If your TAM number is wildly different from that triangulation, you've made an error somewhere.
- 08
Show your working in the plan
Write the formula explicitly: 'SOM = $4,800 ARPC × 2,000 practices (1.7% of 120,000 US dental practices) = $9.6M ARR by year 3.' Investors trust math they can audit.
Key takeaways
- Always size bottom-up: price × number of realistic customers. Top-down is a red flag.
- SOM is the number you'll be measured against. Be conservative — usually 1–5% of SAM in 3 years.
- Show the formula, cite the customer-count source, and triangulate against a public comparable.
- If SOM is too small to fund the next round, the problem isn't the math — it's the market choice.
Frequently asked questions
+What's a healthy TAM size for a venture-backed startup?
Most VCs want to see at least $1B TAM and a clear path to $100M+ in revenue for the company within 7–10 years. Bootstrappers and SBA-funded businesses can build great companies in TAMs of $50–500M; venture math doesn't apply.
+Do I need to size all three (TAM, SAM, SOM)?
Yes. TAM proves the opportunity is big enough to matter. SAM proves you're not delusional about reach. SOM proves you're not delusional about execution. Skipping any of the three weakens the others.
+How do I size a market for a brand-new category?
Find the closest existing behaviour and size that. If you're selling a new productivity tool, size the SaaS productivity category and explain what slice you'll convert. 'New category' rarely means 'no proxy exists'.
+Should I use market data from before COVID?
Try not to. Buyer behaviour in most categories shifted enough during 2020–2022 that pre-pandemic numbers can mislead. Use 2023+ data where available and note the source year explicitly.
+What if my SOM number looks small?
Either the segment is too narrow (broaden the ICP), the ARPC is too low (raise prices, add expansion revenue), or the market simply doesn't support a venture-scale business. All three are useful answers.
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