TAM, SAM, SOM: How to Size a B2B Market

TL;DR. To calculate TAM, SAM and SOM for a B2B market, count the accounts that could use your product and multiply by what each would pay in a year. TAM is every account with the problem. SAM is the share your product and route to market can actually serve. SOM is the share you can realistically win in a defined period. For B2B, build the numbers bottom-up from account counts and price, then use a top-down figure only as a sense check.

What do TAM, SAM and SOM actually mean?

The three figures are nested. Each one narrows the one above it.

Measure	Question it answers	What narrows it
TAM (total addressable market)	How much annual revenue exists if every account with this problem bought a solution like ours?	The problem definition
SAM (serviceable addressable market)	How much of that can our current product, pricing and channels serve?	Product fit, geography, segment, route to market
SOM (serviceable obtainable market)	How much can we realistically win in a stated period?	Sales capacity, competition, conversion, time

All three should be stated as annual revenue, in the same currency, with the same definition of an account. If TAM counts companies and SOM counts seats, the numbers cannot be compared.

A market size is a decision tool, not a trophy. It helps you choose which segments deserve investment, whether a target is plausible, and what you would need to believe for a plan to work.

What is the difference between top-down and bottom-up sizing?

Top-down starts with a broad published figure, such as total spend in a category, and applies percentages to shrink it to your slice. Bottom-up starts with countable units, such as the number of accounts that match your criteria, and multiplies by a realistic price.

	Top-down	Bottom-up
Starting point	A category or industry total	A count of accounts that fit
Main inputs	Third-party estimates and assumed percentages	Account lists, firmographic filters, your pricing
Speed	Fast	Slower; needs research
Strength	Gives a rough ceiling and context	Ties directly to your ICP, pricing and sales plan
Weakness	Category definitions rarely match your product; percentages are often guesses	Depends on the quality of your account counts and price assumptions
Best use	Sense-checking the order of magnitude	Planning, targeting and forecasting

The common failure of top-down sizing is the "if we win a small percentage of a huge market" argument. It hides every assumption that matters: who buys, why, at what price and through which channel. Bottom-up sizing forces those assumptions into the open, which is why it is usually the more useful method for B2B teams.

Use both when you can. If the two methods disagree by a wide margin, the gap tells you which assumption to investigate.

How do you calculate TAM, SAM and SOM bottom-up?

The core formula is simple:

Market size = number of accounts × annual revenue per account

The work lies in defining each input carefully.

1. Define the account. Decide whether you count companies, sites, departments or teams. Pick the unit that actually signs the contract.
2. Set TAM criteria. List the observable characteristics of an account that has the problem: industry, size band, function, technology used. Your ICP is the starting point; How to Build an ICP covers how to define it.
3. Count matching accounts. Use firmographic databases, industry registries or association lists. Record the source and filters so the count can be repeated.
4. Set annual revenue per account. Use your actual or planned pricing, not an aspirational figure. If price varies by size, split accounts into bands and price each band separately.
5. Apply SAM filters. Remove accounts you cannot serve today: unsupported regions, languages, integrations, compliance needs or company sizes outside your purchase path.
6. Estimate SOM. Start from what your team can reach and convert in the period, not from a percentage of SAM.

What does a worked bottom-up example look like?

The numbers below are illustrative only. They do not describe any real market.

Imagine a company selling scheduling software to field-service businesses. It sells to the company, priced by size band.

Step 1: TAM. The team counts all field-service businesses with ten or more technicians, in any region.

Size band	Accounts (illustrative)	Annual price per account	Revenue
10–49 technicians	40,000	3,000	120,000,000
50–199 technicians	8,000	12,000	96,000,000
200+ technicians	1,000	40,000	40,000,000
TAM	49,000		256,000,000

Step 2: SAM. The product supports only English, integrates only with two common accounting tools, and has no features yet for the largest enterprises' approval workflows. The team applies these filters:

Size band	Accounts after filters (illustrative)	Annual price	Revenue
10–49 technicians	12,000	3,000	36,000,000
50–199 technicians	2,500	12,000	30,000,000
200+ technicians	0 (not yet served)	—	0
SAM	14,500		66,000,000

Step 3: SOM. The team estimates what it can win in the next year from capacity, not hope:

• Marketing can generate around 1,500 qualified opportunities in the smaller band and 300 in the mid band.
• Historical or benchmark win rates the team trusts are assumed at one in five for the smaller band and one in six for the mid band.
• That gives roughly 300 small-band wins and 50 mid-band wins.

SOM = (300 × 3,000) + (50 × 12,000) = 900,000 + 600,000 = 1,500,000 in annual revenue.

SOM is a small fraction of SAM here, and that is normal. It reflects the team's reach and capacity this year. As those grow, or as the product clears a SAM filter such as the enterprise workflow gap, SAM and SOM expand.

The useful output is not one headline figure. It is the visible chain of assumptions: which filters cost the most accounts, which band carries the revenue, and which capacity limit constrains SOM. That chain feeds directly into deciding where to focus, which Prioritize B2B Customer Segments covers in detail.

How should you sense-check the numbers?

• Run a top-down comparison. If a credible category figure exists, check whether your TAM sits in the same order of magnitude. A large mismatch usually means a different category definition or an unrealistic price.
• Reconcile price with reality. Compare annual revenue per account against current deals or willingness-to-pay research. B2B Willingness-to-Pay Interview Questions can help.
• Check SOM against funnel capacity. Opportunities, win rates and sales cycle length must be consistent with your pipeline and headcount.
• Show a range. Give low, expected and high cases for the inputs you are least sure of.

What are the common mistakes?

Treating TAM as everyone who could conceivably buy. Count only accounts with the problem and the ability to pay.

Mixing units. Counting companies in one step and users in another inflates or deflates the result.

Using list price for every account. Discounts, smaller packages and size differences change revenue per account. Band your pricing.

Deriving SOM as a flat percentage of SAM. Build it from reach, conversion and capacity instead.

Leaving sources undocumented. If nobody can repeat the account count, nobody can update it.

Sizing once and forgetting it. Revisit the model when pricing, product coverage or channels change.

When does this approach not apply?

Bottom-up sizing from account counts works poorly for genuinely new categories where buyers do not yet recognize the problem. There, size the problem by counting accounts with the underlying workflow or cost, and treat the result as a hypothesis to test with early customers. It also needs adjustment for usage-based pricing, where revenue per account depends on consumption; model a range of usage levels rather than a single price.

What else should you know about TAM, SAM and SOM?

Frequently asked questions

How do you calculate TAM, SAM and SOM?

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Multiply the number of accounts by annual revenue per account at each level. TAM covers every account with the problem, SAM keeps only those your product and channels can serve, and SOM is what you can realistically win in a stated period based on reach, conversion and capacity.

Is top-down or bottom-up market sizing better for B2B?

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Bottom-up is usually more useful because it ties directly to your ICP, pricing and sales plan. Top-down is a helpful sense check on the order of magnitude.

Should TAM, SAM and SOM be revenue or number of customers?

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Report them as annual revenue, but keep the account counts visible. The counts show where the revenue comes from and make the model easier to check.

What time period should SOM cover?

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Choose a period that matches your planning horizon and state it explicitly. A SOM without a time period cannot be checked against a plan.

How often should a market size be updated?

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Update it when pricing, product coverage, target segments or channels change, and review it during regular planning cycles.

If you want help applying this to your own team, talk to us.

Related reading

• How to Build an ICP — define the account criteria that drive your TAM.
• Prioritize B2B Customer Segments — decide which part of your SAM deserves investment first.
• B2B Willingness-to-Pay Interview Questions — test the price assumptions in your model.
• Calculate B2B Funnel Conversion Rates for Long Sales Cycles — ground your SOM in realistic conversion.