Market sizing is where many B2B strategies quietly become fiction. A large TAM looks impressive in a deck. A realistic SOM (what you can actually win with your current offer, channels, and capacity) is what should shape hiring, budget, and go-to-market focus.
This article covers practical B2B definitions, top-down and bottom-up sizing, pricing and serviceability, common errors, and one clearly labelled hypothetical example. Where you need external data, use named public sources or your own commercial evidence, not invented benchmarks.
Market opportunity sits alongside ICP inside a coherent go-to-market strategy. Opportunity without a sharp ideal customer profile is just a big number.
What TAM, SAM and SOM mean in B2B
TAM (Total Addressable Market) The total revenue opportunity if every organisation that could theoretically use a solution like yours bought it. It is the outer boundary of the category, not your plan.
SAM (Serviceable Addressable Market) The portion of TAM you can serve with your current (or near-term) product, geography, segment focus, and delivery model. SAM applies real constraints.
SOM (Serviceable Obtainable Market) The share of SAM you can realistically capture in a defined period, given competition, brand, channels, sales capacity, and execution quality. SOM is a planning number, not a trophy.
Investors sometimes care about TAM as category narrative. Operators should care more about SAM and SOM as decision tools. If SOM cannot fund the plan, the plan is wrong, not the market.
Two ways to calculate: top-down and bottom-up
Use both. When they diverge, investigate rather than picking the flattering answer.
Top-down
Start from a large market figure and narrow.
- Begin with a category or industry revenue estimate from a named public source (government statistics, regulator data, trade body reports, or a research firm you are willing to cite).
- Filter by the segments you can serve (industry, size, geography, use case).
- Apply further filters for product fit and buying reality.
- Arrive at SAM, then apply a obtainability assumption for SOM.
Top-down is fast and good for orientation. It is also where lazy filtering happens, “we’ll take 1% of a huge market” is not analysis.
Bottom-up
Start from countable accounts and your economics.
- Count or estimate the number of organisations that match your ICP in target geographies.
- Estimate average annual contract value (or another revenue unit) for those accounts.
- Multiply for SAM (all serviceable accounts × realistic ACV if they bought).
- For SOM, apply expected reach, conversion, and capacity over a time horizon (e.g. 3 years), not a fantasy win rate.
Bottom-up is usually more honest for B2B, especially when markets are niche. It forces you to confront how many real buyers exist.
Reconcile the two
If top-down SAM is £2bn and bottom-up SAM is £80m, one of the following is true: the category definition is wrong, the ICP is narrower than admitted, pricing assumptions differ, or a data source is being misused. Document the gap. Do not average it into a comforting middle.
Pricing assumptions change everything
Market size is revenue opportunity, not company count alone. Pricing assumptions therefore matter as much as account counts.
Clarify:
- Unit of value: seats, sites, projects, retainers, usage, what do you actually sell?
- Average and median ACV: averages hide that a subset of deals drive the model.
- Expansion: is year-one revenue different from steady-state?
- Discounting and packaging: does the “list” ACV used in sizing ever appear in the real world?
- Currency and purchasing power across markets.
A common error is sizing TAM with enterprise ACV while your current motion sells mid-market packages. That inflates opportunity and mis-sets channel investment. Align pricing used in sizing with the offer you will take to market in the planning period, or show scenarios.
Serviceability and SOM
Serviceability turns TAM into SAM. It is not only “can the product technically work?” It includes geography (sell, support, comply), segment fit without rewriting the product, delivery capacity, channel access at acceptable cost, and trust or proof requirements you do not yet meet. If you cannot serve an account well, it is not in your SAM, even if a competitor could.
SOM answers how much of SAM you can obtain in a defined window, given win rates, sales capacity, marketing reach into the ICP, cycle length, competition, and proof gaps. Keep it time-bound: “obtainable in 36 months under these assumptions,” not destiny forever. When SOM looks too small to justify the plan, that is useful information, narrow the plan, change the offer, expand serviceability for real, or choose a different beachhead. Do not inflate the percentage.
If growth is stuck, the constraint may be opportunity size, or positioning, distribution, or sales friction inside an adequate market. See How to Identify What’s Actually Stopping Your Business from Growing and Why Your B2B Marketing Isn’t Generating Enough Pipeline.
Hypothetical example (illustrative only)
The following numbers are hypothetical. They are not based on a real client, category dataset, or claimed outcome. They exist only to show the logic.
Company: A UK B2B SaaS tool that helps 50–250 person professional services firms manage client onboarding workflows. Planning horizon for SOM: 3 years. Pricing assumption: £12,000 average annual contract value (ACV).
Step A: Bottom-up SAM
Suppose research (company databases + filters for firm size, industry codes, and UK location) yields approximately 8,000 firms that match the firmographic ICP.
If every one of those firms bought at £12,000 ACV:
SAM = 8,000 × £12,000 = £96m annual revenue opportunity at full penetration.
Step B: Narrow further if needed
If only firms that already use a certain adjacent tool category are realistically ready (say 40% of the 8,000 based on a reasoned estimate) then a tighter serviceable view is:
3,200 accounts × £12,000 = £38.4m SAM (behaviourally filtered).
Use the version that matches how you will go to market. If your motion requires that adjacent stack, the tighter SAM is the honest one.
Step C: SOM for 3 years
Assume year-3 capacity can work 400 qualified opportunities per year at a 20% win rate and £12,000 ACV (net-new logos only). Annual new ARR at that run-rate: 400 × 20% × £12,000 = £960,000. With ramp, a simple hypothetical cumulative new ARR by end of year 3 might land around £2m SOM for planning.
Relative to a £38.4m behaviourally filtered SAM, that is a small share, normal, and useful for testing whether hiring and channel spend match obtainable reality. Claiming “5% of a £2bn global TAM” instead would imply £100m with no link to ICP count, capacity, or win rate.
Common errors
Using global category TAM as if it were your market; confusing company count with revenue; applying vanity rates (“1% of TAM”) without a capacity path; ignoring competition and status quo; sizing on an aspirational ICP while selling a narrower offer; treating one desktop study as permanent truth; and hiding assumptions. Update sizing when ICP, pricing, or geography changes, it is an input to How to Build a Go-to-Market Strategy, not a one-off slide.
How to use the numbers in decisions
Use SAM/SOM to prioritise segments, set headcount and channel plans against obtainable pipeline, and pressure-test board narratives with bottom-up logic. Revisit sizing when GTM Performance metrics contradict the model. Do not win arguments with the largest available number, justify unfocused marketing because “the market is huge,” or skip ICP work because the TAM slide looks finished.
Paceject includes TAM/SAM/SOM in the Define stage of GTM Strategy & Implementation when opportunity clarity is part of the constraint, alongside ICP, positioning, and route to market. Related capabilities follow diagnosis.
Closing
TAM describes the category. SAM describes what you can serve. SOM describes what you can obtain with a real go-to-market system in a real time window. Make pricing and serviceability assumptions explicit, and treat large round numbers with suspicion.
If you want help grounding opportunity sizing in evidence, get in touch. Scope sits under GTM Strategy & Implementation; investment framing is on pricing.