Most market-sizing numbers fall apart the moment someone asks how you got them. The two methods worth using are top-down — start from the whole market and narrow down — and bottom-up — start from real customers and build up; the strongest estimates use both and check that they roughly agree. One gives you the ceiling, the other gives you something you can defend. For a B2B revenue team, the point isn't a big number for a slide; it's a believable number you can plan against. Here's the practitioner's read on doing it right.
What do TAM, SAM, and SOM actually mean?
They're three shrinking circles: the whole market, the slice you can serve, and the slice you can realistically win. TAM (Total Addressable Market) is everyone who could ever buy a product like yours if there were no limits. SAM (Serviceable Addressable Market) narrows that to who you can actually reach and serve — the right region, segment, company size, and use case. SOM (Serviceable Obtainable Market) is the share of SAM you can win in a real timeframe, given your team, budget, and competition. The mistake is leading with TAM as if it were the prize. TAM sets context; SAM and SOM are what you build a plan on.
How does the top-down method work?
You start with a published total for the market and apply filters until you reach your slice. Take an industry size from analyst reports or public data, then multiply it down by the percentages that describe your reach — your region, your segment, your buyer type. Worked example (illustrative): suppose a software category is reported at a large industry total. You estimate that companies in your target size band make up a portion of it, that your region is a fraction of that, and that the buyers who need your specific use case are a fraction again. Multiply those down and you land on a SAM. Top-down is fast and good for a first read, but it's only as honest as the percentages you choose — and it's easy to talk yourself into generous ones.
How does the bottom-up method work?
You build the number from the unit you actually sell — count real potential customers and multiply by what each is worth. Start from your own data and the real world: how many companies fit your ideal profile, how many of those would plausibly buy, and what the average deal is worth per year. Multiply customers by price by frequency and you have a market size built from things you can verify. Worked example (illustrative): if you can identify a defined list of companies that match your ideal customer profile, estimate the share that would realistically buy, and multiply by a typical annual contract value, you get a SAM or SOM grounded in named accounts rather than borrowed percentages. Bottom-up takes more work, but it's the number a sales leader will actually trust.
Which method should you use — and when?
Use both, then see if they meet in the middle. Top-down gives you the outer boundary and a fast sanity check; bottom-up gives you a defensible, plannable figure. When they roughly agree, your confidence goes up. When they're wildly apart, that gap is a gift — it usually means one of your assumptions is wrong, and finding which one teaches you something about the market. Early on, lean top-down for direction. As you gain real pipeline and customer data, lean bottom-up, because now you're reasoning from evidence instead of estimates. For most B2B revenue planning, the bottom-up number is the one you commit to; the top-down number is the one you use to argue you're not leaving the market half-served.
The IV-Lead take
Market sizing goes wrong when it's treated as a fundraising prop instead of a planning tool. A giant TAM impresses no one who has to actually hit a number. What we care about with clients is the bottom-up math, because it doubles as a go-to-market plan: the same list of ideal accounts that sizes your SOM is the list your sales team should be working. If your CRM holds clean firmographic data — industry, size, region, fit — you can build a bottom-up estimate from your own pipeline instead of guessing. That's the quiet advantage of a well-organized revenue system: the market size and the target list are the same data, kept honest.
Want your market sizing built from real CRM data instead of borrowed percentages? Book a 30-minute portal audit — we'll show you how clean your data needs to be to size and target at the same time. For the bigger picture, see how we approach revenue operations.
Frequently asked questions
What's the difference between TAM, SAM, and SOM?
TAM is the entire possible market, SAM is the portion you can actually serve, and SOM is the realistic share you can win in a set timeframe. They shrink in that order, from context to plan.
Is top-down or bottom-up better for market sizing?
Neither alone — use both. Top-down is fast and sets the ceiling; bottom-up is grounded and defensible. When the two roughly agree, your estimate is far more trustworthy.
What data do I need for a bottom-up estimate?
A count of companies matching your ideal customer profile, a realistic estimate of how many would buy, and a typical annual deal value. Clean firmographic data in your CRM makes this far easier.
How often should we re-do market sizing?
Revisit it when you enter a new segment or region, change pricing, or your pipeline data shifts meaningfully. For most B2B teams that's at least once a year, tied to planning.
