TAM/SAM/SOM for Non-MBA Founders: A Worked Example

The first time a serious investor (or a serious AI validator) asks for your TAM, SAM, and SOM, the temptation is to make up a number that sounds defensible and move on. Don't. A lazy TAM is the single fastest way to signal to a sophisticated investor that you don't actually understand your market. The good news: sizing a market correctly is not a finance skill. It's a research skill, and the tools are all free.

This post walks through TAM/SAM/SOM from first principles, with a single worked example we use throughout: a US-based B2B SaaS startup that helps independent HVAC companies (heating, ventilation, air conditioning) schedule service calls faster. By the end of this post you'll be able to size your own market the same way.

The 30-second version

• TAM (Total Addressable Market): the total revenue opportunity if every potential customer used your product.
• SAM (Serviceable Addressable Market): the slice of TAM you can actually reach, given your geography, language, channels, and current focus.
• SOM (Serviceable Obtainable Market): the slice of SAM you can realistically win in the next 1–3 years, given your team, capital, and go-to-market motion.

These three numbers tell the same story at different zoom levels. TAM is "how big could this be." SAM is "how big is it for us." SOM is "how big is it for us this year."

Why most TAM slides are wrong

The dominant TAM hack is the "top-down" approach: take a big industry number from a McKinsey report, multiply by 0.01, and declare a $1B TAM. This is wrong in three ways:

1. It overstates the realistic opportunity. A $400B industry number is a category number, not a budget number. The amount of money in that category that flows through software vendors is usually 1–5%, not 100%.
2. It understates the specificity investors want. "We're in the $400B healthcare market" tells the investor nothing about the wedge you're going after. They want to see you understand the small, specific beachhead.
3. It's a single number, not a funnel. TAM, SAM, and SOM are three numbers that should each shrink the previous one by a meaningful factor (usually 5–50x). A one-number TAM slide skips the analysis.

The right approach is "bottom-up": start with a count of real potential buyers, multiply by a realistic per-buyer revenue, and work backwards up the funnel. This is slower, but it produces a TAM you can defend in a 30-minute investor meeting without hand-waving.

Worked example: HVAC field-service SaaS

The startup we'll use throughout: a B2B SaaS that helps independent US HVAC companies schedule service calls, dispatch technicians, and invoice customers. Price point: $300/month per company.

Step 1: Count the potential buyers (the TAM seed)

The smallest unit of analysis. For us, the unit is "one independent HVAC company in the US that employs at least 3 technicians."

Where to get the count:

• The US Bureau of Labor Statistics (BLS) publishes annual counts of establishments by NAICS code. NAICS 238220 is "Plumbing, Heating, and Air-Conditioning Contractors." In 2024, there were approximately 119,000 such establishments in the US with payroll.
• The BLS also publishes the count of establishments by employee size class. About 46,000 of those have 5+ employees, which roughly matches "3+ technicians" (technicians plus a few office staff).

So our TAM seed is: 46,000 potential buyers in the US.

Step 2: Multiply by realistic per-buyer revenue

Our price is $300/month, or $3,600/year per company.

46,000 × $3,600 = $165.6M / year

This is our TAM. If we sold our product to every HVAC company in the US with 3+ techs, at full price, no churn, we'd do $165M ARR.

This number is small, and that's a feature, not a bug. A defensible $165M TAM is more credible to an investor than a hand-wavy $4B TAM, because the investor knows the bottom-up math will hold up in due diligence.

Step 3: Subtract for things we can't reach (the SAM)

TAM assumes we can reach everyone. SAM is what we can actually reach given real-world constraints. Common SAM deductions:

• Geography. Are we US-only, or do we also sell in Canada / the UK? For this example, US-only.
• Channel. Can we reach every HVAC company through our current sales motion (self-serve + inside sales)? Or do half of them require a field-sales rep we don't have?
• Language. Are we English-only?
• Vertical focus. Are we focused on residential HVAC, or do we also do commercial? (Commercial is a different buyer with different needs; we'd be a bad fit at launch.)

For our HVAC example:

• 46,000 US establishments with 3+ techs
• Minus 30% that are commercial-only (we don't serve them well yet) → 32,200
• Minus 15% that are too rural for our inside-sales motion to reach cost-effectively → 27,400
• Minus 10% in non-English-speaking markets we can't serve yet → 24,700

24,700 × $3,600 = $88.9M / year

This is our SAM. We've now cut our TAM roughly in half by acknowledging the buyers we can't actually reach today.

Step 4: Subtract for the realistic share we can win (the SOM)

SAM is what we could serve with infinite sales effort. SOM is what we will serve in the next 1–3 years, given our team, capital, and go-to-market motion.

For a B2B SaaS targeting independent HVAC companies with inside-sales, a realistic SOM calculation:

• Year 1: 50 customers (early adopters, product-market fit validation)
• Year 2: 200 customers
• Year 3: 600 customers

600 × $3,600 = $2.16M ARR in year 3

This is the SOM. The $2.16M number is the one your CFO needs to plan around, and it's the one your investor wants to see a path to. The $165M TAM and $89M SAM are context. The $2.16M SOM is where the company actually lives for the next three years.

Step 5: Sanity-check the funnel

The numbers should shrink by reasonable factors:

• TAM → SAM: 165.6M → 88.9M. That's a 1.86x shrink, which is on the low end. Most investors will tell you they expect TAM-to-SAM to shrink by 5–10x. So either (a) our SAM deductions are too generous, or (b) our TAM is too conservative because we're missing a category of buyer (e.g. plumbing-only shops that also do HVAC, which is ~25% of the addressable market).
• SAM → SOM: 88.9M → 2.16M. That's a 41x shrink, which is in the healthy zone for a seed-stage company. Investors want to see SOM be 1–5% of SAM at this stage, so 2.4% is on point.

If our TAM-to-SAM had been 1.86x but TAM had been $4B (instead of $165M), the SAM would be $2.1B and the SOM at 2.4% would be $50M in year 3 — which would be an absurd trajectory for a 3-person seed team. That's the smell-test version of the analysis. The size of the team has to be commensurate with the size of the opportunity you're claiming you can win.

Common shortcuts (and when to use them)

You don't always have time for a full BLS deep-dive. Here are the three shortcuts that are honest enough for an investor pitch and fast enough for a single afternoon.

Shortcut 1: The "10x rule" sanity check. Take your SOM, multiply by 10. If that number is plausibly TAM, you're in the right ballpark. (For our HVAC example: 10 × $2.16M = $21.6M, which is about 13% of $165M. That's tight; we'd want to revisit.)

Shortcut 2: The competitor-revenue proxy. If you have 2–3 direct competitors, look up their published revenue (most public competitors disclose ARR; private competitors often leak numbers in funding rounds). If their combined revenue is $30M and your SOM is $200M, the math doesn't work — your SAM is overstated. If their combined revenue is $30M and your SOM is $3M, your SOM is under-stated (the existing market is bigger than you're modelling, which is a happy problem).

Shortcut 3: The buyer-count sanity check. For B2B, the "natural" market is usually 1–10% of the count of plausible buyers. If your SAM is 1,000 buyers and the median competitor sells to 50, your SOM is 50. If you have 100 customers in your pipeline and the median deal close rate is 10%, you'll close 10. This is a rough number, but it's the kind of "is-this-realistic" check that seasoned investors do subconsciously.

When TAM/SAM/SOM isn't the right frame

A few startup types are hard to model with this funnel:

• Two-sided marketplaces. TAM is usually the size of the smaller side; SAM is the subset you can access; SOM is the take rate × transactions, not the user count. Model the take rate and the average transaction size, not the buyer count.
• Open-source monetization. TAM is the entire dev-tools category, but SAM is the subset of companies that will pay for managed/SaaS versions. SOM is heavily constrained by the go-to-market (you have to build the SaaS first).
• Long-sales-cycle enterprise. A 12-month sales cycle compresses your SOM dramatically. Model by expected deals per quarter, not by total addressable customers.

In each of these cases the structure of the analysis is the same (count, multiply, subtract for reach, subtract for share) but the unit of analysis changes. The 5-step funnel is the shape; the specific numbers depend on the market.

What to do with your TAM/SAM/SOM once you have it

Three things, in order of importance:

1. Put SOM into your financial model. The $2.16M figure is the one that drives headcount, runway, and fundraising math. The other two are context for the story.
2. Sanity-check against the competitor-revenue proxy. Find your 2–3 closest competitors, look up their revenue, and check that your SOM × your expected market share doesn't exceed the realistic combined market. If it does, your SOM is too aggressive.
3. Run it through a validator. A good AI validator (Giri, for example) will give you a market-size estimate in the $50M–$500M range for most B2B niches, with a citation trail. If your hand-built TAM is wildly different from the validator's, that's a signal to dig deeper into one of the two numbers.

The goal of the TAM/SAM/SOM analysis is not to produce a number that's precisely right. The goal is to produce a number whose methodology an investor can interrogate, and whose underlying facts (NAICS codes, BLS counts, competitor revenues) are publicly verifiable. A defensible $165M is more valuable than an indefensible $4B.

Closing thought

The TAM/SAM/SOM exercise is not about numbers. It's about discipline. A founder who can produce a credible $165M TAM in 30 minutes, who can name the 2 competitors that have $30M between them, who can quote the BLS NAICS code for their buyer, and who can articulate which 30% of the SAM they can't reach today — that founder has demonstrated the kind of analytical rigor that compounds over the next five years.

The TAM/SAM/SOM is the first of many such exercises. Get good at it now, and every later one is easier.