How to Size Your Market: TAM, SAM and SOM for an Investor Pitch

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By Saroj Jha · October 4, 2026 · 13 min read

TAM is the total demand for what you solve, SAM is the part you can serve today, and SOM is the part you can realistically win in the next few years. Build all three from the bottom up, customers you can count times what each pays, and label every assumption so an investor can check it.

Many market-size slides show three nested circles and a very large number. Investors tend to look past the number to the method. A TAM taken from an analyst headline, with a SOM of “just 1%”, tells them you have not done the work. A bottom-up estimate with named sources and clear assumptions tells them you understand who buys and why.

In my view, this is the slide founders find hardest, because there is no single right answer. There is, though, a method an investor can follow and check. Here it is in six steps, with one company’s numbers from start to finish.

Running case: Example Co. Fictional

Example Co. is a seed-stage MedTech startup. It makes a benchtop blood analyzer that runs routine tests, such as HbA1c and a lipid panel, from a fingerstick sample in about 12 minutes, so the clinician can discuss results during the visit. In this fictional case it is FDA-cleared and CLIA-waived. A clinic gets the analyzer under a three-year agreement and buys test cartridges at about $14 each. Its first customers are independent primary-care practices; urgent care centers and health-system clinics come later. It is preparing its investor pitch and needs a market slide with TAM, SAM and SOM. Every Example Co. number below is labelled as a public count or an assumption, and all of them are illustrative.

What do TAM, SAM and SOM mean?

The TAM and SAM definitions come from the OpenStax Entrepreneurship textbook, which calls TAM the total available market^[4]. I have found no authoritative source that defines SOM; the definition above is common practice. Whatever definitions you use, state them on the slide, so the investor reads your numbers the way you meant them.

Should I size top-down or bottom-up?

Bottom-up, with top-down as a check.

• Top-down starts from a published industry total and narrows it with percentages. It is quick, but each percentage is a guess, and the starting total usually measures someone else’s definition of the market.
• Bottom-up starts from the customers themselves: how many there are and what each would pay per year. It is slower, but every input can be checked.

Bill Aulet’s Disciplined Entrepreneurship sizes the first (beachhead) market as the annual revenue you would earn at 100% market share, a figure it suggests calculating both ways: the easier top-down estimate first, then a bottom-up count of customers, which gives the more accurate picture^[5],[6]. In his own summary, the task is to “estimate the revenue per year you will get in your beachhead market if you achieve 100% market share”^[7]. If a careful top-down figure and your bottom-up figure are far apart, find out why before you pitch either.

Step 1: Define the customer and what you charge for

Describe the customer precisely enough to count: industry, size, location and the trait that gives them the problem. “Mid-size companies” is too vague; “independent primary-care practices with 3 to 10 clinicians” can be counted. Then decide the unit you charge for. If you price per seat, count seats; if per company, count companies. Counting the wrong unit is an easy error to make and an easy one for an investor to spot.

Example Co., step 1 Fictional

The first customer is an independent primary-care practice with 3 to 10 clinicians that sends routine bloodwork to an outside lab. The unit is one clinic, meaning one practice location. A typical clinic runs about 150 tests a month at an assumed $14 a cartridge: about $2,100 a month, or about $25,000 a year. Other segments have different units and prices, so they are listed but not sized.

Do this

Write your customer definition in one sentence and your price per unit per year next to it. If you cannot count the thing in the sentence, rewrite it.

Step 2: Count the customers from official data

In the US, County Business Patterns and Statistics of U.S. Businesses count businesses by industry and size; Nonemployer Statistics counts businesses with no paid employees^[8]; the BLS counts jobs by occupation, if you sell per seat^[9],[10]. For hospitals, the American Hospital Association publishes a count from its annual survey^[11]. The Market Research Playbook lists more sources.

Use one source per count and say which. An industry association count and a Census count of the same industry can differ widely, because they define the industry differently.

Example Co., step 2 Fictional

The physician-office count is public. The Census Bureau’s County Business Patterns counts employer establishments, each “a single physical location at which business is conducted or services or industrial operations are performed”^[12]. Its 2023 table counts 204,617 establishments in NAICS 621111, offices of physicians except mental health specialists^[13]. That covers every specialty, so it is a ceiling, not Example Co.’s market. The number of independent primary-care practices with 3 to 10 clinicians is not public: the team uses an assumed 20% of the 204,617, which is about 40,923 practices, and marks it to be replaced with Census size-class and specialty data. Other segments are not counted yet, so they are left out rather than guessed: the AHA’s 6,100 US hospitals (2024 survey) are hospitals, not their outpatient clinics.

Do this

Next to each count, write the source, the year, and “public” or “assumed”. An investor should be able to see which is which at a glance.

Step 3: Multiply to get TAM

The core formula is simple:

Market size per year = number of target customers × annual revenue per customer

For an early-stage company, I treat TAM as the segments on your expansion path that you can count today, each at its own price. Add them up, and list the segments you have not sized yet rather than filling them with a guess. If a credible top-down figure for your exact category exists, compare it with yours and explain any big gap.

Example Co., step 3 Fictional

Independent primary-care practices: an assumed 40,923 × an assumed $25,000 = $1,023,075,000, about $1.02 billion. TAM for the one counted segment, US only: about $1.02 billion a year. Urgent care, retail clinics and health-system clinics would add to it, but they are “not yet sized”.

Do this

Show the multiplication for each segment on the slide or in the appendix, not only the total.

Step 4: Narrow to SAM

SAM is the part of TAM you can serve with today’s product, geography and channels. Remove customers you cannot serve yet: another country, a feature you lack, a buying process you cannot meet, a channel you cannot reach. Each filter should be one short sentence an investor can challenge.

Example Co., step 4 Fictional

Two filters turn TAM into SAM. First, the team serves only the states where its two sales reps and a distributor partner can deliver cartridges today: an assumed 20% of practices, so 40,923 × 20% = about 8,185. Second, it serves only practices that send routine bloodwork to an outside lab, because the others have no result delay to remove: an assumed 80%, so 8,185 × 80% = 6,548. At $25,000 that is $163.7 million, about $164 million a year. Health systems drop out for now: they ask for published accuracy evidence and an integration with their records system, which the company does not have yet.

Do this

Write each SAM filter as a sentence that starts “We exclude ... because ...”. If you cannot finish the sentence, the filter is a guess.

Step 5: Build SOM from your capacity to win

Tie SOM to your capacity to sell, not to a percentage. A SOM of “1% of the market” says nothing about how you will win those customers. Build it from what you can observe:

• Sales capacity. Deals a rep can close per quarter × reps you plan to hire × your win rate.
• Channel capacity. Traffic or leads your channels produce × conversion rates you have measured.
• Evidence of demand. Pilots, pre-orders and a waiting list count; survey answers count for less.

Be careful with survey data here. Across 40 studies, purchase intentions correlated with actual purchases at 0.49 on average, and less closely for new products and longer time horizons^[14]. For consumer goods, hypothetical willingness to pay runs about 21% above real willingness to pay on average, across 77 studies^[2]. A SOM built on “70% of respondents said they would buy” will not survive due diligence.

Example Co., step 5 Fictional

The team’s two-quarter target is 25 clinics signed from a named list of 300, with a win rate above 25% from demo to agreement. It builds SOM from that: 50 new clinics in year 1 (25 every two quarters, which matches its goal of 50 in 12 months), 90 in year 2 (a third rep and a distributor partnership) and 150 in year 3 (a fourth rep and a second distributor). That is 290 wins; less an assumed 10 cancellations, about 280 paying clinics × $25,000 = $7.0 million a year. That is about 4% of SAM, and the team says so, because an investor will ask whether 4% in three years is realistic. All numbers are fictional.

Do this

Build SOM year by year from sellers, channels and win rates, then check what share of SAM it implies. If the share looks too high, your capacity assumptions probably are too.

Free tool

Size your market from counts you can source

The free Bottom-Up Market Sizer builds TAM, SAM and SOM from counted accounts and sourced assumptions, sets SOM by what your team can win, and flags every unsourced number.

Step 6: Put it on one slide, with sources

Investors use the market size to judge how big the company could become, and they check the assumptions. In a survey of 885 venture capitalists, 68% named the market as an important factor in their decisions, though the team was named more often, by 95%^[1]. The survey dates from 2015–16, but a 2025 study of more than 8,000 deals seen by one early-stage investor adds a useful point: the team best explained which startups raised at least $1 million, but market and product had more explanatory power for which raised more than $10 million, and for longer-term outcomes^[15].

Investors also verify what you claim. A study of 722 funding requests to one US venture capital firm found that business plans were only weakly linked to funding decisions; the authors conclude that investors learn this information through other channels^[16]. Expect them to rebuild your market size themselves. Make it easy by showing your sources and inputs.

What Uber’s, Airbnb’s and Zoom’s filings show

Companies going public describe their business and markets in their IPO registration statement, and several use the same layered framing. The clearest is Uber’s 2019 filing. It gave each layer a number and a rule:

• TAM: “Our Personal Mobility TAM consists of 11.9 trillion miles per year, representing an estimated $5.7 trillion market opportunity in 175 countries.” The method was bottom-up, “multiplying the number of passenger cars in each country by our country-level estimates of miles traveled per car”.
• SAM: “Our current Personal Mobility SAM consists of 3.9 trillion miles per year, representing an estimated $2.5 trillion market opportunity in 57 countries.” The rule was stated: “We also include all miles traveled in passenger vehicles for trips under 30 miles in our SAM.”
• Near-term SAM: “Our near-term SAM consists of 4.7 trillion miles per year, representing an estimated $3.0 trillion market opportunity in 63 countries”, adding “Argentina, Germany, Italy, Japan, South Korea, and Spain”.
• Where it stood: “consumers only traveled approximately 26 billion miles on our platform in 2018”, “implying a less than 1% penetration rate of our near-term SAM”^[3].

Airbnb’s 2020 filing shows the SAM filter at work: “We estimate our total addressable market (“TAM”) to be $3.4 trillion, including $1.8 trillion for short-term stays, $210 billion for long-term stays, and $1.4 trillion for experiences.” Its SAM of $1.5 trillion covered short-term stays and experiences only, so long-term stays sat in TAM but not SAM. It also dated its inputs: “We have estimated key components of our SAM and TAM using 2019 actual figures”^[17].

Zoom’s 2019 filing is the top-down contrast. It named the research-firm categories it addresses and quoted their total: “IDC estimated that these segments combined represent a $43.1 billion opportunity in 2022.” It then argued that it addresses “a broader opportunity than is currently captured in third-party market research”^[18].

The pattern to copy is not the size of the numbers. It is that each layer has a stated rule, a year and a source, and that the company shows how much of the market it holds today. These are what the filings say, not an account of how the companies made their estimates internally; none of the three disclosed every input.

Example Co., step 6 Fictional

The market slide follows Uber’s pattern at seed scale: TAM about $1.02 billion (one counted segment, US), SAM about $164 million (practices it can serve today), SOM about $7 million a year by year 3 (from sales capacity). Under each number sits its formula in small type, the physician-office source (Census County Business Patterns, 2023) and the word “assumed” wherever it applies. The full inputs go in the appendix.

Do this

Give every layer on your slide one line that states its rule, like Uber’s “countries where we operate today”, and one line that states its source and year.

What changes by stage?

• Pre-seed — Size your first segment bottom-up. A clear SAM for one segment beats a vague TAM for a whole industry.
• Seed — Add a SOM built from your sales and channel capacity, and show how the market expands to the next segments.
• Series A–B — Rebuild the numbers from your own data: win rates, sales cycle, revenue per customer. Show how each new segment adds to the TAM.

What mistakes should I avoid?

• Borrowing a headline number. An industry total for “diagnostics” is not the market for your product.
• “We only need 1%.” It shows no path to the customers.
• Counting the wrong unit. If you price per seat, count seats; if per company, count companies.
• Mixing sources with different definitions. Use one per count, and say which.
• Using stated intent as demand. It is an upper limit.
• Hiding assumptions. Label every number “public” or “assumed”.
• No date. Say which year each figure is from.