TAM, SAM, SOM: How to Size the Market You Can Actually Reach

A food truck parks downtown in a city of a million people. On a hopeful morning, the owner could call that a million-person market. Only the office workers within a few blocks walk past at lunch, though, and one grill can serve about 150 of them before the rush ends.

Founders size markets the way that owner does on the hopeful morning. The pitch deck says the industry is worth billions, and the plan claims one percent of it. An investor reads that slide and learns nothing about who will buy.

TAM, SAM and SOM fix that by counting down from the city to the line at the window. Two questions matter more than any of the totals. Who exactly sits inside each number, and what limits how many of them you can serve?

TAM, SAM, SOM defined

Each number sits inside the one before it, the way the lunch line sits inside the few blocks and the blocks sit inside the city:

• TAM (total addressable market): the annual revenue you'd earn if every company matching your customer definition bought from you. The whole city.
• SAM (serviceable addressable market): the part your product and sales motion can reach today, after real limits such as geography or a required integration. The blocks within walking distance.
• SOM (serviceable obtainable market): the share of SAM you can realistically win in the next three years. The line one grill can serve.

Each step down should come from a limit you can name. A cut chosen to land on a tidy figure is a guess wearing a number.

Top-down vs. bottom-up sizing

You can reach those three numbers from either end.

Investors have seen a hundred decks claim one percent of a giant number, and they discount that move on sight. Build bottom-up as your main figure. If you have an industry total, use it only as a ceiling check: when your bottom-up TAM comes close to the whole industry's spend, your count or your price is too high.

A bottom-up sizing example

The bottom-up method is easier to see on one company. Picture PawDesk, a hypothetical startup selling front-desk software to independent US veterinary clinics with three or more vets. Every figure below is invented for illustration.

1. Define the customer: "independent US clinics with three or more vets" can be counted. "Pet businesses" can't.
2. Count the matches: PawDesk pulls 8,000 matching clinics from a business database and records the source.
3. Set the price: PawDesk plans to charge $6,000 a year per clinic.
4. Filter to SAM: the product only connects to two practice-management systems, and 40% of the clinics run one of them.
5. Estimate SOM two ways: a demand rate (what share of SAM will buy) and sales capacity (how many deals the team can close).

The arithmetic runs like this:

Take the lower of the two SOM figures. Demand says PawDesk could win 160 clinics in three years, but two reps can close only 144. Capacity binds, so SOM is 144 clinics, or about $864,000 a year by the end of year three, before churn. That's 4.5% of the serviceable clinics.

Founders like to lead with the $48M. The $864,000 is what the next three years look like, so build the plan on that one.

Then test the shakiest input. If only 20% of clinics turn out to run a compatible system, SAM halves to $9.6M. Check the integration rate on a sample of real clinics before the number goes in a deck.

When the SOM looks small: a small SOM can still be a good business, especially one funded from its own revenue. Venture investors need a market that could make the company many times its size, so for a raise, a small SAM is the harder problem.

Market sizing mistakes

The arithmetic is simple. The mistakes come from what goes into it:

• SAM equal to TAM: no limits applied, which usually means nobody thought about them.
• A customer nobody can count: "small businesses" spans a food truck and a 200-person software firm, so no database can count it.
• Demand without capacity: a 10% capture rate means nothing if two reps can't close that many deals.
• One number, no range: a single figure reads as false precision. Show how the total moves when the weakest input is wrong.
• Sizing before segmenting: the count depends on a customer definition, so choose the segment first.

A beachhead market narrows the first segment, and the ideal customer profile template sets the account-level filters your count depends on.

Using the numbers

The food truck owner who counts the lunch line knows how many burgers to prep and whether a second grill would pay for itself. SOM does the same job for a startup. It sets how many reps to hire and how much pipeline the plan needs, and it's the number an investor will press on first.

The sizing then feeds the go-to-market plan, where the reachable market becomes a channel and a target. In my fractional CMO engagements, the buyer definition behind those counts gets written down in the company brain, so the sizing, the plan and every campaign start from the same customer.

Count the line at the window before you count the city.