Almost every pitch deck has a slide with three concentric circles. The biggest says TAM, the middle says SAM, the smallest says SOM. Founders fill them with large numbers and hope the sheer size of the outer ring will impress. When investment capital is harder to come by, that approach tends to fall flat. Experienced investors have seen too many slides claiming a slice of a trillion-dollar market. Sizing your market with care is how you stand apart from that crowd.
What the three letters mean
TAM stands for total addressable market: the total revenue available if every possible customer bought your type of product. SAM is the serviceable addressable market, the portion you could realistically reach with your current product, pricing, geography and sales model. SOM is the serviceable obtainable market, the share you can plausibly win in the near term given competition and your resources.
Each step narrows the focus. TAM answers whether the category is big enough to matter. SAM answers whether your specific offering fits a meaningful segment. SOM answers whether you can build a real business in the next few years.
The top-down trap in sizing your market
The most common mistake is pulling a headline figure from an industry report and multiplying by a tiny percentage. The logic goes: this global market is huge, so even one percent is a fortune. Investors hear this and tune out. It tells them nothing about who your customer is, how you will reach them, or what they will pay.
Top-down figures for sizing your market are also fuzzy. Analyst reports define markets differently, often include adjacent categories, and may be built on assumptions nobody on your team can check. If you cannot explain where a number comes from, it will not survive a polite question.
Sizing your market: build it from the bottom up
Sizing your market from the bottom up starts with your customer. Count how many of them exist, estimate what each might pay per year, and multiply. For example, if you sell scheduling software to independent dental practices, you would identify how many such practices exist in the regions you serve, decide on a realistic annual subscription, and calculate from there.
Some practical steps:
- Define one clear customer type, not a vague “small businesses” bucket
- Find a reliable count from public registries, trade bodies or your own data
- Price based on what early customers actually pay, not what you hope for
- Note which assumptions are guesses and which are measured
Sizing your market bottom-up gives numbers that are usually smaller, and that is fine. A modest, defensible number beats an enormous, shaky one.
Using SOM to tell a believable story
SOM is where discipline shows when sizing your market. It forces you to connect market size to your go-to-market plan: how many salespeople, how long deals take, what conversion rates you expect. If your plan says you will win a quarter of the market in three years with two sales reps, an investor will notice the gap.
A good SOM calculation ties to capacity. You might say that with a given acquisition channel and a realistic close rate, you can reach a certain number of customers by a certain stage. That makes the revenue forecast feel like a plan instead of a wish.
It also helps you decide what to build. If the obtainable market is small, you may need to expand into adjacent segments, raise prices or add products. Better to learn that on a spreadsheet than after burning a year of runway.
Why caution makes this matter more
When funding is plentiful, investors may accept big stories on the strength of momentum. When it is scarce, they look harder at fundamentals: can this company reach profitability, how efficient is its customer acquisition, and does the market support a business of the size their fund needs? Sizing your market becomes a test of founder judgment as much as a calculation.
A founder who says “our market is smaller than people assume, but here is why we can own it” often sounds stronger than one who claims everything. Specificity signals that you understand customers and have done the work of sizing your market.
A worked example of sizing your market
Imagine a founder selling route-planning software to independent plumbing firms. Rather than quoting the global home-services industry, she counts the plumbing firms in the three regions her team can serve, multiplies by a realistic yearly subscription, and calls that her SAM. Her SOM comes from capacity: how many demos two salespeople can run each month and what share of them close.
This kind of sizing your market exercise takes an afternoon, not a week, and you walk into the meeting able to defend each line. For a second take on the same problem, this founder-focused guide to TAM, SAM and SOM is a useful companion.
Common questions
Do I still need a TAM? Yes, but keep it short and sourced. Its job is to show that the category has room to grow.
What if my market does not exist yet? Then size it by analogy: look at the spending your customers already make on the problem today, even with clumsy workarounds, and build from there.
How often should I revisit sizing your market? Whenever your pricing, target customer or sales channel changes, and at least before every fundraise.
How precise should I be? Round numbers are fine. Show your assumptions and ranges rather than false accuracy.
Wrapping up the slide
The three circles are not decoration, and sizing your market is not a box-ticking exercise. Used honestly, they are a compact argument: the category is large enough, our segment is reachable, and here is the share we can win. Build from real customers, show your working, and keep the story proportionate.keep the story proportionate. Numbers are only half of a pitch, so it helps to learn how the story and the figures support each other. In a cautious funding climate, believable beats big.
