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There is a slide that gets more founders torn apart than any other. It is the market slide, the one with a giant number and the words “if we capture just 1% of this market.” Every investor has seen a thousand of them, and they stop believing the number the moment they see it.
Market sizing done right does the opposite. It signals that you understand your market well enough to be honest about it. Here is how to build that.
What TAM, SAM, and SOM actually mean
They are three concentric circles, from the whole world down to what you can realistically win.
TAM, the Total Addressable Market, is the total demand if every possible buyer bought from someone. It is the size of the whole pond.
SAM, the Serviceable Addressable Market, is the slice of TAM your product and business model can actually serve, given your geography, segment, and channel. It is the part of the pond you can fish in.
SOM, the Serviceable Obtainable Market, is the share of SAM you can realistically capture in a defined period, given your resources and competition. It is the fish you can actually catch.
The mistake is stopping at TAM. A big TAM tells an investor the opportunity exists. SAM and SOM tell them you understand your real constraints, which is what they are actually buying.
The two ways to calculate it, and why you need both
Top-down starts from a published market figure and narrows it: total market, then your segment, then your reachable share. It is quick, but it is only as trustworthy as the report you started from, and it tends to flatter.
Bottom-up builds from your own unit economics: number of potential customers, times what each pays, times how many you can realistically reach. It takes more work, but it is far more defensible because every input is something you can point to.
The strongest market slides do both and show where the two numbers land. When a top-down estimate and a bottom-up estimate roughly agree, that convergence is the credibility. When they diverge wildly, you have found something you need to understand before an investor finds it for you.
The mistakes that get a market slide rejected
Three patterns get flagged instantly. Sizing off a single top-down report with no source anyone can check. The “1% of a huge market” claim, which tells an investor nothing about whether you can actually win those customers. And a number with no sources at all, which reads as a guess no matter how confident the font.
How investors evaluate market size
Investors are not looking for the biggest number. They are looking for a number that is triangulated (built more than one way), sourced (traceable to real research), and realistic (a SOM that matches your stage and resources). A defensible mid-sized market beats an enormous unsourced one every time, because the enormous one signals you have not done the work.
Where this is heading: sourced beats impressive
Investor diligence is getting more data-driven, not less. Around 82% of firms now use AI for deal-sourcing research, which means your market claim is increasingly checked against independent data before you even get to a second meeting. Diligence that once took a week can now stretch to one or two months of deeper scrutiny for the companies investors take seriously. EqvistaQubit Capital
The direction of travel is clear: as it gets cheaper for investors to verify a market number, an unsourced one becomes a liability rather than an aspiration. The market slide that wins is the one where every figure names its source and the methods agree.
FAQ
Got Questions? We’ve Got Answers
How do I calculate TAM, SAM, and SOM?
Size TAM as the whole market, SAM as the part your model can serve, and SOM as the share you can realistically win. Calculate each top-down and bottom-up, and show where they agree.
What is the difference between TAM, SAM, and SOM?
TAM is the entire market, SAM is your serviceable segment, and SOM is the slice you can capture in a set period. Each is a narrower, more realistic circle than the last.
How do investors evaluate market size?
They look for a triangulated, sourced, realistic number rather than a single top-down guess, and they increasingly verify it against their own data.
Getting a market number that names its sources is exactly what Sizoru is built for: TAM, SAM, and SOM calculated both ways, with most figures cited to Tier 1 and Tier 2 research, in about three minutes.
