SOM is the share of the market you can realistically capture in the near term, and the number your forecast should actually be built on. Here is how to size it.
Serviceable obtainable market (SOM) is the portion of your serviceable market that you can realistically capture in the near term, given competition, resources, and reach.
SOM is the grounded number. It takes your SAM and asks what share you can actually win in the next year or few, accounting for competitors, your sales capacity, and your go-to-market reach. It is the smallest of the three figures and the one a sober forecast is built on, sitting below TAM and SAM.
If your SAM is $144M and you can credibly win 3 percent of it in the next two years given your team and competition, SOM = 144,000,000 x 0.03 = $4.3M. That is the number to plan headcount and pipeline against, not the TAM.
SOM is where ambition meets reality. It is the number that should drive hiring, targets, and investor forecasts, because it reflects what you can actually capture, not what exists in theory. Founders who plan against TAM overspend; founders who plan against SOM build a model that holds up. It flows directly from your GTM strategy and capacity.