Most marketing plans open with an invented number: "the market is big". That number helps with no decision at all, because it never tells you what share of it you could plausibly win.
Three different numbers
TAM is the total market if everyone were your customer. SAM is the portion your product can actually serve. SOM is what you can win in a set period on your current budget.
Where the numbers come from
Official sources such as the statistics authority, sector reports, and published financials. Every figure needs a link you can open — not an estimate off the top of your head.
A sign the number is wrong
If your SOM is more than 5% of your SAM in year one, it's probably inflated. Revisit the assumptions.
Any figure in your plan without a source you can open is an assumption, not information.
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