Sizing a market investors will actually believe
TAM, SAM, SOM is not the point. The point is a bottom-up number a partner can rebuild on the back of a napkin and still find defensible.
Every deck has a market slide. Most of them do the same thing: cite a Gartner or Statista figure in the tens of billions, draw three nested circles labeled TAM, SAM, SOM, and move on. A partner reading that slide sees one of two things. Either the founder does not understand their market yet, or the founder is hoping the number will do the arguing.
The market slide is not asking how large the universe is. It is asking how large the wedge you can actually reach is, and whether that wedge is big enough that a good outcome for you is a good outcome for the fund.
Top-down numbers are guilty until proven innocent
A top-down TAM ("the global HR software market is $81B") is easy to source and easy to dismiss. Investors have seen the same three research firms' numbers hundreds of times. What they cannot dismiss is a bottom-up number built from units they can count.
The strong version does three things the weak one does not: it names a countable universe, prices it in your own units, and puts the huge top-down number in second position where it belongs, as context rather than as the argument.
The bottom-up formula
- Count the customers: number of legal entities, sites, seats, transactions, or vehicles that fit your ICP. Cite the source.
- Multiply by price: your own price per unit per year, not an industry average.
- Discount for reachability: what fraction can you actually sell to in the first 5 years, given channel and geography.
- State the wedge: this is your SOM, and it should be at least 10x your Series A revenue target.
Three failure modes
1. The trillion-dollar TAM
Any number with a T in it triggers a reflex: the founder is telling me the size of the ocean, not the size of the boat. The larger the top-down figure, the more skeptical the reader becomes. A believable $400M wedge beats an unbelievable $400B TAM every time.
2. The imaginary adjacency
Founders often add adjacent markets to inflate the number: "scheduling plus payroll plus benefits plus tips." Unless you have a credible product plan for each, the additions read as a wish list. Show the wedge you are actually selling this year and put the adjacencies on a labeled roadmap.
3. The penetration assumption nobody stated
"If we capture just 1% of the market" is the phrase every partner has heard 500 times. 1% of a huge market is still an enormous business, and no one has ever gotten 1% by accident. State the penetration you can defend: how many customers per year, through which channels, with what CAC.
What the slide should look like
- A single sentence in the title with the wedge number in your own units and price.
- One diagram: three nested numbers with the wedge highlighted, not three abstract circles labeled TAM/SAM/SOM.
- One footnote with the source of the unit count.
- One line on the adjacent market you will move into after the wedge, with the year.
The problem slide is where most decks lose the meeting. The fix is not more empathy, it is a specific person, a specific cost, and a specific reason it exists today.