Top-down vs bottom-up: why it matters
"The global SaaS market is $250B" is a top-down TAM. It tells investors nothing about your actual opportunity because you're not selling to all of SaaS. A bottom-up TAM starts from your specific customer: "There are 40,000 Series A–C CFOs in the US who manage treasury manually. We charge $12K/year. That's a $480M addressable market." That's credible. Investors can verify it. Top-down is a shortcut that signals you haven't done the work.
Inconsistent TAM across slides
This is the most common AI Consistency Checker finding in pitch decks on CAPLINK. Slide 4 says "$42B market." Slide 11 appendix says "$38B." The financial model implies a 0.1% market share that equates to $200M revenue by 2027 — which doesn't match either number. Investors notice. Pick one number, derive it bottom-up, use it consistently everywhere.
TAM too small — the other direction
Overcorrecting into false modesty is just as damaging. A $30M TAM with no clear expansion path tells institutional investors the company can't return a fund. If your initial beachhead is small, show the expansion logic: adjacent markets, geographic rollout, product extensions. The SAM/SOM framework exists precisely for this — start small, show the path to large.
Unrealistic market share assumptions
"We'll capture 10% of the market in year 3" is a statement that exits the room immediately. No company with a three-year history has captured 10% of a multi-billion market. Even 1% of a $1B market is $10M ARR — already an exceptional outcome. Be specific about your target segment and realistic about penetration rates. Investors model this themselves; if your numbers don't survive their model, the meeting ends.
How to build a market size slide investors believe
Start with your ideal customer profile. Count how many of them exist. Multiply by your annual contract value. That's your SAM. Then show your 3-year path to capturing a credible slice of it. Source your numbers (Statista, industry reports, your own primary research). One slide, three numbers (TAM / SAM / SOM), one sentence of methodology each. Clean, simple, defensible.