What is market size realism and why do founders overestimate TAM?
Short Answer
Founders often claim 10% of a £10B market (£1B TAM) but can realistically capture <1%. Honest TAM estimates focus on year-5 realistic revenue potential.
Full Explanation
Total addressable market (TAM) is the revenue opportunity if you capture 100% of a market. Founders rightfully point to large TAM—but capturing market share is exponentially harder than TAM size suggests. Reality check: a £1B TAM doesn't mean you'll do £1B revenue. Even dominant players (Slack, Figma) have captured single-digit percentages of their TAM. Honest TAM thinking: 1) Segment the market (not all £1B is addressable to you). 2) Estimate realistic market share (for year 5, what's realistic—1%, 5%, 10%?). 3) Model revenue from realistic share, not from TAM. Example: email marketing platform. TAM might be £5B (all businesses sending marketing emails). Your segment: mid-market B2B SaaS (£500M). Realistic share (year 5): 2%. Revenue (year 5): £10M. This is honest. Claiming £5B TAM and £50M revenue (1% of TAM) is unrealistic. Investors are sceptical of massive TAM claims because they signal market naivety. They want to see: evidence of real demand in your specific segment, realistic market share assumptions, and clear path to £10-50M revenue (VC cheques). Opagio's questionnaire helps companies calculate realistic TAM and segment-specific revenue potential, avoiding the overestimation trap.
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