Top-Down Approach
Start with a large, credible market number from a third-party report and work down to your segment. This approach is fast, uses authoritative sources, and is intuitive for investors — but it can mask weak assumptions about what fraction of the market you can actually reach.
Example — B2B SaaS HR tool for Indian SMBs:
- NASSCOM reports India's HRtech market at ₹8,500 Cr in 2024, growing at 22% CAGR
- Your product targets companies with 50–500 employees → MCA21 shows ~180,000 such companies registered in India
- Filter to companies with structured HR departments (estimated 40% based on CRISIL MSME survey) → 72,000 potential accounts
- Average ACV you can charge: ₹1.2L per year → SAM = 72,000 × ₹1.2L = ₹864 Cr
Bottom-Up Approach
Build the market size from the ground up using your own unit economics and a count of addressable customers. This is harder to fake, more credible to sophisticated investors, and directly ties market sizing to your GTM model.
Same example — Bottom-Up:
- Your current ACV: ₹1.2L. Based on pricing research, this is acceptable to companies with 50–500 headcount in Tier 1 and metro Tier 2 cities.
- LinkedIn Sales Navigator query: "HR Manager OR CHRO, company size 50–500, India" → 68,000 matching companies
- Apply 35% filter for companies already using a competing solution they are unlikely to switch from → 44,200 genuinely addressable accounts
- At a 15% annual win rate with your current GTM capacity (2 AEs at ₹1.5 Cr quota each) → realistic SOM Year 1: ₹3 Cr ARR
- SAM = 44,200 × ₹1.2L = ₹530 Cr
Which to use in your investor deck?
- Use top-down to establish credibility of the TAM with a third-party citation
- Use bottom-up to validate SAM and SOM — this is what sophisticated VCs actually scrutinise
- Show both in the same slide: "Top-down puts our SAM at ₹864 Cr; our bottom-up model from 44,200 addressable accounts confirms ₹530 Cr SAM — we use the conservative bottom-up figure in all our projections."
- Never rely on top-down alone — it signals that the founder has not done the hard work of understanding their actual customer universe