Market sizing answers one question: is this opportunity big enough to justify the investment? Investors want to see it. Strategy teams need it for resource allocation. The problem is that most market size estimates are fiction dressed up as analysis.
The typical approach: find a report from Gartner or Grand View Research that says your market is worth $47 billion, put it on slide 3, and move on. This tells you nothing useful. A $47 billion market means nothing if your realistically addressable slice is $5 million.
Top-down starts with the total market and narrows down. Take the global CRM market ($80B), filter by geography (Europe = 25% = $20B), filter by company size (SMB = 40% = $8B), filter by your specific niche (manufacturing SMBs = 5% = $400M). The problem with top-down is that every filter is a guess, and the errors compound.
Bottom-up starts with your unit economics and builds up. Count the number of potential customers you can identify (say 15,000 manufacturing SMBs in Europe), multiply by your average contract value ($12,000/year), and you get a serviceable addressable market of $180M. This is more grounded because it starts from data you can actually verify.
Use both methods. If they produce wildly different numbers, dig into why. The bottom-up number is usually more reliable for planning purposes. The top-down number is useful for fundraising context.
TAM (Total Addressable Market): Everyone who could theoretically buy your product if you had infinite resources and no competitors. This is your "if everything goes right and we dominate the world" number. Useful for investor context, useless for planning.
SAM (Serviceable Addressable Market): The subset of TAM you can actually reach with your current business model, geography, and capabilities. This is your realistic playing field.
SOM (Serviceable Obtainable Market): The subset of SAM you can realistically capture in 2-3 years given your resources, team, and competitive position. This is what your revenue plan should be based on.
Most startups make SOM too large. A good rule of thumb: if you are entering a market with established competitors, your SOM in year one is 0.5-2% of your SAM. If you are creating a new category, the number is higher but the SAM itself is smaller and harder to define.
Five mistakes that lead to meaningless market size estimates:
0.5-2% of your SAM if entering an established market with competitors. Higher if you are creating a new category. If your year-one revenue plan implies capturing more than 5% of your SAM, your SAM is probably too small or your plan is too optimistic.
Count the number of people or companies with the problem you solve, estimate what percentage would pay for a solution, and multiply by your price point. This is inherently uncertain, so present a range rather than a single number.
Use them for context and top-down validation, but do not rely on them exclusively. Their definitions of market boundaries may not match yours. Always supplement with your own bottom-up sizing based on countable potential customers.
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