Top-down market sizing
In short
Top-down market sizing is a method that starts with a broad market figure, such as total industry spend, and applies filters to reach the part relevant to your product. It works from the whole toward your slice.
It is quick and useful for a first estimate, but it depends on the quality of the starting number and on assumptions about each filter. Bottom-up sizing builds the number from account counts and prices instead.
How the method works
1. Find a starting figure
Use a credible report, public filings, or government data for the broad category. Record the source and year.
2. Narrow by relevant filters
Apply geography, segment, company size, and use case in turn. Each step should have a stated reason.
3. Apply a share assumption
Estimate the part of that segment you could serve, based on product scope and channels.
Worked example
Say a team starts with a hypothetical software category worth 10 billion dollars a year. Only the North American portion is relevant, at 40 percent, giving 4 billion. Mid-sized companies are 25 percent of that, giving 1 billion. The product covers one of two use cases, so the serviceable market is about 500 million. The numbers are invented for illustration; real inputs need sources.
Top-down vs. bottom-up
- Top-down: Fast, uses existing research, but is sensitive to assumptions and may overstate the market.
- Bottom-up: Counts real accounts and multiplies by price. Slower, but easier to defend and turn into target lists.
- Best practice: Run both. If the results are far apart, one of the assumptions is wrong.
Common mistakes
- Using a vague starting number: A category that does not match your product inflates the result.
- Stacking untested percentages: Each guess compounds error.
- Stopping at the total: A market figure does not tell you which accounts to call.
Top-down market sizing FAQs
What is top-down market sizing?
Top-down market sizing starts with a large figure, such as total industry spend, and narrows it with filters like region, company size, and use case to estimate the portion your product could serve. It is fast but depends heavily on assumptions.
What is the difference between top-down and bottom-up market sizing?
Top-down starts from a broad market total and narrows it. Bottom-up counts the actual accounts you could sell to and multiplies by expected price. Bottom-up is usually more defensible and yields lists; top-down is quicker for a first estimate.
When should you use top-down market sizing?
Use it for early estimates, investor or planning conversations, and sanity checks when account-level data is not yet available. Pair it with a bottom-up calculation before making headcount or territory decisions.
What are the weaknesses of top-down market sizing?
It relies on third-party totals that may not match your category, and each percentage filter is an assumption. Errors compound across steps, and the result tells you nothing about which specific accounts to target.
