Top-down Market Sizing
A market sizing method that starts from the total size of an industry and narrows it, filter by filter, to the revenue your company could realistically capture.
Updated September 2026
Definition
Top-down market sizing begins with a macro number, usually the size of an industry as reported by an analyst firm or a statistics office, and applies a chain of percentages to it: which geographies you serve, which segment you sell to, which use case your product covers, and what share of that you can plausibly win. Each filter shrinks the number until you arrive at your serviceable addressable market and, one step further, your obtainable market.
It is the fast way to size a market and the right way to show its ceiling. It is also the method most often abused on pitch decks, which is why investors read a top-down number with one question in mind: where did each percentage come from?
The top-down formula
Your market = Total industry size × Geography % × Segment % × Use-case % × Achievable share %
- • Total industry size is your TAM, from a cited report.
- • Geography, segment and use case filters give you your SAM.
- • Achievable share, over a stated time horizon, gives you your SOM.
Every percentage needs a source or an explicit assumption. “Conservatively, 1 percent” is not an assumption; it is a guess with an adjective.
How to do a top-down market sizing
1. Pick the right starting number
Choose the narrowest industry figure that still contains your whole market. A B2B payroll tool should start from "HR software", not "enterprise software". A wider starting point means more filters, and every filter is a place to be wrong.
2. Filter by geography
Restrict to the countries you can sell into in the period you are sizing for. Use the report's own regional split where it exists; otherwise use GDP or company-count ratios and say so.
3. Filter by segment
Company size, vertical, or customer type. Statistics offices publish counts of firms by size band in most countries, which makes this the easiest filter to source.
4. Filter by use case
The share of the segment's spend that goes to the problem you solve. This is where most sizings become fiction. Anchor it to a line item in a budget, a comparable product category, or a survey you can cite.
5. Apply an achievable share and a horizon
State what share you can win in three to five years and why: competitor market shares, your sales capacity, or a comparable company's trajectory. Then check the result against a bottom-up build.
Worked example: a project management tool for construction SMBs
| Step | Filter | Source or assumption | Result |
|---|---|---|---|
| TAM | Global project management software | Analyst report, 2025 | $7.5B |
| Geography | Europe, 28% | Same report, regional split | $2.1B |
| Segment | Construction firms, 9% | Eurostat share of firms by sector | $190M |
| Use case | SMBs with 10 to 250 staff, 60% | Eurostat firm-size bands, weighted by seats | $114M (SAM) |
| Share | 8% in five years | Comparable vertical SaaS reached 6 to 10% | $9M (SOM) |
The number investors will underwrite is $9M, not $7.5B. The $7.5B belongs on the slide too, because it shows the room to expand beyond construction and beyond Europe once the wedge is won.
Top-down vs bottom-up market sizing
| Top-down | Bottom-up | |
|---|---|---|
| Starts from | Total industry size | One customer, one price |
| Answers | How big could this get? | How much can we actually sell? |
| Time to build | An afternoon | Days, needs pricing and reach data |
| Typical failure | Unsourced percentages, inflated TAM | Optimistic conversion and pricing |
| What investors use it for | Sanity check on the ceiling | The number in the model |
Do both. If the top-down SOM and the bottom-up total land within a factor of two of each other, you have a market slide that survives questions. If they differ by ten times, one of your assumptions is wrong, and it is better to find out before the meeting.
Common mistakes on the market slide
- Starting too wide. “The global software market is $700B” tells an investor nothing about a payroll tool for dentists.
- Percentages without sources. Each filter should be traceable to a report, a dataset, or a stated assumption you can defend.
- Confusing spend with revenue. Industry reports often measure total spend including services and hardware. Your product captures software licence revenue only.
- Showing only the TAM. Investors want the wedge and the ceiling. One without the other reads as either unambitious or unserious.
- Stale data. Cite the report year. A 2019 figure on a 2026 slide is a red flag in fast-moving categories.
Frequently asked questions
What is top-down market sizing?
Top-down market sizing starts from the total size of an industry, usually taken from a research report or government statistics, and narrows it with a chain of percentages (geography, segment, use case, realistic share) until you reach the revenue your company could plausibly capture. It answers "how big is the pond" before you count the fish.
What is the top-down market sizing formula?
Your market = total industry size × geography % × segment % × use-case % × achievable share %. Each percentage should have a source or a stated assumption. The product of the first three filters is usually your SAM; multiplying by achievable share gives your SOM.
What is the difference between top-down and bottom-up market sizing?
Top-down starts with the whole industry and filters down. Bottom-up starts with one customer, multiplies the price they pay by the number of customers you can realistically reach, and builds up. Top-down is faster and shows scale; bottom-up is more defensible and shows execution. Investors expect both, and they expect the two numbers to be in the same ballpark.
Which market sizing method do investors prefer?
Bottom-up, for the number they underwrite. Top-down, for context on how big the opportunity could become. A deck that shows only a top-down number ("1 percent of a $50B market") is the most common market-slide mistake, because it says nothing about how you get the first 1 percent.
Where do I find data for a top-down market sizing?
Industry analysts (Gartner, IDC, Forrester, Statista, IBISWorld, Euromonitor), government statistics offices and census data, trade associations, public-company annual reports and investor presentations, and sizing figures quoted in competitors' fundraising announcements. Cite the source and the year on the slide.