Market sizing has a reputation problem. It sounds like something that requires a consulting firm, a subscription database, and a slide deck with a lot of blue.
For most small businesses, it requires arithmetic and about two hours. The goal is not precision. The goal is to find out whether the number is plausibly big enough to bother, and whether your revenue assumptions are inside the realm of physics.
Start from the bottom, not the top
The classic mistake is top-down: the industry is worth 40 billion dollars, and if we capture just one percent of it, we make 400 million.
Nobody has ever captured one percent of a large market by deciding to. Top-down sizing produces a number that feels enormous and tells you nothing about whether you can sell to eleven people next month.
Bottom-up works differently. You start from a unit you can actually observe and multiply upward.
The arithmetic
You need four numbers.
- How many of your specific buyers exist in the area you can serve. Not everyone in the category — the narrowed buyer from your blueprint.
- What percentage of them are plausibly reachable through the one channel you named.
- What percentage of reachable people would plausibly buy in a year. Be pessimistic. Low single digits is normal for cold audiences.
- Your price.
Multiply. The result is a rough annual revenue ceiling for your current plan.
The number itself matters less than what happens when you look at it. Usually one of two things: either it is large enough that the plan is worth running, or it is so small that no amount of execution fixes it, which means the price, the buyer, or the channel has to change. Both outcomes are useful. Only one of them is discoverable by working harder.
Where to find the inputs for free
- Census and Bureau of Labor Statistics data for counts of businesses or households by geography and category.
- Industry association membership numbers, which are often published and are a decent proxy for how many serious operators exist.
- Search volume tools, including the free tier of most keyword planners, to see how many people are actively looking for the thing.
- Marketplace and directory counts — how many providers are listed in your category on the platforms your buyers use, and how many reviews the busy ones have.
- Your competitors' public signals: headcount on LinkedIn, number of reviews, posting frequency. A competitor with four hundred reviews is telling you the market supports at least four hundred transactions.
The competitor count is the most honest signal
Founders treat competitors as bad news. They are usually the best free market research available.
A category with no competitors is more often a category with no customers than a category with no competition. A category with several competitors who have been operating for years and appear to be busy is telling you that money changes hands here reliably.
What you want to look for is not absence of competition but a seam — a buyer everyone is serving poorly, a price point nobody occupies, a geography that is underserved, a delivery method that is stuck in 2011.
Do the two hours
Write your four numbers down with the source next to each one. Note which is the shakiest. That is the assumption to test first once you start selling, because it is the one most likely to move the whole estimate.
This is not a document you show anyone. It is a sanity check that costs you an afternoon and occasionally saves you a year.
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Tomorrow: your first financial projection will be wrong — build it anyway.