Market sizing
In short
Market sizing is the process of estimating how many buyers exist in a market and how much revenue they could generate for you. The result is usually expressed as TAM, SAM, and SOM.
GTM teams use it to decide which segments deserve investment, how many reps to hire, and how many accounts each rep should cover. A rough, honest estimate beats a precise number built on guesses.
Two ways to size a market
Use both methods and compare the results. A large gap means one set of assumptions is wrong.
Bottom-up
Count real companies that match your ICP and multiply by expected annual contract value. Say a team finds 8,000 matching companies and expects $12,000 per year from each. That gives roughly $96 million. This is the more credible method for B2B because it starts from real accounts.
Top-down
Start from an industry-wide figure and narrow it with filters such as region, size, or use case. It is quick but depends on the quality of the source and on how arbitrary your percentages are.
How to size a market step by step
- Define the buyer: Write down the ICP so you know what counts as a potential customer.
- Count the accounts: Use firmographic filters on a company database to get a real number.
- Set a realistic deal value: Base it on current average contract value, not on list price or hope.
- Narrow to reachable and winnable: Remove regions, segments, or sizes you cannot serve to get SAM, then estimate the share you can win for SOM.
- Write down assumptions: Each number should be traceable so it can be revisited when facts change.
What teams use it for
- Segment choice: Compare the size of two segments before committing a team to either.
- Capacity planning: Divide accounts by what one rep can work to estimate headcount.
- Targets: Check that a revenue goal does not require an unrealistic share of the market.
Common market sizing mistakes
- Treating TAM as a forecast: TAM is a ceiling. Most of it is not reachable in the near term.
- Using one big percentage: Saying "we only need 1% of a huge market" hides whether the 1% is actually winnable.
- Ignoring who can actually buy: Counting companies that lack budget, tools, or the problem inflates the number.
- Never updating: Markets shift. Refresh the count at least yearly.
Market sizing FAQs
What is market sizing in business?
Market sizing is estimating the number of potential customers and the revenue available in a market. It helps teams decide whether a market is worth entering and how to divide effort between segments. Results are typically expressed as total, serviceable, and obtainable market.
How do you do market sizing for a B2B product?
Define your ideal customer, count the companies that match it using a company database, and multiply by your expected annual contract value. Then narrow the total to the accounts you can serve and realistically win. Cross-check with a top-down estimate.
How do top-down and bottom-up approaches to market sizing differ?
Top-down starts with a large industry figure and narrows it by assumptions. Bottom-up counts actual target accounts and multiplies by deal value. Bottom-up is usually more credible in B2B because each input can be checked against real companies.
How accurate does market sizing need to be?
It needs to be accurate enough to support the decision in front of you, such as choosing between two segments. Directional answers with clear assumptions are more useful than false precision. Revisit the estimate whenever key assumptions change.
