An industry report puts the market at $400 million, but when you count the customers yourself, you get closer to $70 million. Both numbers can't be right for the decision in front of you, and market sizing is how you work out which one to trust and why.
TL;DR: Market sizing estimates how much customers spend on a product or service in a given market and period. There are two main methods:
- Top-down starts from a total market figure and narrows it to the part that matters.
- Bottom-up counts the potential customers and multiplies by what each one spends.
Good market sizing uses both, explains why they differ and ends with a range you can defend rather than a single precise number.
What is market sizing?
Market sizing is the process of estimating the total value or volume of a market. In practice, that means working out how much all customers in a defined market spend each year on a particular kind of product or service, or how many units they buy.
It answers questions that sit behind most big business decisions. Investors use it to judge how much room a company has to grow. Strategy teams use it to decide which markets to enter, and founders use it to show the opportunity is worth funding. In due diligence, it tests whether a business plan's growth assumptions are realistic.
The result, the market size, is usually expressed in one of two ways:
- By value, as total annual spending, for example "$120 million a year".
- By volume, as the number of units, customers or transactions, for example "12,000 clinics".
The number only means something once the market is clearly defined. "The dental software market" could include imaging tools, billing systems and patient apps, while "practice management software for independent dental clinics" is far more specific. The narrower and clearer the definition, the more useful the result.
Is market size the same as revenue?
No. Market size is what all customers in a market spend, while a company's revenue is the part of that spending it captures. A company's revenue divided by the market size gives its market share.
This difference matters when reading a business plan. A company with $10 million in revenue in a $50 million market already holds 20% of it, so further growth has to come from taking share, raising prices or expanding into new markets. The same company in a $500 million market has much more room to grow, as long as that larger market is one it can actually serve.
TAM, SAM and SOM explained
Market sizing often uses three layers to separate the whole market from the part a company can realistically win:
| Layer | Stands for | What it means | Example |
|---|---|---|---|
| TAM | Total addressable market | Everything customers spend on this kind of product | All dental clinic software spending in the country |
| SAM | Serviceable addressable market | The part the company's offer and reach can serve | Practice management software for independent clinics in the regions it covers |
| SOM | Serviceable obtainable market | The share it can realistically win in the next few years | 10% of SAM over five years, based on its sales capacity and competition |
TAM is the headline number, but SAM and SOM are usually more useful for decisions. A large TAM with a small SAM often means the company is counting customers it can't actually reach.
Top-down market sizing
Top-down sizing starts with a large, published figure and narrows it step by step to the market you care about.
How it works:
- Find a total market figure from an industry report, government statistics or a trade association.
- Apply filters to narrow it down, such as product type, customer segment and region.
- Multiply through the filters to reach your market.
Example: a report says dental clinics in a country spend $400 million a year on software. Practice management software makes up about half of that, which gives $200 million. Independent clinics account for about 60% of spending, which brings the estimate to $120 million.
When to use it: top-down is quick and useful early on, when you need a rough sense of scale. It also shows how your market fits into the wider industry.
Where it goes wrong: the result is only as good as the starting figure and the filters. Published totals often define the market more broadly than you need, and the filter percentages are frequently guesses. Small errors in each filter multiply together.
Bottom-up market sizing
Bottom-up sizing builds the market from the ground up, starting with the customers.
How it works:
- Count the potential customers in the market.
- Estimate how much each one spends, or how many units each one buys, in a year.
- Multiply the two, adjusting for different customer types if they spend very differently.
Example: the same country has about 20,000 dental clinics, and around 60% of them are independent, which gives 12,000 clinics. If a typical independent clinic pays $6,000 a year for practice management software, the market is $72 million.
When to use it: bottom-up is better when you need a number to act on, such as a sales plan or an investment decision. It is easier to check, because every input can be tested against real data like customer counts, pricing and interviews.
Where it goes wrong: it can miss customer groups you didn't think to count, and average spend per customer is easy to get wrong. It also takes longer, because each input needs its own evidence.
Top-down vs bottom-up: which should you use?
| Top-down | Bottom-up | |
|---|---|---|
| Starts from | A published total market figure | A count of customers and their spend |
| Speed | Fast | Slower |
| Best for | Early sense of scale, context | Plans, forecasts and investment decisions |
| How credible | Depends heavily on the source | Easier to check and defend |
The best answer is usually to use both. In the dental example, top-down gives $120 million and bottom-up gives $72 million. That gap is the most useful part of the exercise, because explaining it tells you what the market really is.
Here, the report's total probably includes imaging software and services that independent clinics don't buy as part of practice management. Some clinics also use free or very basic tools, which lowers real spend per clinic. After adjusting for both, a defensible estimate might be $75–90 million, with the reasons written down next to it.
How to size a market, step by step
A simple framework keeps the work focused and makes the result easier to defend.
Step 1: Define the market precisely
Write down exactly what you are sizing: the product or service, the customer type, the geography, the time period and whether you are measuring value or volume. Most market sizing disagreements turn out to be disagreements about this definition.
Step 2: Build a market sizing tree
Break the estimate into a tree of smaller parts that multiply together, such as the number of clinics, the share that are independent and the spend per clinic. A tree shows which inputs the result depends on and makes it easy for others to follow your logic. It works the same way as a hypothesis tree, turning one big question into smaller ones you can answer.
Step 3: Gather evidence for each input
Find a source for every branch of the tree. Where no source exists, make a clear assumption and label it as one, so readers know which numbers are estimates.
Step 4: Calculate and compare
Work out both a top-down and a bottom-up estimate where you can, then compare them. If the two are far apart, find out why before relying on either one. If data is limited and you can only do one, use the method with the stronger inputs and note the weakness of the other.
Step 5: Test the assumptions
Change each key input up and down to see how much it moves the result. This is called sensitivity analysis, and it shows which assumptions matter most. Those are the ones to check hardest, for example through customer interviews.
Step 6: Present a range, with the reasoning
Give a range rather than a falsely precise single number. Show the tree, the main assumptions and their sources, so anyone reviewing it can see how you got there.
Where to find market sizing data
Market sizing research usually draws on a mix of sources:
- Industry and analyst reports, for top-down totals and growth rates.
- Government statistics, such as business counts, census data and trade figures, for counting customers. In the US, the Census Bureau's County Business Patterns data counts businesses by industry and location.
- Trade associations and industry bodies, which often publish member counts and surveys.
- Public company filings, which show what competitors earn and how they describe their markets.
- Pricing pages and sales data, for realistic spend per customer.
- Customer interviews and surveys, for checking adoption rates and spending.
- Expert calls, for a reality check on assumptions you can't verify from published data.
Wherever you can, note the year, region and definition behind each number. Mixing figures from different years or regions is one of the easiest ways to end up with a wrong result.
Common market sizing mistakes
- Defining the market too broadly, which makes it look bigger than the part a company can serve.
- Using only one method, so there's nothing to check the result against.
- Double counting, for example counting a clinic chain and each of its clinics as separate customers.
- Mixing value and volume, or mixing currencies and years, in the same calculation.
- Treating TAM as the opportunity when the company can only reach a small part of it.
- Hiding assumptions, so nobody can tell which numbers are facts and which are guesses.
- False precision, such as presenting $87.4 million when the inputs only support a range.
Market sizing in consulting and due diligence
In consulting, market sizing is a core skill for strategy work, from market entry decisions to growth plans. In commercial due diligence, it plays a specific role: testing whether the market a business plan claims matches the market the company can actually serve.
Plans often quote an impressive total market while the company sells to a narrow segment of it. Sizing the served market from the bottom up, and comparing it with the plan, is one of the fastest ways to find out whether forecast growth is realistic. Our guide to commercial due diligence shows where market sizing fits in the wider process.
Where AI helps with market sizing
AI is useful at several points in the work:
- Building the first version of the tree, by suggesting the inputs a market depends on.
- Finding and summarising sources for each input, such as reports, statistics and filings.
- Running the numbers in a spreadsheet, including the sensitivity analysis.
- Challenging the result by asking what would make the estimate wrong.
The main risk is that AI can produce confident numbers with no real source behind them. Ask for a source next to every figure, and treat any number without one as an assumption to check. Our guide to market research with AI covers how to turn AI-assisted research into recommendations you can defend.
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Frequently asked questions
What is market sizing in simple terms?
It is working out how much customers spend on a particular product or service in a particular market each year.
Is market size the same as revenue?
No. Market size is what all customers in the market spend. A company's revenue is the share of that spending it captures.
What is the difference between top-down and bottom-up market sizing?
Top-down starts from a total market figure and narrows it down. Bottom-up counts customers and multiplies by what each one spends. Using both and comparing the results gives a more reliable estimate.
What are TAM, SAM and SOM?
TAM is the total spending in the market, SAM is the part a company can serve and SOM is the share it can realistically win in the next few years.
What is a market sizing tree?
It is a diagram that breaks a market estimate into smaller inputs that multiply together, such as the number of customers, the share that buy and the spend per customer. It makes the logic easy to follow and check.
What is market sizing in consulting?
In consulting, market sizing is used to judge the size of an opportunity for decisions like entering a new market, planning growth or testing a business plan during due diligence.