Best tools to estimate market size for a B2B SaaS idea, with a worked example

Most market-sizing advice points to company-list databases, web-traffic estimators and analyst subscriptions, then hands you a formula with placeholder numbers. A formula is not the hard part. The hard part is the price buyers already pay, the complaint volume that shows they want something better and the rate at which businesses in the vertical change hands. That gap is what this page closes.

For the full TAM, SAM and SOM method, read how to calculate market size for a startup. This page is the tool roundup plus one worked SaaS example; we do not repeat the method there. If you want to run the arithmetic yourself, use the market size calculator.

BigIdeasDB is #1 of the 7 approaches below. The ranking test is one question: does the tool help you set a defensible number for every input of a bottom-up model, not just the count? The Census and BLS files are the best free sources for the count and sit second and third. The rest are honest general-purpose helpers, each useful at one step.

Top-down TAM starts from an industry total and applies a percentage. A solo founder cannot audit the total, cannot defend the percentage, and still does not know how many accounts will pay. A bottom-up number replaces both guesses with counts. For the formulas and a longer critique, see the market size guide; here is the short version from the people doing it.

“imo the hardest part isnt the calculation itself, its getting honest about how narrow your actual ICP is. most founders inflate TAM because a smaller number feels scary when you're pitching investors” – r/B2BSaaS

“count the accounts you could actually name, not the analyst report. pull a list of companies that match your buyer from a directory or a job board, then multiply that count by what one account pays a year. if it comes out small, the market is small, however good the top-down slide looks” – r/B2BSaaS

The second comment is the whole method in three sentences. The tools below are ways to get the count, the price and the proof that accounts are not happy where they are.

A bottom-up SaaS market size needs four inputs: how many businesses exist, what they already pay, how many are unhappy, and how many are changing hands. The first is a Census file. The other three are what BigIdeasDB measures in one place, which is why it leads the ranking.

For US HVAC and plumbing contractors with 5+ employees, the serviceable market is about $4.2M a year at a 25% buying share and $30 a month. The steps below show where each number comes from, so you can swap in your own vertical.

Step 1. Count the businesses (Census)

The Census Bureau's County Business Patterns lists 111,207 establishments in NAICS 238220 (plumbing, heating and air-conditioning contractors) in 2023, employing 1,214,761 people. We cut the file by size class. 64,871 have under 5 employees. That leaves 46,336 with 5+ employees, the businesses large enough to run dispatch, quotes and invoicing in software.

Step 2. Set the price (TrustMRR price points)

The price input should be what buyers pay today, not what you hope to charge. Across 790+ B2B startups with revenue above zero and 5+ subscriptions, monthly revenue per subscription is $14 at the 25th percentile, $30 at the median and $75 at the 75th. B2C startups sit at $7 (n=1,700+). Read the full distribution in SaaS revenue benchmarks by category and the TrustMRR revenue data. These are indie-priced products, so a vertical tool with real workflow depth can charge more. Treat $75 as the optimistic case, not the base case.

Step 3. Set the buying share (the one input you must test)

No database tells you the share of contractors that will buy a new tool in your category. Set three scenarios, 10%, 25% and 40%, and let interviews narrow them. The evidence that a share exists at all is dissatisfaction. HVAC categories on Capterra show 5.0% of 3,200+ reviews at 1-2 stars, 1.5x the 3.3% median across 850+ categories. Buyers are paying and a visible minority are unhappy.

Step 4. Multiply

SAM equals establishments with 5+ employees (46,336) x buying share x monthly price x 12. The table shows annual SAM in each scenario.

The base case (25% share, $30 a month) is 11,584 accounts and $4.2M a year. A realistic first-three-years SOM is 2% of those accounts: 232 customers and about $83,000 a year. That number is the honest answer, and it is a good one for a solo founder: a profitable product, no venture round needed. If you need $1M a year, the model says you need either a higher price or a wider vertical.

Step 5. Cross-check with complaints, density and exits

Three checks turn a calculation into a decision. Density: the Stripe Index lists 39 B2B software companies matching HVAC or field service and 7 are micro-SaaS, so the space is thin on small specialists. Check the Stripe Index for your own vertical. Exits: the Main Street Index holds 590+ US HVAC businesses for sale with a median ask of $498,250 and a median SDE of $229,584 at about 5 employees. That is roughly 1.3 listings per 100 establishments with 5+ staff, a rough ratio because the Census base includes plumbing. Of 330+ HVAC listings that state a reason, 62.1% say retirement. Each sale is a new owner re-choosing software. Pain: the top systemic complaints in Capterra's field-service category are weak customer support, no lead tracking and scheduling errors. Our field service software complaints report has the reviews.

The Software Gap Score marks HVAC as underserved, 50.0 of 100, with 712 listings against the software supply. That is the gap-scored version of the same finding. For the surrounding market logic, see the vertical SaaS gap report and boring industries begging for micro SaaS.

Yes, and it gives a different answer. Census counts 163,894 real estate agent and broker establishments (NAICS 531210) in 2023, but 151,101 of them have under 5 employees, leaving 12,793 with 5+. The buyer pool is much smaller than the headline count suggests, because most agents are solo and the Census count excludes the self-employed.

The data lines up with what reviewers say. Capterra's Real Estate Agency category shows 6.5% of 1,100+ reviews at 1-2 stars, 2.0x the median, the worst of the five categories we checked. TrustMRR's Real Estate startups earn a median of $132 MRR across 45 companies and $21 per subscription (n=25, below the n=30 floor, so directional only). The Stripe Index lists 220+ real estate companies with 13 micro-SaaS. At the base-case 25% buying share and a $30 monthly price, 12,793 establishments give about 3,200 accounts and a $1.2M annual market. Smaller than HVAC, with louder complaints. See real estate transaction software complaints.

One card per tool, in rank order, with the fact that decides where it sits.

The same thread names the gap between an eligible account and a reachable one. A commenter writing from inside a target company described it:

“I'd just expect a verified count of technically eligible accounts to overstate the reachable market by a lot, with the gap being mostly timing. Which is fine if the team treats the list as a sequencing problem rather than a forecast.” – r/B2BSaaS

That is why the buying share is a scenario, not a fact, and why the Main Street Index exit data matters: a sale is the moment an owner is most open to changing tools. For revenue reality checks on the price input, see the best SaaS research tools.

A bottom-up market size misleads when the buying share is guessed, the price is aspirational, or the count includes businesses that will never buy. Four limits to hold in mind.

• Establishments are not firms. A contractor with three locations counts three times in Census, but buys software once. Divide when you can.
• Price is not a constant. $30 is an indie median. A tool that replaces a $300 a month incumbent can charge far more; a tool bolted onto a free one cannot.
• Asking is not selling. The 590+ HVAC listings are asking prices, and the 62.1% retirement figure is the sellers' stated reason.
• Market size is not demand. Complaints show demand exists, not that your wedge wins. Pair this page with the saturation tools roundup.

The Small Business Administration's market research guide makes the same point in plainer terms: size the customers you can name, not an industry abstraction.

Every number on this page comes from a live query or file run on November 14, 2026, or from the source named. BigIdeasDB corpus counts are rounded and carry a plus sign; Census figures are quoted exactly as published.

More ranked guides in this series: best tools to find a SaaS niche. Vertical and buyer guides: best tools to research a field service software idea and best tools to research a fintech software idea.

Market size FAQ

Sizing a specific vertical? Scan it in BigIdeasDB or read the full TAM, SAM and SOM method.