How to Estimate Market Size for Your SaaS Idea (Without a Research Team)

The first time a VC asked me for the TAM, SAM, and SOM of NeedSonar, I made up a number that sounded big. The number was 4.2 billion dollars. I had no idea where it came from. It was a placeholder dressed as analysis. Most indie founders do the same thing, and it is a waste of time because the answer does not actually help you decide whether to build. What helps is a market-size estimate that is honest, small enough to be defensible, and grounded in a number you can actually verify. This post walks through four methods, with a worked example for each. None of them requires a research team. All of them can be done in an afternoon.

The underlying principle is convergence. No single method will give you a number you can trust. Four methods that produce answers within 2x of each other will. If one method gives you a wildly different answer, you have a problem with the inputs to that method, not a problem with the size of the market. The point of doing all four is not to be thorough. It is to find the one that is wrong and fix it before you build.

Method 1: Top-Down (TAM / SAM / SOM)

The top-down method starts with a global number and narrows. The global number usually comes from a Gartner or Statista report. For SaaS, a common starting point is "global SaaS market" at roughly $200B in 2026, growing to $300B by 2028. You then narrow. The narrowing is where most of the error lives. Every narrowing step is an assumption, and the assumptions compound.

SAM is the slice of that market you can address. For a vertical SaaS in dental practice management, the SAM might be "dental practice software in the US," which is around $1.8B. SOM is the slice you can realistically capture in three years. For a solo founder, that is usually 0.1% to 0.5% of SAM, so $1.8M to $9M in revenue. The 0.1% to 0.5% assumption is itself a guess. A solo founder with no distribution might capture 0.01%. A solo founder with strong distribution and a viral wedge might capture 1%. The range is wide.

The method is fast and produces impressive numbers. The accuracy is low because the global figure is usually not relevant to your niche, and the narrowing is full of assumptions. Use it to communicate with investors who speak this language, not to make your own decisions. The moment you find yourself defending a $4.2B number in a pitch, you have spent the credibility you needed for the rest of the deck.

Worked Example: Dental Practice Software

| Layer | Definition | Estimated Size |

|---|---|---|

| TAM | Global dental software | $4.5B |

| SAM | US dental practice software | $1.8B |

| SOM | Capture in 3 years at 0.3% | $5.4M |

The SOM in the example is the number an investor will accept. It is also the number you should treat as your realistic upside, and the question to ask is whether $5.4M ARR is a business you want to build. For most indie founders, the answer is yes. For a venture-backed team that needs $50M ARR, the answer is no. The method tells you whether your ambition and the market are aligned.

Method 2: Bottom-Up (Competitor Revenue Stacking)

The bottom-up method starts with competitors and adds their revenue. If there are 3 to 5 incumbents in your niche, find each one's estimated revenue. For private companies, this means looking at LinkedIn headcount, Glassdoor revenue estimates, Crunchbase totals, and public customer counts. Add the totals to get the niche size. The math is simple. The estimation is hard, and the estimation is the whole game.

The advantage is that the number is grounded in real companies. The disadvantage is that you have to estimate private revenue, and estimates are noisy. The accuracy is moderate, and the answer is usually smaller than top-down suggests, which is a feature, not a bug. The bottom-up number is the one you can defend in a room, because every component has a source.

The sources to use, in order of reliability, are: SEC filings for public companies, official revenue disclosures on company websites, LinkedIn headcount multiplied by industry average revenue per employee, Glassdoor revenue estimates (with a discount for optimism), and Crunchbase total funding raised multiplied by an industry average multiple. The multiple varies by stage and by niche, but for SaaS it is usually between 3x and 8x annual revenue. A company that raised $30M and has been around for 5 years is probably at $5M to $10M ARR, not $30M.

Worked Example: Solo Founder CRMs

Suppose you are building a CRM for solo founders. You identify 5 competitors. Their estimated revenues are $4M, $2.5M, $1.8M, $0.9M, and $0.6M. The stacked total is $9.8M. If you can capture 5% of that in three years, your SOM is $490k ARR, or about $41k MRR. That is a real number you can build a plan around. The 5% capture rate is itself a guess, but it is a guess that is grounded in the observed behavior of similar niches.

"We estimated the niche at $12M ARR by stacking four competitors. We are now at $180k ARR after 14 months, which is 1.5% of the stack. The estimate was right." - u/metricsfirst on r/SaaS

A useful sanity check is to look at the smallest competitor. If the smallest competitor has $0.6M ARR and 200 customers, the average revenue per customer is $3,000/year, or $250/month. If your product is similar in scope, your pricing is constrained by the same number. If you want to charge 2x, you need a 2x better product. The math is unforgiving.

Method 3: Value-Based (WTP × Potential Users)

The value-based method starts with a willingness-to-pay assumption and a user count. You estimate the number of potential users (e.g., 50,000 solo founders in the US), multiply by an assumed monthly WTP (e.g., $19/mo), and multiply by 12 for ARR. The result is $11.4M ARR as a ceiling. The ceiling is a useful upper bound. The realistic number is much lower, because the assumption is that 100% of the potential users would buy at $19/mo, which never happens.

The accuracy depends entirely on the WTP estimate. If you overestimate WTP, the number is fantasy. If you can ground WTP in a survey or a smoke test, the method is more reliable. Discount the result by 70% to account for the gap between stated and revealed WTP. In the example above, the realistic SOM is around $3.4M ARR. The discount is the most important part of the method, and the part most founders skip.

A second discount to apply is for awareness. Even if 100% of the potential users would buy, you still have to reach them. The realistic awareness for a new product in year 1 is 1% to 5% of the addressable market. Apply that discount on top of the WTP discount and you get a much smaller number. For the example above, 1% awareness of 50,000 users is 500 buyers, and at $19/mo that is $114k ARR. That is the realistic number. The $11.4M is the ceiling.

Worked Example: AI Copywriting Tool for E-commerce

| Input | Estimate | Source |

|---|---|---|

| US e-commerce stores | 2.4M | Census, BigCommerce |

| Solo / SMB segment | 1.2M | 50% assumption |

| WTP per month | $25 | Survey median |

| Annual ceiling | $360M | 1.2M × $25 × 12 |

| Realistic SOM (10% capture) | $36M | 10% × $360M |

| Discounted for stated-vs-revealed | $10.8M | 30% of SOM |

The table walks through the discount layers. The first layer is the capture rate. The second is the WTP discount. A third layer, not shown, would be the awareness discount. With all three, the realistic number for a year 1 is closer to $1M ARR. The point is not to despair. The point is to know what you are actually planning for.

Method 4: Community-Based (Reddit + HN + Discord Subscribers)

The community-based method uses community size as a proxy for market size. The logic is simple: if 80,000 people subscribe to r/SaaS, and 10% of them would buy your tool, your addressable market inside that community is 8,000 buyers. Multiply by your price, and you have a market estimate grounded in observed behavior. The observed behavior is the most valuable part, because it is not a survey. It is a real subscription to a real community.

The method is messy but honest. You start by listing the 3 to 5 communities your buyers live in. You note the subscriber count for each. You apply a conversion rate (1% to 5% is realistic for cold traffic from a community). You sum. The result is usually 20% to 50% of the bottom-up estimate, which is a useful reality check. The number is conservative, which is what you want at the planning stage.

This is the method NeedSonar is built to support. The platform crawls those same communities, scores the pain intensity in each, and gives you a per-community addressable estimate that you can use in your SOM calculation. It is not a substitute for revenue stacking, but it is the fastest way to ground a number in observed community behavior. The output is a number you can use in a deck without blushing, because every input is a real community with a real subscriber count.

Worked Example: Tools for Indie Game Developers

|---|---|---|---|---|---|

| r/gamedev | 1.2M | 1.5% | 18,000 | $19 | $4.1M |

| r/IndieGaming | 380k | 2% | 7,600 | $19 | $1.7M |

| TIGSource forums | 90k | 3% | 2,700 | $19 | $615k |

| Total | | | | | $6.4M |

That $6.4M is the community-based SOM. Compare it to the bottom-up stack of competitor revenue and the two should be within 2x. If they are off by 10x, you have a problem with the niche definition, not the math. The community-based number is the floor. The bottom-up number is the ceiling. The truth is somewhere in between.

A refinement is to weight communities by intent density. r/gamedev has 1.2M subscribers, but only a fraction of them are actively building games. The high-intent subset is probably 5% to 10% of the subscriber count. If you restrict the calculation to that subset, the per-buyer conversion rate is much higher. The total addressable market shrinks, but the conversion rate rises to compensate, and the result is usually a similar number with more confidence.

Which Method Should You Use?

Use all four and look for convergence. If top-down says $50M, bottom-up says $8M, value-based says $12M, and community-based says $6M, the answer is somewhere between $6M and $12M. That is a real number you can defend. The point of market sizing is not to impress an investor. It is to know whether your niche is large enough to support your revenue goal and small enough that you can win it.

For most indie SaaS, the right answer is: a community-based SOM of $3M to $15M ARR, validated by a bottom-up competitor stack within 2x, and a top-down ceiling that is not embarrassingly wrong. Anything more precise is theater. The number that matters is the one that lets you decide whether to start, and the answer is almost always yes if the bottom-up and community-based methods both come back above $3M ARR. Below that, the niche is too small for a solo founder to make a living.

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