Market size reality check: how big does your market need to be?

‍"It's a massive market."

Four words that appear in roughly 99% of pitch decks, usually on the slide with an enormous number pulled from a Statista report and very little explanation of what it actually means. And look, we get it - you're excited, the opportunity feels huge, and someone told you that investors want to see big numbers. They do. But not like this.

Here's the thing most founders get wrong about market size: it's not about finding the biggest number you can defensibly put on a slide. It's about understanding whether enough people will pay enough money for your specific thing to make the business you want to build actually work.

That's a different question entirely. And a much more useful one.

You don't need a billion-dollar market

This might sound like heresy if you've been consuming a steady diet of startup Twitter and Y Combinator content, but hear us out. The market size you need depends entirely on the kind of business you're building.

A VC-backed startup chasing a £100M+ exit? Yes, you probably need a large and growing addressable market - investors need to believe the ceiling is high enough to generate outsized returns. But a bootstrapped SaaS product that could comfortably make you £1M a year in recurring revenue? You need a much smaller slice of a much smaller pie, and that's perfectly fine.

The problem is that founders often treat market sizing as a one-size-fits-all exercise. It isn't. Your goals shape the maths, and the maths should shape your strategy.

TAM, SAM, SOM - without the jargon

You've probably seen these acronyms floating around. They get thrown about in pitch meetings like everyone should just know what they mean, so let's break them down plainly.

1. TAM (Total Addressable Market) is the entire revenue opportunity if you somehow captured every single customer in your space globally. It's a ceiling, not a target. Think of it as the theoretical maximum if you had zero competition and infinite resources - which you don't, obviously.
2. SAM (Serviceable Addressable Market) is the portion of that TAM you could realistically reach with your current business model, geography, and product. If your TAM is everyone who buys coffee, your SAM might be independent coffee shops in the UK who buy beans from specialist roasters.
3. SOM (Serviceable Obtainable Market) is the bit you can actually capture in the near term. This is the number that matters most when you're starting out, and it's the one founders spend the least time thinking about. Your SOM is your real, honest, first-few-years market - and if this number doesn't work, neither does your business.

The question isn't "how big is the market?" It's "how much of this market can I realistically win, and is that enough?"

A worked example (because this stuff needs one)

Say you're building a scheduling tool for independent personal trainers in the UK. Your research tells you there are roughly 40,000 of them. That's your SAM - not every PT in the world, just the ones your product is designed for and your sales efforts can reach.

Now, you're charging £30 a month. If you captured 5% of that market in year three - which would be a solid result - that's 2,000 customers paying £360 a year, giving you £720,000 in annual recurring revenue.

Is that a good business? For a lean, bootstrapped startup with two founders and low overheads, that could be a brilliant business. For a VC-backed company that's raised £2M and needs to show a path to ten times that? Probably not.

Neither answer is wrong. But you need to know which game you're playing before you can tell whether the numbers work.

So what should you actually do?

Rather than starting with the biggest number Google will give you and working backwards (the classic top-down approach that makes investors' eyes glaze over), try this:

1. Start from the bottom up. How many potential customers can you actually identify? Not theoretically - actually. Can you name them, find them, reach them?
2. Be honest about conversion. What percentage will realistically buy? Not in your optimistic scenario - in your "things are going okay but not amazingly" scenario. For most early-stage products, a 1-3% conversion rate from awareness to paying customer is a reasonable starting assumption.
3. Do the maths on your unit economics. What will each customer pay, and what does it cost you to acquire and serve them? If the gap between those numbers is thin or non-existent, a bigger market won't save you. It'll just mean you lose money at scale.
4. Pressure-test the growth ceiling. Even if year one works, is there room to grow? Can you expand into adjacent segments, move upmarket, or add products? A small SOM is fine if the SAM gives you somewhere to go.

Context is everything

We've worked with founders who had a clear path to £500K in revenue with a niche product and a well-defined audience - and that was exactly the right business for them. We've also worked with founders who needed to demonstrate a route to £50M+ because their model demanded it. Both required sharp thinking about market size, but the conversations looked completely different.

The danger isn't in having a small market. It's in not knowing your market is small - or worse, convincing yourself it's large based on a TAM number that has almost nothing to do with your actual business.

"But what if an investor asks me about market size and my numbers aren't huge?" Then you tell them you've done the work. You explain your SOM, show your assumptions, and articulate why the opportunity is compelling at the scale you're targeting. That's infinitely more impressive than a £4.7 billion TAM slide with no explanation of how you'd capture any of it.

Know your numbers, know your game

Market sizing isn't a box-ticking exercise for your pitch deck. It's one of the earliest and most important reality checks you can do as a founder. It tells you whether your idea can sustain a business, what kind of business that might be, and where the real risks and opportunities sit.

You don't need a billion-dollar market. But you do need a real one - and you need to be honest about what slice of it is actually yours to win.

Not sure whether your market stacks up? Book a free 30-minute discovery call with one of Rise's founders. No pitch required - just bring your idea and your questions. You'll walk away with a clearer picture of whether the numbers work, even if you decide we're not the right fit.