Do the numbers even work? The first honest revenue maths to do before you build
Let's say you've done the hard part. You've spoken to real people, you've seen demand that cost them something, and the idea has survived contact with reality. Now you're itching to build, and you've earned that feeling, because most founders never get this far.
But there's one question you've probably not asked out loud yet, and it's the one that quietly decides whether all of this becomes a business or an expensive hobby. Can the numbers even work? Not "will people want it", you may already have good evidence for that, but "once they do, is what's left after costs actually enough to pay me?"
I spent my career on the capital side, first across three of the UK's largest corporate banks and then advising founders through their raises. On that side of the table you learn to do one unglamorous thing before you let yourself fall for any idea: a rough sum on whether the economics can hold. It takes about ten minutes on the back of an envelope, and it can save you a year.
When CB Insights went through 431 venture-backed companies that shut down from 2023 onwards, "unsustainable unit economics", which is a tidy phrase for the maths never adding up, was a primary cause in 19% of the failures it could categorise (CB Insights, The top 9 reasons startups fail, published March 2026). Sit with that for a second. These were companies with money in the bank, some of them millions, and close to one in five still died because the sum underneath the business didn't work. You get to do that sum first, for free, before you've built a thing.
Why founders skip the maths
Almost nobody does this early, and the reasons are understandable. It feels premature, because pricing is surely a "later" problem. The spreadsheet feels made up, because every number in it is a guess. And underneath both of those there's usually a quieter fear that if you look too hard, the answer might be no.
So let me take the pressure off. The point of early maths isn't to be accurate. It's to find out whether the idea has any chance of clearing the bar, using numbers rough enough to work out in your head. You're not forecasting the future. You're checking whether the shape of the thing makes sense before you pour months of your life into it.
The only sum you need to start
Forget the five-tab model. At the very start, a business is one line: revenue is your price multiplied by the number of customers, and profit is whatever is left after what it costs to serve them and keep the lights on. That's the whole machine. To see if it can work, you only need three honest numbers.
The three numbers that decide it
Here they are, and not one of them needs a spreadsheet.
Your price. What one customer pays you, and how often. A one-off £300 and £30 a month are very different businesses, even when they sound similar.
Your cost to serve one customer. What it actually costs you to deliver to one person: the hosting, the fees, the materials, your time on support. Not your fixed overheads yet, just the cost that comes with each extra customer.
The customers you can realistically win. Not the size of the market. The honest number you can reach and convert through the channels you actually have, this year.
The gap between your price and your cost to serve is your contribution per customer, the part of each sale that's left over to pay for everything else and, eventually, you. Multiply that by the customers you can realistically win. If the result can't cover your fixed costs and pay you something you can live on, the idea doesn't work yet at those numbers. Far better to learn that on an envelope than in your bank account.
Work it backwards from the number that matters
Guessing your revenue is hard. Working backwards from what you need is easy, and far more honest. Start with the number that would make this worth doing. Say you want it to earn you £3,000 a month to begin with.
Now the price does the talking. If you charge £30 a month and keep £25 after costs, you need 120 paying customers, and you need to keep them. If you charge a one-off £300 and keep £250, you need to make twelve sales a month, every month, from a standing start. Suddenly "get customers" has a concrete shape, and you can ask the only question that matters: which of those can I actually do, through the channels I actually have?
This is also where pricing stops being an afterthought. Watch what a low price does to the same target. At £5 a month you'd need 600 customers for that same £3,000. The number of customers is usually the hardest thing in the whole business to move. Your price is often the easiest. Founders reach for "I'll just get more customers" when "I'll charge a bit more" is the lever sitting right in front of them.
The costs that quietly eat your margin
The most common mistake I see is modelling the price and forgetting the leaks. A few that catch people out, roughly in order of how often they're missed:
Payment fees. Card processing takes a small slice of every single sale. Small, but it comes off the top of everything.
Refunds and failed payments. Some of the money you've already counted never actually lands.
The cost of winning each customer. Ads, offers, and above all your own time count, even when no cash visibly changes hands.
Your time to deliver and support. Doing it yourself feels free, right up until it quietly eats a full working week.
Churn, if you sell a subscription. Customers leave, so you're forever refilling a leaky bucket just to stand still.
VAT, further down the road. Once your taxable turnover passes £90,000 in any rolling twelve months you have to register for VAT (the threshold is held at £90,000 for the 2025/26 and 2026/27 tax years, gov.uk). If you sell to consumers you often can't simply add 20% on top, so it comes out of your margin instead. You may be nowhere near that yet, but it's worth knowing the number exists. This is information, not tax advice.
Run it pessimistic, then decide
Here's the one capital-side habit worth stealing for your own idea. Once you have your honest numbers, make them worse on purpose. Halve the customers. Nudge the costs up. Assume some of them leave. Does the business still clear the bar you set? If it only works when everything goes right, it doesn't really work, because in the early days very little goes exactly right. If it still stands up when you're being pessimistic, you have something genuinely worth building.
If the numbers don't work yet
Say you run it and it doesn't add up. That's not a dead end, and it's certainly not a reason to reach straight for "I'll just sell more", because customers are the hardest number to move. Look at the easier levers first:
Raise the price, and test whether your demand holds at a number that actually works.
Cut the cost to serve, by automating the expensive part or removing the hand-holding.
Change who you sell to. One business paying £300 can be worth more, and easier to find, than sixty consumers paying a fiver.
Change the model itself, from one-off to recurring, or from fully self-serve to a smaller number of higher-value customers.
And sometimes the honest answer is that you have a lovely product that's not yet a good business at these numbers. Finding that out on the back of an envelope, rather than a year into building, isn't a failure. It's one of the most valuable things the maths can hand you.
Do this before you build, not after
The revenue maths isn't there to talk you out of your idea. It's there to show you the real shape of the business you'd be signing up to run, while that shape still costs nothing to change. Do it now, roughly, honestly, and in your own handwriting if you like. The founders who struggle are rarely the ones whose sums came out wrong. They're the ones who never did the sum at all.
Before you build a thing
If you'd rather not wrestle a spreadsheet to do this, working out your numbers is one of the things the Launchology AI Co-Founder is built for. Tell it your price, your rough costs and who you're selling to, and it will help you put together a simple, honest revenue picture and then pressure-test it the way someone on the capital side would, before you commit months to building. It's free to start, and it's built for founders rather than guessing at your business. Try the AI Co-Founder.