Ask most vendors how they got to the saving figure on their homepage and the honest answer is usually a spreadsheet nobody kept. Our own free AI ROI calculator makes the same kind of estimate, so it deserves the same scrutiny we'd apply to anyone else's. Here is exactly what it multiplies, the constants sitting quietly behind the sliders, and what the arithmetic cannot see no matter how carefully you fill in the boxes.
The four numbers it multiplies
You give the calculator four things: how many people are on the team, how many hours each person loses to repetitive work in an average week, a fully loaded hourly cost for that time, and a share of that work you believe AI could realistically take on, chosen from three preset tiers rather than typed in freehand.
From there the arithmetic is plain multiplication, nothing hidden underneath it. Hours saved in a week is team size times hours lost per person times the automation share. Hours saved in a year is that weekly figure times a fixed working-weeks constant. The pound saving is annual hours times the hourly cost you entered. The full-time-equivalent headcount is annual hours divided by a standard full-time-hours constant. That is the whole model. No compounding multiplier, no productivity ramp, no discount for the months it takes a team to actually trust a new system before leaning on it.
The constants under the bonnet
Two figures never appear in the input boxes, and both are worth naming because they quietly do a lot of work. The first is a working-weeks-per-year constant, with UK bank holidays and standard annual leave already stripped out, used to turn your weekly saving into an annual one. The second is a standard working week length, used only to convert an annual hours figure into a full-time-equivalent count. Both are fixed. They apply the same way to a five-person agency and a hundred-person warehouse, because neither is measured against your business, only assumed for it. A calculator that asked for your actual holiday allowance and your actual contracted hours would need several more fields, and a form nobody finishes is worse than a form that makes one clean assumption and says so.
The automation share works on the same principle. Rather than asking you to guess a percentage out of thin air, the tool offers three labelled tiers. Conservative is what's achievable with off-the-shelf tools and no change to how the team currently works. Typical is what we see from a focused first automation project. Ambitious needs the workflow redesigned around AI rather than AI bolted onto the process as it already exists. Choosing a tier picks the number behind the label for you. You're not estimating a percentage, you're picking a level of ambition and letting the calculator hold the number.
What the tool already says it leaves out
To its credit, the page is upfront about two exclusions, right there in the copy under the sliders. It measures time saved only, so it leaves out revenue upside that's genuinely harder to price: leads answered while they're still warm, invoices chased before they go stale, fewer mistakes needing rework later. And it leaves out implementation cost entirely, on the stated reasoning that for most small businesses that cost is smaller than the annual saving it's weighed against. Both of those are honest, declared omissions, sitting in the FAQ for anyone who reads past the number.
Three things the arithmetic assumes away
None of that is where the model actually gets fragile. The real gaps sit somewhere the formula never looks at all.
Whether anyone keeps using it. The maths assumes the automation share you selected holds steady, every week, for the rest of the year. There's no field for a system that gets built, demonstrated once in a meeting, and then quietly stops being opened because it doesn't fit how the team actually works day to day. That gap, between what a tool is capable of and what a team keeps reaching for, is where most automation projects actually stall, and no multiplication catches it.
What it costs to keep working. The calculator treats the automation share as free to hold onto once you have it, a number set and forgotten. In practice something has to keep it working: a workflow that breaks when a connected app changes its interface without warning, a prompt that needs retuning when the underlying task shifts, an owner who notices when the output has quietly got worse before a customer does. None of that shows up in the four inputs, because there's nowhere in the formula for it to go.
Where the freed hours actually land. Hours saved only turn into pounds saved if someone deliberately redirects that time to something worth the hourly cost you typed in. Left alone, freed time tends to get absorbed back into a slightly slower version of the same day, the inbox still takes as long to clear, meetings still run to their old length. The calculator has no way of knowing what your team would actually do with time back each week. That's a decision your business makes, not a step in the arithmetic.
What the number is actually for
None of that makes the estimate worthless, it just marks it as a sizing tool rather than a business case. A figure the calculator produces tells you whether a topic is worth an hour of proper investigation, not what to put in front of a board or a lender. Treat it the way you'd treat a sharp guess from a colleague who knows your business reasonably well: a reason to look closer, never a reason to stop looking.
That's also the honest limit of a page like this one. We could add fields for adoption, maintenance, and redirected time, but a calculator that asked you to forecast your own team's habits would stop being a two-minute tool and start being a guess dressed up as precision, which is exactly what we're trying not to build.
If you want to see the formula in front of you, open the calculator and put your own team size, hours, and hourly cost through it, the working shown above is exactly what runs behind the sliders. And if the result makes automation worth investigating and you'd rather talk through which process would actually survive adoption, upkeep, and a place to send the freed hours, book the free audit and we'll work through it on the call instead of in a spreadsheet.