Can I afford it?
A free calculator. Nothing to sign up for, nothing stored.
Most people answer this by looking at their bank balance, which is the one number guaranteed to be wrong. Your balance still has your rent in it, the bill going out on Friday, and whatever you have promised to a goal. It is a record of what has already happened, not a statement of what you can spend.
Put your own figures in below. The arithmetic is the same rule the Xfin app runs.
Why your bank balance is the wrong number
A balance is a record, not a forecast. It does not know your rent is due on the 28th, that a subscription comes out on Friday, or that £400 of it is your emergency fund. Everything you have already committed is still sitting inside that figure right up until the moment it leaves — which is exactly when it is too late.
The useful number is what remains once the money that is already spoken for is taken out. Cash you hold, plus pay that genuinely lands before the money leaves, minus the bills coming in that window, minus what your goals are holding.
How much should be left after bills?
There is no universal figure, but a practical rule: treat anything under about 15% of your remaining reserve as thin. Below that, one unexpected cost puts you into money you had already assigned somewhere else. That is the threshold this calculator uses to say “fits, but it is thin” rather than simply yes.
What if it goes over?
Then the money comes from somewhere — usually a bill you still owe or a goal you were saving toward. A warning is not much use at that point; a price is. If you tell the calculator what you put toward a goal each month, it converts the overshoot into how much later that goal arrives. A few hundred dollars over can be weeks or months off something you actually wanted.
That is the whole idea behind Xfin: not a rule that tells you no, but an honest account of what a decision costs, before you make it.
Doing this without the arithmetic
This page asks you for the figures. The app works them out: it finds your recurring bills, notices when pay lands, keeps track of what your goals are holding, and keeps the number current on its own. See how it works →
Related: what’s actually left · asking Fin about a purchase · where your data lives