The formula
How to calculate credit card interest
Credit card interest is calculated daily on the balance outstanding, then charged monthly. Because the APR is already a compounded annual figure, the daily rate is its 365th root rather than a simple division.
Most cards charge interest from the transaction date on cash advances and from the statement date on purchases, and clearing the statement balance in full each month usually avoids purchase interest entirely.
The inputs, one by one:
- Balance
- Purchase APR (%)
- Days in the billing period (days)
Everything recalculates as you type, and the numbers in the address bar update with it, so a link to this page carries your figures with it.
Some of the fields above will accept figures that seem unusual for your own situation, and that is deliberate: the formula behind credit card interest works the same way regardless of scale, so the calculator does not stop you testing a hypothetical scenario a long way from your actual numbers — often the fastest way to see which input the result is most sensitive to.
Why credit card interest matters
The formula behind credit card interest is standard and has not changed in decades; what changes is the situation it gets applied to. Two households can run the identical calculation and land on very different conclusions once their own numbers — income, rate, term, balance — are dropped in, which is why a generic textbook example is less useful than a calculator you can adjust to match your own circumstances.
Beyond a one-off check, the same calculation is worth revisiting whenever the underlying numbers change — a new interest rate, a change in income, a different term. Because the figures live in the page's own web address, coming back to update just one field and compare the new result against the old one takes seconds rather than starting again from a blank page.
It is also worth being clear about what a single figure like this can and cannot settle on its own. It answers the specific question the formula was built to answer, and nothing more — a favourable credit card interest result does not automatically mean a decision is a good one overall, since plenty of other factors that a formula cannot capture, from personal circumstances to how comfortable a commitment feels, usually matter just as much as the arithmetic. Use the number as one solid input among several rather than the whole of the decision.
Where the same calculation needs to be run for several different scenarios side by side — three loan offers, two savings plans — the fastest approach is usually to open the calculator in a second browser tab for each one, so that the results can be compared directly rather than overwriting each other in a single set of fields.
Worked example
A concrete run-through, using the values already in the fields:
- Balance: 2,500
- Purchase APR: 24.9 %
- Days in the billing period: 30 days
That gives:
- Interest this period: 45.7
- Interest per day: 1.52
- Interest over a year if unpaid: 622.5
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
The daily figure makes the cost tangible. A £2,500 balance at 24.9% costs about £1.53 a day — around £46 a month for nothing at all.
Where this goes wrong. Losing the interest-free period on purchases. Carrying any balance forward typically means new purchases start accruing interest immediately, so the effective cost of the next month's spending jumps.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
Because interest compounds daily and the APR already reflects that compounding. The monthly figure on a 24.9% card works out slightly above 2.075%, and any fees charged in the period add to it.
It reduces it proportionally but does not stop it. Only clearing the full statement balance by the due date restores the interest-free period on new purchases.
The answer it gives you is interest this period. With 2,500 balance, 24.9 % purchase APR and 30 days days in the billing period, that comes to 45.7. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.