Credit card interest calculator

How this calculator works

The reason a credit card behaves so differently from a loan is that the minimum payment is a moving target. It's set as a percentage of what you owe, so as the balance falls, the payment falls with it - you pay a little less each month, the balance drops a little slower, and the whole thing stretches out for decades. This calculator recalculates that minimum every single month rather than treating it as a fixed figure, which is what makes the result so much starker than most free calculators show.

We use the standard UK rule: the minimum is the greater of a percentage of the balance plus that month's interest, or a £5 cash floor. Since 2011 the minimum has had to at least cover interest and charges, so a compliant card's balance can never actually grow while you pay it - it just clears agonisingly slowly, and the tail end is paid off £5 at a time.

Alongside it we run the same balance with a payment that never changes. The gap between the two is the entire lesson of the page, and the most useful version of it costs you nothing extra: simply keep paying today's minimum every month instead of letting it shrink. You pay no more this month than you already were, and typically cut decades off the payoff.

Interest is charged monthly at the rate that reproduces your card's APR properly - the twelfth root of the annual rate, since a UK APR is an effective annual figure that already includes compounding. Dividing the APR by twelve, as many calculators do, overstates the monthly charge. We assume no new spending on the card, no fees beyond interest, and no promotional 0% period; adding purchases while repaying will push every figure here out considerably.