A card is not a loan with instalments. The minimum payment is a percentage of what you owe, so it shrinks every month as the balance drops — and the smaller it gets, the more of it is interest. That is why a balance that looks like it should clear in a couple of years takes fifteen, and why the total paid can be double what was borrowed.

Using it in three steps

1. 1 Enter the balance and the annual rate — both are on your statement.
2. 2 Choose a fixed monthly payment, or switch to minimum-only to see what the card issuer's own schedule costs.
3. 3 Compare the interest figure between the two. That difference is what paying a bit more each month is worth.

The payment that changes everything

Every month, interest is charged on the balance first, and only what is left of your payment reduces the debt. On a 5,000 balance at 24%, the interest alone is 100 in the first month — so a 120 payment removes 20 of the debt and a 220 payment removes 120, six times as much. This is why raising the payment by half often cuts the payoff time by three quarters: the extra money goes entirely against the principal.

When the tool says it never clears

If the payment is smaller than one month's interest, the balance grows even though you paid. There is no number of months that clears it, so the tool refuses to invent one and tells you the interest instead — that figure is the floor your payment has to beat before any of it touches the debt. Card issuers set the minimum above that line, but not by much, which is exactly how a small balance survives for a decade.