Because the minimum is a percentage of the balance, it shrinks as the balance shrinks — the payment glides down and the payoff horizon stretches out. On the default card, minimums-only means about 18 years and $9,341 of interest on an $8,000 debt. Lenders are required to show this on statements for exactly this reason.

Any amount above the interest line goes straight at principal, and the effect compounds: on the default inputs, raising the payment from $250 to $350 saves over a year of payments and hundreds in interest. A fixed payment that never glides down — the way this calculator models it — is itself the trick most people miss.

Then the balance grows instead of shrinking, forever. The calculator flags it and shows the minimum payment that makes progress — on $8,000 at 19.99% the interest alone runs about $133 a month, so anything below that is treading water at best.

Any fixed-rate balance you can pay freely — cards, lines of credit, personal loans. It assumes no new spending on the account; add purchases and every number gets worse, which is an argument for moving the balance somewhere you can't tap it.