How this is calculated
Each month the balance grows by one month of interest (APR ÷ 12), then shrinks by your payment. The simulation runs month by month until the balance hits zero — once with your regular payment, once with the extra added — and compares the two.
new balance = balance × (1 + APR/12) − payment
Why small extras work so hard
Every extra dollar goes entirely to principal, and principal you remove today stops accruing interest for every remaining month. That's why $100 extra on a 22% card often saves multiples of itself.
If the payoff time says "never"
A payment smaller than the first month's interest means the balance grows instead of shrinking. The calculator will warn you and show the minimum payment that makes progress.
Notes
Assumes a fixed APR, no new charges, and no fees. Cards compound daily in practice; monthly compounding is a close, slightly optimistic approximation.