How a card minimum is commonly figured
Card issuers use different formulas, and the exact one is in the cardholder agreement. One common version is 1% of the balance plus that month's interest, with a floor such as $25 when that total is small. The calculator uses that formula by default, and every piece of it can be edited. Some issuers use a flat percentage of the balance instead. That version shrinks too, for the same reason.
On a $5,000 balance at 24% APR, the month's interest is about $100. Add 1% of the balance, $50, and the minimum is about $150.
Why a shrinking minimum stretches things out
Under that formula, only 1% of the balance goes to principal each month. As the balance falls, the minimum falls with it. Once that same card is down to $2,500, the minimum is about $75, and only $25 of it reduces the balance.
So the payment keeps getting smaller right along with the balance. The balance drops quickly in dollars at first, then more and more slowly. That long tail is where most of the years come from.
Near the end, 1% of the balance plus interest drops below the floor, and the floor becomes the minimum. From there, the payment stops shrinking and the balance clears faster. A higher floor ends the tail sooner; a lower one lets it run longer. Changing the floor in the calculator shows how much it matters for a given balance.
The fixed-payment comparison
The second result keeps paying today's minimum, the same dollar amount, every month until the card is gone. Interest still shrinks as the balance falls, but the payment does not, so a growing share of each payment goes to principal.
The calculator shows the years and total interest for both versions, plus the difference between them. On many balances that gap is large, because the fixed payment never gets pulled into the long tail.
Where the interest piles up
In the shrinking-minimum version, the balance spends many months at fairly low levels, and each of those months adds a small interest charge. None of them looks large on its own. Added together over years, they can come to a large share of the original balance.
The fixed payment passes through those low balances much faster, so far fewer of those small charges ever happen.
Reading the results
The difference line is the cost, in years and dollars, of the payment falling along with the balance. Both results assume no new purchases and a fixed APR, with interest charged monthly at the APR divided by 12. They are estimates, not a statement. The credit card payoff calculator tests any fixed payment or target month, the credit card interest calculator shows the daily and monthly charge, and the extra payment calculator shows what extra on top of minimums does across several debts.