How the payoff estimate works
Each month the model adds interest and any new purchases before applying the fixed payment. It switches from a promotional APR to the regular APR after the entered duration.
A payment that does not cover interest and new purchases cannot reduce the balance, so the calculator reports the problem instead of showing a misleading payoff date.
Worked example
A $5,000 balance at 22% APR with a $250 fixed payment and no new purchases is simulated month by month. The final payment is limited to the remaining balance plus that month's interest.