THE NUMORIX GUIDE
How to use the Credit Card Calculator
Last reviewed September 14, 2026
What this calculator does
The engine converts APR to a monthly rate, then advances the balance month by month.
Formula and method
The engine converts APR to a monthly rate, then advances the balance month by month. Fixed-payment mode uses the entered dollar payment; percent-of-balance mode uses the greater of the entered balance percentage and monthly interest plus $1. Timeframe mode solves a fixed payment with the standard amortization formula. Payments are capped so the final row does not overpay the balance plus that month's interest.
Variables and inputs
Enter credit-card balance, APR, payment type, fixed monthly payment or payment percentage, and optionally a payoff timeframe in years and months. Balance and payment are dollars; APR and percentage-of-balance are percentages; timeframe is monthly.
Worked example
For a $5,000 balance at 20% APR with a $200 fixed payment, monthly interest in month 1 is 5,000 x (0.20 / 12) = $83.33. Principal paid is 200 - 83.33 = $116.67, leaving about $4,883.33 before any new charges or fees.
How to interpret the result
The result shows an estimated payoff month count, total paid, total interest, and optional schedule rows. A payment that barely exceeds interest can create a very long payoff. The result assumes no new purchases, fees, or changes in APR unless those effects are reflected in the entered numbers.
Common mistakes to avoid
Use the card's purchase APR rather than a promotional rate that has already expired. Do not count a minimum payment formula as a fixed dollar payment for every month. Add new charges separately; this engine assumes the balance only declines.
Assumptions and limitations
The model uses monthly APR/12, ignores grace periods, daily average balance, fees, penalty APRs, new spending, and issuer minimum-payment rules. Percent-of-balance mode includes a hardcoded extra $1 floor, so a real card's minimum can differ.