Interest, then payment
Each month, we multiply the balance by the annual rate divided by 12. We add that interest, subtract your regular payment, then subtract any extra. The rate stays fixed and there is no new borrowing.
A quick example
At 12% APR, a 1,000 balance adds 10 in interest in month one. A payment of 100 leaves 910. With zero interest, the whole payment reduces the balance.
Dates and rounding
The first payment is one month after the start date. Short months use their last valid day. Interest rounds half-up to 0.01 each month in every currency, including JPY. The last payment is reduced to exactly what is owed. Currency changes never convert amounts.
Where lenders may differ
Daily interest, fees, changing rates, promotional periods, and payment timing can change the result. This is not a payoff quote. Inputs are limited to 1 billion per amount, 0–100% APR, and 1,200 months. The target mode verifies its payment with the same monthly schedule.