Debt Payoff Calculator

The monthly interest rate r is the annual rate divided by 12. Months to payoff uses the closed-form amortisation formula: n = −ln(1 − r × P ÷ PMT) ÷ ln(1 + r), where P is the current balance and PMT is the monthly payment. Math.ceil rounds up to the next whole month. Total interest is then computed by simulating each month step-by-step: interest this month = remaining balance × r; new balance = old balance + interest − payment. This simulation handles the smaller final payment correctly — if the balance goes negative in the last month, the excess interest is subtracted back. If the monthly payment is less than or equal to r × P (the monthly interest alone), the debt can never be paid off and an error is shown.