Loan Total Interest Calculator

Total interest paid is the true cost of borrowing — and it often shocks borrowers to see how much interest accumulates over the life of a long-term loan. A $300,000 mortgage at 6.5% over 30 years generates over $380,000 in interest, nearly doubling the original loan amount. This calculator reveals that full picture: monthly payment, total interest, total repaid, and interest as a percentage of principal. It is invaluable when comparing loan offers with different rates and terms, deciding whether to choose a 15-year or 30-year mortgage, or evaluating the cost of carrying high-rate consumer debt. Works for any fixed-rate installment loan including mortgages, auto loans, student loans, and personal loans.

The monthly payment is computed using the standard amortization formula: M = P × r(1 + r)^n / [(1 + r)^n − 1], where P is the principal, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments (term in years × 12). Total amount paid = M × n. Total interest = total paid − original principal. The interest-to-principal ratio = total interest ÷ principal × 100, which illustrates how expensive longer terms or higher rates are relative to the amount borrowed. This calculation assumes all payments are made on schedule with no extra payments or early payoff.