Mortgage Amortization Calculator - Payment Schedule

The mortgage amortization calculator converts a fixed-rate home loan into a monthly principal-and-interest payment, then rolls that payment forward until the balance is gone. Buyers, refinancers, and housing counselors use it to see how much of a loan is interest over 15 or 30 years and how a small extra monthly principal amount changes the payoff date. Enter the loan amount (the principal, not the purchase price), annual interest rate, term in years, and an optional extra monthly payment. The scheduled payment follows the standard annuity formula: P × i / (1 − (1 + i)^(−n)), where i is the monthly rate r/12 and n is the number of months. Each month, interest accrues on the remaining balance and the payment (plus extras) first covers that interest. Leftover dollars reduce principal. Total interest is the sum of those monthly interest charges. Total payment is principal plus total interest. Payoff time is the number of months until the balance falls below one cent, expressed in years. With no extra payment, payoff time matches the original term aside from rounding. Use the mortgage amortization calculator to compare a 15-year loan with a 30-year loan, to test a $100–$300 extra principal habit, and to explain why early payments are mostly interest. It does not build a printable month-by-month table, but the totals match a full amortization schedule that uses the same rules. Taxes, insurance, mortgage insurance, and HOA dues are excluded so the interest cost of the loan is easy to isolate. Assumptions include a fixed rate, monthly compounding, and extras applied every month from the first payment. ARM resets, interest-only periods, and biweekly servicer drafts are out of scope. If the extra payment is so large that interest cannot be covered—an unusual case—the form returns an error. Recalculate after a refinance or recast, and treat the result as a planning estimate rather than a servicer payoff quote, which can include per-diem interest and fees.