How fixed-rate amortization works
A level principal-and-interest payment is calculated from the starting balance, monthly interest rate, and number of scheduled months. Each month, interest is the outstanding balance multiplied by the monthly rate. The remainder of the payment reduces principal.
Extra payments change time and interest
The entered extra amount is added after calculating the scheduled payment and is applied to principal in this simplified model. Reducing balance earlier usually shortens payoff time and reduces later interest. The last payment is limited to the remaining principal plus that month’s interest.
What the annual view shows
Monthly rows are grouped into years for the visible table. The principal-progress bar compares the ending balance with the original principal. The downloadable CSV retains every modeled month, including payment, principal, interest, and ending balance, so the arithmetic can be inspected.
What is not included
The model excludes property tax, homeowners insurance, mortgage insurance, association fees, closing costs, points, escrow changes, late fees, prepayment rules, variable rates, daily interest, and rounding conventions used by a particular servicer. The interest rate entered is treated as a nominal annual rate divided by 12, not APR.
Compare with actual loan documents
Lenders may round monthly values, accrue interest on different schedules, or apply extra payments according to account instructions. Confirm that an extra amount is designated for principal and that no prepayment restriction applies. Use the simpler Loan Payment Calculator when only the standard payment and totals are needed.
Choose the next mortgage question separately
Use the Mortgage Calculator when property tax, insurance, HOA dues, and mortgage insurance belong in the monthly housing cost. Use the Mortgage Recast Calculator when a lender may lower the required payment after a lump-sum principal reduction, or the Mortgage Refinance Calculator when replacing the loan changes rate, term, costs, and balance.
This is an educational estimate, not mortgage, lending, investment, tax, legal, or financial advice. Verify rates, fees, payment application, required escrow, contractual terms, and an official lender schedule before making decisions.