Mortgage overpayment calculator

Enter your balance, rate and remaining term, a monthly overpayment and a lump sum. You get the same loan run three ways, month by month, with the months and interest each strategy saves. Same logic as the Bindler workbook, verified against it.

The loan
Overpayments
Interest is charged monthly at the annual rate over twelve on the opening balance. The contractual payment stays the same and the term shortens, which is how most lenders treat overpayments unless you ask for the payment to be recalculated. Check your overpayment allowance and any early repayment charge.
The workbook: all three schedules, month by monthMortgage Overpayment Comparison, $12: three 480-month schedules side by side, the comparison sheet, interest saved per unit of extra, and a Guide. Verified through a formula engine before listing.
See the workbook

Method: each month, interest on the opening balance at the annual rate over twelve, the contractual payment (never more than balance plus interest), then the extra; the closing balance carries forward. Scenario B adds a fixed extra every month, C a lump sum in the chosen month. Overpayment allowances, early repayment charges, rate changes at the end of a fixed period and offset accounts are outside this page. Same logic as the Bindler workbook. Arithmetic on your own numbers, not financial advice.