Mortgage Refinance Calculator
Compare two complete payment paths: Table 1 shows the original loan, and Table 2 shows the refinanced loan.
Original Loan
Past Extra Principal Payments
Enter one-off principal payments made before the refinance decision. Payments are applied after scheduled principal in their payment month.
| Date | Extra principal amount (USD) | Row actions |
|---|---|---|
Refinance Offer
Optional Future Extra Payments
How the two tables are calculated
The same monthly mechanics apply to both tables
In each table, the monthly rate is the annual rate divided by 12 and then by 100. Each row's interest equals the prior balance times that monthly rate. The rest of the scheduled payment reduces principal. For a fixed-rate loan, the scheduled payment is calculated as P × r × (1 + r)^n / ((1 + r)^n − 1). At a 0% rate, payment is principal divided by the number of months.
How extra principal appears in the tables
An extra payment is applied after scheduled principal and directly lowers the ending balance. Because the next month's interest is calculated from that lower balance, the historical extra payments in Table 1 change the balance at the decision date. If a future monthly extra is enabled, it is included in both tables from the decision point forward. Each table's final payment is reduced so its balance closes at exactly zero.
Why there are two tables
Table 1 shows the original loan from its first payment through its projected payoff if it is kept. Table 2 shows the replacement loan from its first payment through payoff. From the refinance decision date onward, the calculator compares the remaining payments in Table 1 with the payments in Table 2.
Why historical paid interest is context only
Interest already paid cannot be recovered. It helps explain how the highlighted balance in Table 1 was reached, but the decision starts at the refinance date. The forward comparison therefore uses only the remaining Table 1 payments and the Table 2 payments, along with upfront closing costs, to compare future interest outcomes.
Worked example
See the mortgage refinance worked example for a step-by-step reading of a 30-year loan, two irregular principal payments, and a 15-year refinance offer.