Mortgage Overpayment Calculator
Compare scheduled payments with a fixed monthly extra principal payment to estimate modeled mortgage interest and payoff time.
Earlier editorial review
- Reviewed
- 2026-09-21
- Methodology
- 1.1
Sources
How the schedule works
Each month, interest is calculated from the unpaid principal balance and the monthly rate. The scheduled payment covers that interest first, and the rest reduces principal. An extra payment is then applied to principal in the same monthly cycle.
The calculator derives the regular principal and interest payment from the entered balance, nominal annual rate, and remaining whole month term. It runs a baseline schedule and a second schedule with the same regular payment plus the entered extra amount.
The comparison reports total interest under each schedule, the difference in interest, and the number of monthly payments until payoff. It does not value the cash used for extra payments or estimate an alternative investment return.
Both schedules use a fixed rate and monthly payments. The accelerated schedule applies the extra amount to principal in the same monthly cycle after interest is calculated. The final payment is limited to the balance and interest still due.
Step by step
- Enter the current unpaid principal, nominal annual rate, and remaining term.
- Enter the fixed amount of extra principal paid each month.
- Compare monthly payments, total interest, and payoff months for both schedules.
Worked example
For a principal balance of 250,000 at a 6% nominal annual rate with 30 years remaining, the scheduled principal and interest payment is 1,498.88 per month. Add a fixed 200 extra each month. The accelerated schedule pays off in 267 months instead of 360, saving 93 months.
Scheduled interest is 289,595.47. Interest with the extra payment is 203,362.51, so modeled interest savings are 86,232.96. The final payment is capped at the remaining balance plus that month’s interest. These figures are reproduced by the default inputs.
When to use this calculator
Use this calculator to compare a regular monthly mortgage schedule with one that adds the same extra principal amount every month. Enter the current balance and remaining term rather than original loan figures.
Use the result as a mathematical comparison under the displayed assumptions. For daily interest, irregular payments, changing rates, a lump sum, fees, or a lender recast, request a schedule from the loan servicer.
Common input errors
Use the current unpaid principal balance and remaining term, not the original purchase price or original loan amount. The interest rate is an annual percentage, not a monthly rate.
The accelerated schedule assumes the extra amount is applied to principal after monthly interest. A servicer may process additional payments differently. Confirm how an extra payment is applied before relying on the modeled schedule.
Mortgage overpayment formula
Monthly rate r = nominal annual rate / 100 / 12
Regular payment = P × r / (1 − (1 + r)^(−n)), or P / n when r = 0
Monthly interest = opening balance × r
Accelerated payment = regular payment + fixed extra principal amount
Principal paid = payment − monthly interest, capped at the remaining amount due
Interest saved = baseline schedule interest − accelerated schedule interest
The annual nominal rate must be from 0% through 100%. The term must represent 1 to 600 whole monthly payments. Principal must be positive and the extra payment cannot be negative. Results outside the supported numeric range are unavailable.
Assumptions and limits
This model assumes a fixed nominal annual rate divided by 12, monthly interest accrual, and payment at the end of each month. It assumes each extra amount reaches principal in that monthly cycle and that the regular payment stays unchanged. It does not model lender rounding, daily interest, rate changes, recasting, one time payments, fees, prepayment penalties, escrow, or taxes. Actual servicing rules and payoff statements can differ. Check the loan terms and payment instructions with the servicer.
Key terms
- Principal balance
- The balance still owed at the start of the modeled schedule.
- Monthly rate
- The nominal annual interest rate divided by 12 for the monthly schedule.
- Extra principal payment
- The fixed amount added to the regular payment each month and directed to principal after interest is calculated.
Related tools
Mortgage Refinance Break Even compares a lower rate with upfront refinancing costs while keeping the remaining term the same. This calculator keeps the current rate and term, then models a recurring extra principal payment.
Debt Snowball versus Avalanche compares payoff schedules for several debts. This calculator models one mortgage and one fixed monthly extra payment.
FAQ
What inputs does the calculator use?
Enter the current unpaid principal balance, annual nominal interest rate, remaining term in years, and a fixed extra principal amount paid each month. The term must correspond to a whole number of months.
How is the regular payment calculated?
The calculator divides the nominal annual rate by 12 and calculates the level monthly principal and interest payment over the remaining term. At a zero rate, it divides principal evenly across the monthly payments.
When is the extra amount applied?
The model calculates monthly interest first, then applies the regular payment and the fixed extra amount to the balance. The extra amount is assumed to reduce principal during the same monthly cycle.
What happens to the final payment?
The last payment is limited to the remaining principal plus that month’s interest. The model does not count a full scheduled payment when a smaller amount pays off the balance.
Does the calculator recommend an extra payment amount?
No. It compares the amount entered with the regular payment schedule. It does not assess affordability, emergency cash needs, or the value of another use for that money.
Related tools
Mortgage Refinance Break Even compares rates and upfront refinancing costs for the same remaining term. Debt Snowball versus Avalanche compares several debt balances. Both answer different questions from this single loan overpayment schedule.