Decision tools built for clarity
Early Debt Payoff Calculator
Estimate how an extra monthly payment changes the payoff date and total interest on a fixed-rate debt.
Enter the required values, select the applicable options, and calculate.
This educational estimate assumes no early-payment penalty and excludes fees, missed payments, tax effects, variable rates, and lender-specific rules.
Methodology
Formula or transformation
Reuses the fixed-payment amortization kernel, applies the declared extra amount each month, and closes the balance through one bounded schedule. The final payment is capped at the amount then due, so any fraction left by cent rounding is settled in that last month. A solved extra payment is published as the least whole-cent amount that closes the debt by the target month.
Variables
- Outstanding debt balance
- Annual interest rate as a percentage
- Remaining term in whole months
- Extra amount paid each month
Units
- Currency
- Percent
- Month
Rounding policy
Canonical arithmetic retains 24-place precision; the solved extra payment is rounded up to the cent and reconciled through the same schedule, and the last scheduled payment is limited to the balance plus interest then due.
The calculator reuses the fixed-payment amortization formula, compares the original schedule with the extra-payment schedule, and closes each balance within the declared remaining term.
Estimate how an extra monthly payment changes the payoff date and total interest on a fixed-rate debt.
Reviewed sources
- California State Board of Equalization — Periodic repayment and loan payment calculations Reviewed on 2026-09-20
Formula review history
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Initial shared-kernel, bounded-schedule, reverse-solution, and disclosure review.
Reviewed 2026-09-08 for formula version fixed-rate-extra-payment-payoff-v1
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Solved extra payment published as the least whole-cent amount that pays off by the target month when re-entered (deep audit 2026-09-20).
Reviewed 2026-09-20 for formula version fixed-rate-extra-payment-payoff-v1
Assumptions
- Inputs are interpreted using the displayed units and options.
- The calculation follows the methodology and factors listed in the sources.
Limitations
- Display rounding can hide additional decimal places.
- The calculation cannot add accuracy that is absent from the submitted inputs.
- This educational estimate assumes no early-payment penalty and excludes fees, missed payments, tax effects, variable rates, and lender-specific rules.
Worked example
For a balance of 1,200 at 0% over 12 months, adding 20 to the scheduled 100 monthly payment closes the debt in 10 months.
Frequently asked questions
How is the extra payment applied?
It is added to the scheduled payment every month and reduces the principal after that month's interest; the final payment is capped at the balance plus interest then due, so cent rounding settles in the last month.
What does the target payoff month solve?
The least whole-cent extra payment that closes the debt by that month; re-entering it as the extra payment gives the same month, and one cent less takes a month longer.