Extra payment calculator
A small monthly extra goes straight to principal, which shortens the loan and cuts total interest.
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How to use it
- Enter loan amount or balance, annual interest rate, remaining term and extra per month.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Payment = P × r ÷ (1 − (1 + r)^−n). Months with the extra = −ln(1 − r × P ÷ (payment + extra)) ÷ ln(1 + r), rounded up; the last payment covers only what is left.
Example
Inputs: Loan amount or balance = 100,000, Annual interest rate = 6 %, Remaining term = 360 months, Extra per month = 100. Result: Interest saved: 39,900.25; New payoff time: 252 months; Months saved: 108 months.
Limits and caveats
- Assumes every extra payment goes to principal each month with no fee.
- Some lenders need you to mark the extra as principal, or they treat it as a prepaid installment.
Questions about this tool
Why are savings so large on long loans?
Each amount of principal repaid early stops interest that would have built up on it for years.
Is the last payment a full one?
Usually not. The last payment only covers what is left, and the calculator works it out exactly.
Monthly extra or one lump sum?
Both cut interest. This tool handles a fixed monthly extra; the early payoff calculator finds the payment for a target date.