Calculate mortgage overpayment

Compare a repayment mortgage with and without a fixed extra monthly payment.

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ToolMortgage Overpayment Calculator
Input
Output
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How it works

The standard monthly repayment comes from the annuity formula for the outstanding principal, fixed rate, and remaining term. A second schedule advances month by month, charging interest before paying the standard amount plus overpayment, until less than one penny remains; no mortgage values are assumed.

Formula
monthly payment = principal × monthly rate ÷ (1 − (1 + monthly rate)⁻ᵐᵒⁿᵗʰˢ)
  • Interest saved is standard lifetime interest minus interest accumulated by the overpayment schedule.
  • The final payment is capped at the remaining balance rather than charging a full scheduled amount.

Worked example

200,000 at 5% over 25 years
With an extra 200 a month on top of the standard payment
Input
											Loan amount: 200000
Rate percent: 5
Term years: 25
Monthly overpayment: 200
Output
												Standard monthly payment: 1169.18
Standard term months: 300
Standard total interest: 150754.02
Overpaid months: 226
Overpaid total interest: 108911.43
Interest saved: 41842.6
Years saved: 6.17

When to use this

Borrower overpayment plans, remortgage reviews, and household surplus-cash budgets compare interest cost and payoff dates on a repayment mortgage.

Edge cases

  • At 0% interest, the standard payment is principal divided by term months and interest saved remains 0.
  • A 0 overpayment reproduces the standard term and interest rather than inventing a saving.
  • A term shorter than one month is rejected before it can round to zero repayment periods.