What is the mortgage payoff calculator — extra payment savings?
In short
Adding $200/month to a $320,000 mortgage at 6.5% with 25 years remaining saves $47,000 in interest and pays the loan off 4.2 years early. The formula: new payoff months n = −ln(1 − r·B / (M + E)) / ln(1 + r), where M is the standard payment, E is the extra amount, B is the balance, and r is the monthly rate.
This mortgage payoff calculator shows how much interest you save — and how many years you cut — by adding extra monthly payments or a one-time lump sum to your mortgage. Enter your current balance, rate, and months remaining, then try different extra-payment amounts to see the impact.
How to use this calculator
- 1Enter your current remaining loan balance (check your latest mortgage statement).
- 2Enter your interest rate and how many months remain on your loan.
- 3Add an extra monthly payment — even $100/month makes a significant difference.
- 4Optionally enter a one-time lump sum (bonus, tax refund) applied immediately to principal.
- 5Read the interest saved, years saved, and new payoff date.
The formula
- M
- — Standard monthly P&I payment
- E
- — Extra monthly payment amount
- P / B
- — Original loan amount / current remaining balance
- r
- — Monthly interest rate (annual rate ÷ 12 ÷ 100)
- n
- — Original months remaining
- n_new
- — New months to payoff with extra payment
The one-time lump sum is applied to the principal balance immediately, which also slightly reduces the required monthly payment — but the calculator keeps the payment the same and directs the savings toward an even earlier payoff.
Worked example
The scenario
$320,000 balance, 6.5% rate, 300 months (25 years) remaining, $200/month extra.
The result
Standard payment: $2,137/month. Total interest without extra: $321,000. With $200 extra: total interest $274,000 — saving $47,000 and paying off 50 months (4.2 years) early.
Common use cases
- Deciding whether to make extra mortgage payments vs investing the difference
- Planning a lump-sum paydown with a work bonus or inheritance
- Seeing the interest impact before choosing a biweekly payment plan
- Calculating how many extra payments are needed to pay off by a target date
Limitations & assumptions
- Assumes a fixed interest rate — adjustable-rate mortgages (ARMs) will have different outcomes after the fixed period.
- Does not account for prepayment penalties — check your loan agreement before making large extra payments.
- Results are for the principal & interest portion only; property tax, insurance, and PMI are not included.
Frequently asked questions
How much does one extra mortgage payment a year save?
Is it better to pay extra on a mortgage or invest the money?
What happens if I pay an extra $500 a month on my mortgage?
Does a lump-sum payment reduce monthly payments?
What is the biweekly mortgage payment trick?
Disclaimer: KalkWise calculators are provided for general informational and educational purposes only and do not constitute financial, investment, tax, or legal advice. Results are estimates based on the figures you enter and the assumptions described above. Actual outcomes will vary. Consult a qualified professional before making financial decisions.