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Extra Mortgage Payment Calculator

Find out how extra monthly or lump-sum payments slash your interest and shorten your mortgage.

Updated June 2026 · Editorial standards

Your details

$
%
$
$
Interest saved
$103,449
Time saved
83 months / 6y 11m
New payoff date
July 2049
Total interest with extra
$279,185

Paying $200/month extra saves you $103,449 in interest and pays off your mortgage 83 months (6.9 years) early.

Without extra vs with extra

Without extraWith extra
Total interest$382,633$279,185
Payoff dateJune 2056July 2049
Total paid$682,633$579,185
By the KalkWise Editorial Team Reviewed for accuracy Updated June 2026

What is the extra mortgage payment calculator?

In short

An extra $200/month payment on a $300,000 30-year mortgage at 6.5% saves approximately $72,000 in interest and cuts the loan term by about 6 years.

This extra mortgage payment calculator shows how adding extra monthly payments or a one-time lump sum to your mortgage reduces total interest and shortens your payoff. Enter your balance, rate, and remaining term, then test different extra-payment amounts.

How to use this calculator

  1. 1Enter your current loan balance from your latest statement.
  2. 2Enter your interest rate and how many months remain.
  3. 3Add an extra monthly payment — even $100/month makes a real difference.
  4. 4Optionally add a one-time lump sum applied immediately to principal.
  5. 5Read the interest saved, time saved, and new payoff date.

The formula

M=P×r(1+r)n(1+r)n−1
nnew=−ln(1−r·BM+E)ln(1+r)
Standard payment M = P·r·(1+r)^n / ((1+r)^n − 1). New payoff months with extra E: n_new = −ln(1 − r·B / (M+E)) / ln(1+r). Interest saved = original interest − interest with extra.
M
— Standard monthly P&I payment
E
— Extra monthly payment amount
B
— Current remaining balance
r
— Monthly interest rate (annual ÷ 12 ÷ 100)
n
— Original months remaining
n_new
— New months to payoff with the extra payment

A one-time lump sum is applied to principal immediately, accelerating payoff even further.

Worked example

The scenario

$300,000 balance, 6.5% rate, 360 months remaining, $200/month extra.

gives

The result

Total interest drops from ~$382,000 to ~$310,000 — saving about $72,000 and paying off roughly 6 years early.

Common use cases

  • Deciding between extra mortgage payments and investing the difference
  • Planning a lump-sum paydown with a bonus or tax refund
  • Setting a target payoff date and finding the extra payment needed
  • Quantifying the savings before committing to a prepayment plan

Limitations & assumptions

  • Assumes a fixed interest rate — ARMs differ after the fixed period.
  • Does not account for prepayment penalties — check your loan agreement.
  • Covers principal & interest only; tax, insurance, and PMI are excluded.

Frequently asked questions

On a $300,000 30-year mortgage at 6.5%, paying $200/month extra saves about $72,000 in interest and cuts roughly 6 years off the term.

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.