Mortgage Extra Payment Impact Calculator
See how extra payments shorten your mortgage and save interest.
All calculations happen locally in your browser — nothing is uploaded, stored or tracked.
Enter your mortgage
Balance, rate and remaining term.
Choose the extra payment
Monthly, annual or one-time.
Read the impact
Time saved, interest saved, new payoff date.
What Is an Extra Payment Calculator?
Making extra payments — even small ones — dramatically reduces mortgage interest because every extra dollar goes straight to principal, skipping future interest entirely. This calculator quantifies the effect: months cut from the term and total interest saved.
It supports three patterns: a fixed extra amount every month, an annual lump sum, or a one-time payment.
How It Works
The standard amortization is simulated month by month. Each extra payment reduces the balance immediately, so the interest charged the following month is lower — the savings compound over the life of the loan.
Your data never leaves your device.
Every calculation on this site runs in your browser using vanilla JavaScript. Nothing is uploaded, stored or tracked.
- 100% local, client-side calculations
- No data stored on any server
- No tracking, no analytics, no ads scripts
- Works offline once loaded
Frequently Asked Questions
How do extra payments help?
They reduce the principal directly, so less interest accrues on every future payment — that compounds into large savings.
How much does an extra $100/month save?
On a $300k, 6.5%, 30-year loan, about $100/month extra cuts roughly 4 years off the term and saves around $75,000 in interest.
Is it better to pay extra monthly or yearly?
Monthly extra payments reduce the average balance sooner, so they save slightly more than an equivalent annual lump sum.
Should I invest instead of paying extra?
Compare the mortgage rate with your expected investment return after tax — if the investment return is higher, investing may win. This is a math comparison, not advice.
Does the calculator assume the rate stays fixed?
Yes — it simulates your current fixed-rate loan unchanged.