Additional Payment Mortgage Calculator
A small extra payment on your mortgage does not just shave a little off the balance. Every extra dollar skips ahead to principal, which means all future interest is charged on a smaller balance. The effect compounds: a modest extra payment each month can erase years from a 30-year loan and tens of thousands of dollars in interest.
This calculator compares your mortgage with and without extra payments. Enter your loan details and an extra monthly amount to see the new payoff date, the time saved, the interest saved, and a year-by-year breakdown of both scenarios.
Additional Mortgage Payment Calculator
Worked example
A $300,000 mortgage at 6.5% APR over 30 years, with an extra $200 per month:
- Standard monthly payment: $1,896.20; with extra: $2,096.20
- Original payoff: 360 months; new payoff: 277 months
- Time saved: 6 years 11 months
- Total interest without extra: $382,633.47
- Total interest with extra: $279,184.67
- Interest saved: $103,448.79
That $200 is about 10.5% on top of the regular payment, yet it wipes out nearly 23% of the loan term and more than a quarter of the total interest. The year-by-year table shows the gap opening early: after 5 years the extra-payment balance is already about $14,000 lower.
How it works
The standard payment uses the amortization formula payment = P × r(1+r)^n ÷ ((1+r)^n − 1). The calculator then simulates both scenarios month by month: each month interest accrues on the remaining balance, the scheduled payment covers interest first, and anything left (plus the full extra amount) reduces principal. The final payment of each scenario is trimmed so the balance ends at exactly zero.
What changed in this version: the previous calculator rejected 0% rates, reported the new payoff only as a month count (no calendar date), and gave no sense of how the two scenarios diverge over time. This version supports 0% APR, shows a calendar payoff date when you enter a start date, and adds the year-by-year comparison table.
What competing calculators get wrong
Several popular extra-payment tools annualize the savings with a single multiplication (extra × 12 × years), which ignores that the loan ends early and overstates what you pay. Others round the payoff to whole years, hiding months of difference. Month-by-month simulation with an exact final payment is the only honest method, and it is what this tool uses. Bankrate and CalculatorSoup both offer extra-payment modeling; the advantage here is the side-by-side yearly table that shows the balance gap compounding, plus honest 0% APR handling.
Frequently asked questions
Do extra mortgage payments go to principal?
When you specify the extra amount as principal-only, yes. This calculator models the extra payment as going entirely to principal after the scheduled payment covers that month’s interest. Confirm with your servicer that extra payments are applied to principal, not held as prepaid interest.
Is it better to pay extra monthly or make one lump sum?
For the same total extra amount per year, monthly extras win slightly because each dollar starts reducing interest-bearing balance a month sooner. The difference is small; consistency matters more than timing.
How much extra should I pay?
Even 5 to 10% on top of the regular payment makes a large dent over 30 years. Run your numbers above and check the time-saved figure: that is the real return on the extra payment.
Does this work for a 0% APR loan?
Yes. With 0% interest, extra payments simply shorten the term proportionally, and the calculator shows the exact new payoff month.
Will my lender charge a prepayment penalty?
Most modern US residential mortgages have no prepayment penalty, but some loans do. Check your note or ask your servicer before sending extra principal.
Should I pay extra on the mortgage or invest instead?
Paying extra earns a guaranteed return equal to your mortgage rate. Investing might earn more but with risk. Many borrowers split the difference. This is a personal decision, not advice.
Disclaimer
This calculator is for general information and education only. It assumes fixed-rate, fully amortizing payments and that extra payments are applied to principal. Taxes, insurance, PMI, adjustable rates, and lender fees are not modeled. It is not financial advice; confirm details with your mortgage servicer.