A mortgage payoff calculator shows how quickly extra monthly principal payments retire a loan, and how much interest those extra payments save over the life of the mortgage. Enter the current balance, interest rate, scheduled monthly payment, and the extra amount you'll add each month, the calculator returns the new payoff timeline, months shaved off the original schedule, and total interest saved.
For a fresh-loan payment estimate, start with the Mortgage Calculator; this page is for an existing mortgage where you're deciding whether (and how much) extra to pay. The result usually surprises people: even modest extras have outsized effects, because every extra dollar permanently removes the future interest that dollar would have generated.
Key takeaway
Extra principal payments are mathematically equivalent to a risk-free, tax-equivalent return at your mortgage rate. At 6.5%, paying an extra $200/month is like earning a guaranteed 6.5% return on that money, competitive with stock-market expected returns, with none of the volatility. The earlier in the loan you start, the more powerful the effect: an extra dollar in year 1 prevents 29 years of compounding interest charges; an extra dollar in year 25 prevents only a few months.
How it's calculated
The math solves the standard amortization formula for number of periods (n), given a fixed payment that includes the extra principal:
n = −ln(1 − P × r ÷ M_total) ÷ ln(1 + r)
where P is the current balance, r is the monthly rate (annual ÷ 1200), and M_total is monthly_payment + extra_monthly.
Run the same formula twice, once with the scheduled payment alone, once with the scheduled-plus-extra, and the difference is months saved. Total interest in each case is M × n − P; the difference of those two interest figures is interest saved.
Edge case: if monthly_payment ≤ balance × r, the scheduled payment doesn't cover the monthly interest accrual and the loan never amortizes, the formula returns NaN. Real mortgages always have payments above this threshold by construction; the edge case only appears in mis-entered inputs.
Source: Standard amortization formula solved for the number of periods (n) given a fixed payment
Examples
$200K balance, 6.5% rate, $1,500/mo + $200 extra
- Current loan balance $200,000
- Interest rate 6.5%
- Scheduled monthly payment $1,500
- Extra monthly payment $200
On a $200,000 balance at 6.5% with a scheduled payment of $1,500/month, adding an extra $200 each month pays the loan off in about 188 months (15.6 years) instead of
237 months (19.8 years), a saving of roughly 49 months, or 4 years shaved off. Total interest paid drops from ~$155,700 (at $1,500/month alone) to ~$119,200 (with extra), about $36,500 saved in interest for a cumulative extra outlay of ~$37,600 spread over 16 years.$350K balance, 7% rate, $2,329/mo + $300 extra
- Current loan balance $350,000
- Interest rate 7%
- Scheduled monthly payment $2,329
- Extra monthly payment $300
On a near-fresh $350,000 30-year mortgage at 7%, the scheduled $2,329/month payment alone takes
360 months to clear. Adding $300/month extra cuts the timeline to **258 months (~21.5 years)**, over 8 years shaved, and saves roughly $160,000 in interest. The extra outlay over the payoff period is ~$77,300, so each extra dollar saves about $2 in future interest. That's the leverage of paying down a high-rate loan early.
Frequently asked questions
Should I make extra mortgage payments or invest the difference?
It depends on your mortgage rate vs. expected investment return. At a low fixed rate (3–4%), most savers come out ahead investing the extra, long-run stock returns of 7–10% comfortably exceed that hurdle. At higher rates (6%+), a guaranteed risk-free "return" equal to your mortgage rate rivals expected stock returns and beats them on a risk-adjusted basis. Tax treatment matters too: investment gains are taxable; avoided interest is not. Many borrowers split, keep matching the 401(k), build an emergency fund, then route excess to mortgage extras.
How does the lender apply my extra payment?
Most US mortgages apply extra principal payments to the outstanding balance immediately, which shortens the loan without lowering the monthly payment. Some lenders default to applying extras as a prepayment of next month's payment, which doesn't help. Always confirm with your servicer that extras are applied "to principal only"; many require a memo on the check or a specific online-banking option to ensure that.
What's a biweekly mortgage payment, and is it worth it?
A biweekly schedule has you pay half the monthly amount every two weeks, which works out to 26 half-payments per year, equivalent to 13 full monthly payments, not 12. The extra payment shaves about 6 years off a 30-year loan and saves substantial interest. Same effect as adding one-twelfth of your payment as an extra each month. Avoid third-party "biweekly programs" that charge fees; ask your servicer to set it up directly or simply add the equivalent extra each month.
Will paying off my mortgage early hurt my credit score?
Briefly, yes, slightly. Closing a long-running installment account reduces your credit mix and shortens active account history, which can drop your score by a few points temporarily. For most people the impact is small (5–20 points) and recovers within a few months. The financial benefit of being mortgage-free massively outweighs a temporary score dip, and if you're not applying for new credit, the score doesn't matter functionally.
How is this different from a standard mortgage calculator?
A mortgage calculator prices a new loan from scratch, given a price, rate, and term, what's the monthly payment? This calculator works on an existing mortgage with a known balance, and asks the inverse question: given that you can pay extra each month, when does the loan get paid off and how much interest do you avoid? The output is months-to-payoff and interest saved; the input is the extra you can comfortably commit each month.