Mortgage Payoff Calculator

How this is calculated

The monthly payment comes from the standard fixed-rate amortization formula:

M = P × i / (1 − (1 + i)^−N)

• M — the monthly principal-and-interest payment.
• P — the loan amount (principal).
• i — the monthly interest rate: the annual rate as a decimal, divided by 12 (6.5% → 0.065 ÷ 12).
• N — the total number of monthly payments (years × 12).

When the rate is 0% the formula is undefined, so the payment is simply P ÷ N. The payoff schedule is then simulated month by month: interest accrues on the remaining balance, your payment (plus any extra) is applied, and the final payment is capped at what's actually owed. The payoff date assumes your first payment is due one month after the start date, and extra payments are applied every month starting with the first. Interest is compounded monthly, the convention for U.S. mortgages.

How mortgage amortization works, and why extra payments punch above their weight

A fixed-rate mortgage has a level payment, but what's inside that payment changes every month. Interest is charged only on what you still owe, so in the early years — when the balance is at its peak — interest eats most of the payment. On a $300,000 loan at 6.5%, the payment is $1,896.20, and the first month's interest alone is $1,625. Just $271 goes to actually reducing the debt. Thirty years later the proportions have flipped: the final payments are nearly all principal.

What the loan really costs

Run that same loan to the end and the total interest comes to about $382,633 — more than the amount borrowed. That isn't a trick or a bad deal; it's simply what borrowing $300,000 for three decades at 6.5% costs. But it's exactly why the payoff timeline responds so strongly to prepayment: there is a lot of scheduled interest available to cancel.

Where extra payments get their power

Every extra dollar you send goes entirely to principal — and principal you eliminate today stops charging interest every month for the rest of the loan. Add $200 a month to the example above and the numbers move dramatically: the loan pays off in 23 years and 1 month instead of 30, and total interest drops to about $279,185. That $200 a month saves roughly $103,449 in interest. You contribute about $55,000 of extra payments over those years and cancel nearly twice that in interest charges.

Timing matters too. The same extra dollar does more work in year 2 than in year 22, because it has more remaining months of interest to cancel. If you ever receive a windfall — a bonus, a tax refund — applying it early in the loan's life is where it has the most effect.

Common misconceptions

First, extra payments do not lower your required monthly payment. The obligation stays the same; the loan just ends sooner. (Lender arrangements that actually reduce the payment — refinancing or recasting — are different transactions.) Second, make sure extra amounts are applied to principal: some servicers will otherwise treat them as early payment of next month's bill, which cancels no interest at all. Third, the popular biweekly-payment trick isn't magic — paying half the mortgage every two weeks simply results in 13 full payments a year instead of 12, which you can replicate by adding one-twelfth of a payment monthly. And finally, prepaying isn't automatically optimal: it earns a guaranteed return equal to your loan's rate, which you should weigh against high-rate debts, emergency savings, and what investing might earn — decisions this calculator informs but can't make for you.