How much does one extra mortgage payment a year save?

On a typical 30-year loan one extra payment a year — for example via biweekly half-payments — cuts roughly 4 to 6 years off the term and saves tens of thousands of dollars of interest, depending on the rate and balance.

Do extra mortgage payments lower my monthly payment?

No — the required payment stays the same. Extra payments shorten the loan and cut total interest instead. To lower the monthly payment itself you would need to refinance or recast.

Is it better to pay extra monthly or one lump sum?

The earlier money hits the principal, the more interest it kills, so a lump sum today beats the same total spread over years. But a steady monthly extra is easier to keep up — this calculator shows both, together.

Mortgage Overpayment Calculator — Pay Off Years Early

The interest picture

Before and after, line by line

Your balance, year by year

The solid line is your plan; the dashed line is the original schedule. The gap between them is the interest you never pay. Hover any point.

What each move is worth

How we calculate

What is included

Standard fixed-rate amortization: your required payment is computed from the balance, rate and years left, interest accrues monthly, and everything extra goes straight to principal. Biweekly plans are counted as 13 full payments a year (one extra payment spread across the months). The one-time payment is applied in month one.

What is estimated or left out

A planning estimate, not advice. Property tax, insurance and escrow are not part of this picture — they continue regardless and do not shrink with the loan. Adjustable rates, recasting and prepayment penalties (rare in the US, but check your note) are not modeled. Interest is compounded monthly, which matches how US mortgages actually work.

The formula, in plain words

required payment = amortization payment for (balance, rate, years left)
each month:  interest = balance × rate ÷ 12
             balance  = balance + interest − payment − extra
biweekly on: extra grows by required payment ÷ 12

Common questions

Does extra money really go to the principal?: Only if it is marked that way. When you send extra, tell your servicer “apply to principal” — otherwise some will happily hold it as an early next payment, which saves you nothing.
Is paying the mortgage early always smart?: It is a guaranteed return equal to your rate — at 6.5% that is a very good guaranteed return. But money in the house is hard to get back out. Fill your emergency fund and any employer 401(k) match first; both usually beat overpaying.
What about biweekly payment services?: Never pay a fee for this. The whole trick is one extra payment a year — you can do the same for free by adding 1/12 of your payment to each month, which is exactly what the “+ Biweekly payments” switch models.

Mortgage Overpayment