How extra mortgage payments really work

Last updated: October 2026

Start with how one payment splits

On a fixed-rate amortising loan, each monthly payment is split into interest and principal. The interest part is simply the outstanding balance multiplied by the monthly rate — it has nothing to do with how many years are left or how large the original loan was.

Take $300,000 at 6% over 30 years. The monthly rate is 0.5%, so the first payment of $1,798.65 contains $1,500 of interest and just $298.65 of principal. In month one you are renting the money, not buying the house. By the final year the same payment is almost entirely principal, because the balance it is calculated on has shrunk.

Why extra money behaves differently

An extra payment does not "go towards future payments". It reduces the balance today, and the balance is the number every later interest charge is computed from. The saving therefore is not a one-off: it is the sum of a slightly smaller interest charge in every single remaining month. On a 26-year-old loan that is a lot of months, which is why a modest extra amount produces a total saving that looks out of proportion.

What the numbers look like

$300,000 at 6% over 30 years, with the extra amount added from the first payment:

Two things to notice. First, the saving is not linear — doubling $250 to $500 adds another $54,000 of saving, but the last doubling adds less relative improvement because you are running out of interest to avoid. Second, every row still leaves the borrower thousands of dollars of interest: the loan does not become free, it just ends sooner.

Timing beats size

The same $100 a month, started at different moments, is not worth the same thing:

• Started with the first payment: saves about $53,300 and finishes 3.9 years early.
• Started in year 11 instead: saves about $19,700 and finishes 1.9 years early.

Waiting a decade removes roughly 63% of the benefit, even though the monthly amount is identical. The reason is exactly the mechanic above: the extra money can only prevent interest that has not been charged yet, and every year of delay permanently writes off the interest that was charged in the meantime.

The same principle applies to lump sums. A $1,200 payment applied to principal in month one saves roughly $5,400 across the life of this loan; spreading the same $1,200 across twelve months saves marginally less, because the later slices arrive after the balance they would have reduced has already been charged for a while. Neither is dramatic on its own — the $53,000 figure comes from keeping the extra payment up for decades, not from paying a large amount once.

The biweekly myth

Biweekly payment plans are sold as though the fortnightly rhythm itself does something clever. On a fixed-rate loan it does not. Twenty-six half payments make thirteen full payments a year, so the real change is that you pay about 1/12 of a payment extra each month.

On this loan, 1/12 of $1,798.65 is about $150 a month. The result is the same as any other route to $150: roughly $73,700 of interest saved and the loan cleared about 5.4 years early. If a servicer charges a setup fee for a biweekly programme, you are paying for arithmetic you can do yourself — pay half the monthly amount every two weeks into your own account, or simply add $150 to the monthly payment.

What to check before sending extra money

• Where it lands. Many servicers default to holding extra money against your next scheduled payment instead of reducing principal. Check the statement after the first attempt: principal should fall by more than the scheduled portion.
• Prepayment penalties. Rare on modern prime mortgages, common on some older or subprime loans. The penalty can exceed the interest saved.
• Escrow. If your payment includes taxes and insurance, make clear that the extra is for principal, not for a larger escrow cushion.
• Recasting. Some lenders will re-amortise the loan on request, lowering the monthly payment instead of shortening the term. That is a different goal and usually a separate, sometimes paid, process.
• Rate type. On an adjustable-rate loan, prepaying before a reset is more attractive than after one.
• Order of operations. An emergency fund and any debt charging more than the mortgage rate come first — a credit card at 20% is a better target than a mortgage at 6%.

The tax angle, briefly

In the United States, mortgage interest is deductible only if you itemise, and the standard deduction is high enough that many households do not. If you do deduct it, the guaranteed return on prepaying is your rate multiplied by one minus your marginal tax rate — because you are also giving up the deduction on the interest you no longer pay. At 6% interest and a 24% marginal rate, the effective return is closer to 4.6%.

Prepay or invest?

Prepaying is a guaranteed, irreversible return equal to your after-tax rate. Investing has an expected return that may be higher, with variance and with the option to sell. There is no universal answer, and anyone who claims otherwise is selling something. The defensible compromise for most people: hold enough cash to cover several months of payments, clear any higher-interest debt, then decide how much of the remaining surplus goes to the mortgage versus a diversified account — knowing that the mortgage half is the low-risk half of the portfolio.

Run your own numbers

Our early payoff calculator takes your balance, rate, current payment and any extra amount, and shows the month the loan actually ends plus the interest avoided. The mortgage calculator produces the baseline schedule to compare against, and the debt payoff calculator applies the same logic when the debt is a card or a personal loan. The guides index lists the rest of these write-ups.

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