How Extra Mortgage Payments Work

Your monthly payment is a fixed amount, but what it buys changes every month. Each payment is split in two:

Interest is calculated first and it is calculated on your balance. On our example loan — $400,000 at 6.25%, payment $2,462.87 — the first payment splits like this:

You paid $2,462.87 and your debt fell by $379.54. That is not a trick or a bad deal — it is what borrowing $400,000 costs. But it explains everything that follows.

An extra principal payment is different in one crucial way: no interest is charged on it. It is not a payment for the use of money. It is a repayment of the money itself. Every cent lands on the balance.

$200 extra increased the amount of debt you retired that month by 53%.

Here are the first six months, side by side. Pay attention to the principal column, not the extra column:

In month 2, the borrower paying extra sends $382.56 of their regular payment to principal instead of $381.51 — an extra $1.05 they did not pay for. It came from the interest line, which shrank because the balance was smaller.

That $1.05 looks trivial. It is the entire mechanism. Every month it happens again, slightly larger, on a slightly smaller balance, for the rest of the loan. After six months the balance gap is $1,215.75 while you have only contributed $1,200 — the extra $15.75 is money the loan generated for you.

After five years:

$12,000 of your money produced $14,044 of debt reduction. The extra $2,044 is interest you were never charged.

Run that forward 30 years and the numbers stop looking small:

Two separate things produce that $104,950:

1. You skip 66 payments entirely. The loan ends 5 years 6 months early, so 66 months of interest charges never happen.
2. Every payment you do make is more efficient. Because your balance is always lower than it would have been, every single one of the 294 payments carries a smaller interest charge.

The second effect is invisible on a statement, and it is roughly half the benefit.

1. Earlier is worth more than bigger

An extra dollar's value equals the interest it prevents, which depends on how many months remain. The same $2,462.87 payment saves $13,294 in year one and only $2,120 in year twenty — six times less, for identical money.

2. A higher rate makes extra payments more powerful

You are buying a guaranteed return equal to your rate. At 6.25% an extra $200 saves $104,950; on a 4% loan the same $200 saves far less, because there was less interest to prevent.

3. Consistency beats size

$100 every month for 30 years saves $59,744. A single $10,000 payment — which most people find much harder — saves $50,967. The steady trickle wins because it starts sooner and never stops.

The mechanism only works if the money reaches principal. Servicers do not assume that is what you want.

• Use the "principal only" field. Almost every servicer portal has one, sometimes labelled "additional principal" or "curtailment."
• Never just send a bigger cheque. Unlabelled extra money is commonly applied to your next scheduled payment (which pays future interest, not principal) or held in a suspense account earning you nothing.
• Do not let it "pay ahead." Being a month ahead feels like progress but does not shorten your loan. Principal reduction does.
• Exclude escrow from the calculation. If your payment includes taxes and insurance, only the principal-and-interest portion is your loan. Extra escrow pays future tax bills.
• Verify it worked. Next month's statement should show a balance lower than the schedule predicts, and a slightly smaller interest charge. If it does not, the money went somewhere else.
• Check for a prepayment penalty in your note — rare on modern US conforming loans, but not extinct.