What an overpayment buys

Published 2026-09-04

Advice about overpaying a loan ends in the same slogan: pay a little extra, be free years early. It is true and useless, because it never says how much extra buys how many years. Everything below comes from one loan — 250,000 at 6.5 percent over 30 years, the example the loan calculator works from — and every figure is what the calculator returns for those inputs.

On schedule, that loan costs 1,580.17 a month and 318,861.22 in interest. The interest is 127.54 percent of the amount borrowed: the rent on the money costs more than the money.

Interest is rent on the balance

Each month’s interest is the outstanding balance times one twelfth of the annual rate. Nothing else enters into it. Of the first payment, 1,354.17 is interest and only 226.00 reduces the debt. Of the last payment, 8.51 is interest.

That single fact explains the whole subject. Any amount taken off the balance stops paying rent from that month to the end, so what an overpayment is worth is set by how many months stand behind it, not by how large it is.

The same 200, early and late

Put 200 against this balance with the first payment and the calculator removes 1,190.83 of future interest, almost six times the 200 itself. Put the same 200 in with payment number 300 and it removes 76.56. Neither shortens the term by a whole month.

Scale the amount up and the shape is identical. A single payment of 10,000 against the same loan:

The 10,000 does not change. What changes is how much interest stands behind it, waiting to be cancelled.

A monthly extra against one lump sum

The calculator takes both, and they are not the same instrument. An extra 200 every month clears this loan in 265 payments instead of 360 and brings the interest down to 221,243.10 — 97,618.12 removed, 95 months early.

Comparing that with a lump sum honestly means holding the money constant. The extra stops when the loan does, so the total handed over is 200 across 265 payments: 53,000. Paid instead as a single 53,000 with payment number one, the loan runs 209 months and 185,946.16 of interest disappears. Nearly twice the saving, for exactly the same money.

That is not an argument for lump sums. It shows that timing does most of the work, and that anyone able to produce 53,000 in month one was never choosing between these two. The comparison worth running is between real options: the 200 a month you can spare, or the bonus you will receive. Both fields work together, so 200 a month plus a 10,000 bonus at payment 12 prices as one plan: 244 months, 124,720.09 removed.

The term and the saving are one fact

“Clears the loan 95 months early” and “saves 97,618.12” are the same sentence twice. They are not two benefits to be added together, and a leaflet that lists both is counting once.

Months are the honest measure when the goal is to stop paying by a particular date; money is the honest measure when the overpayment competes with another use for the same money. Note which months are removed: the last ones, the cheap end of the schedule, where the payment is almost entirely principal. The saving does not come from them. It comes from all the interest between now and then that never accrued.

The other direction: what a payment buys

Nobody shopping for a house holds the question as “what does 250,000 cost”. They know what they can pay. Read backwards, at 6.5 percent over 30 years, 1,580.17 a month borrows 249,999.99.

Not 250,000. The principal is floored to the cent rather than rounded, because rounding up would sometimes name a loan whose payment lands a fraction above the budget, and “you can afford this” is the one answer that must never overshoot. The missing cent is deliberate.

An overpayment comes out of that budget first, because it leaves the same account in the same month. A budget of 1,780.17 with 200 of it going in as an overpayment borrows the same 249,999.99; the same 1,780.17 with no overpayment borrows 281,642.15. The 31,642.16 between them is the whole decision — identical money buys either a larger loan or a shorter one, never both.

What can make it not worth doing

Two things break the arithmetic above, and neither is in the calculator.

An early-settlement charge. Fixed-rate loans often charge a percentage of anything repaid ahead of schedule, or permit only a set fraction of the balance each year without penalty. Set the charge against the interest removed before deciding; against a late overpayment worth 76.56, almost any charge wins.

A rate you are about to leave. Every figure here assumes 6.5 percent holds for thirty years. If the fixed period ends in two, the numbers past that point describe a rate you do not have. Overpaying during the fix is still real — it lowers the balance you refinance — but the thirty-year saving is a projection, not a promise.

And plainly: an overpayment cannot be taken back, savings can. If a deposit account pays more than the loan charges, the arithmetic points the other way. Enter your own balance, rate and remaining term — for a loan with four years left the answer is often “not worth arranging”, which is a perfectly good answer.