An Extra $300 a Month Saves $158,826 in Year One and $4,349 in Year 25

Most advice about paying extra on a mortgage focuses on how much. The more important variable is when. An identical $300 monthly overpayment produces wildly different results depending on which year of the loan you start it, and the gap is far larger than most borrowers expect.

Using a $400,000 loan at 6.75% over 30 years, with a scheduled payment of $2,594.39 and $533,981.26 of total interest if left alone:

The same $300, applied to the same loan, at the same interest rate, is worth $158,826.39 if you start immediately and $4,349.49 if you start in year 25. That is a factor of thirty-six, and it is not caused by the number of payments alone.

Why the early dollar is worth so much more

An amortized loan charges interest on the outstanding balance each month, then applies whatever is left of your payment to principal. Early on the balance is enormous, so interest consumes nearly everything.

On this loan, the first scheduled payment of $2,594.39 splits into $2,250.00 of interest and $344.39 of principal. Interest takes 86.7% of it. That means an extra $300 in month one buys you almost an entire additional month of principal reduction, and every month of principal you remove early is a month whose interest never accrues at all.

The practical consequence

This inverts a common plan. Many households intend to overpay the mortgage later, once the children are through school, once the car is paid off, once income rises. By the time later arrives, the balance has already amortized down and most of the interest has been paid. The plan is sound in principle and applied at the point of least effect.

The mirror image is also true and more encouraging: a small overpayment you can genuinely sustain from month one beats a large one you intend to start in a decade. Sustained $150 a month from the beginning outperforms $600 a month begun in year twelve on this loan.

Where prepayment sits against everything else

None of the above establishes that prepaying is the right use of the money. Extra principal is a guaranteed, risk-free return equal to your mortgage rate, which on this loan is 6.75%. That is a genuinely good risk-free return, and it is still beaten by several things.

1. Any employer retirement match. A 50% match is an immediate 50% return, roughly seven times the mortgage rate, and it is the only guaranteed return of that size available to most people.
2. Any debt above the mortgage rate. A 22.9% credit card balance or a 12% personal loan should be cleared before a single extra dollar goes to a 6.75% mortgage.
3. A funded emergency fund. Extra principal is the least liquid place your money can go. It reduces what you owe without reducing what you must pay each month, and you cannot withdraw it when the boiler fails.
4. Tax-advantaged retirement contributions, if your expected return exceeds the mortgage rate and you have a long horizon.

Below a mortgage rate of roughly 4%, prepayment is usually the weakest option on this list and a fully funded retirement account is better. Above roughly 7%, prepaying starts to compete seriously with investing, because you are comparing a certain 7% against an uncertain one.

Mechanics that quietly matter

• Direct the extra amount to principal explicitly. Many servicers apply unlabelled overpayments to the next scheduled instalment instead, which parks your money and earns you nothing. Check the following statement to confirm the principal balance dropped by the full extra amount.
• Prepaying does not lower your required monthly payment. The term shortens instead. If your goal is a smaller monthly obligation rather than a shorter loan, prepayment is the wrong tool and you need a recast or a refinance.
• Ask about recasting. Some servicers will re-amortize the loan over the remaining term after a large lump-sum principal payment, lowering the required payment for a fee of a few hundred dollars. It is far cheaper than refinancing and rarely advertised.
• Confirm there is no prepayment penalty. Rare on modern conforming mortgages, still present on some portfolio and non-qualified loans.

The summary that fits on an index card

If you are going to prepay, the value is concentrated in the first third of the loan, so start now and start small rather than later and large. If you have not captured your employer match, cleared high-interest debt, and built an emergency fund, do those first, because each one returns more than the mortgage rate. And if your mortgage rate begins with a 3, the arithmetic favours investing almost every time.