How Overpayments Reduce Your Total Interest
The mechanics of overpayment work as follows. Your standard monthly payment is calculated to repay both the interest charged that month and a portion of the outstanding capital, so that the balance falls to zero on the final payment date. When you make an overpayment — whether a regular additional amount each month or a one-off lump sum — the extra payment goes entirely to reducing the capital balance above and beyond the standard repayment.
This reduces the balance on which next month's interest is calculated, which in turn reduces the interest portion of next month's payment, which means more of your standard payment goes towards capital, which reduces the balance further — and so on throughout the remaining term. The compounding effect means that the true saving from an overpayment is always greater than simply the interest avoided in the month the payment is made.
As an illustration: on a £25,000 loan at 9.9% APR over ten years, a regular monthly overpayment of £100 from the outset would reduce the total interest paid by several thousand pounds and cut the loan term by two to three years. A single lump sum of £3,000 made at the end of year two would have a similarly significant effect. Your lender or broker can run these calculations for your specific loan on request.
Reduce Term vs Reduce Monthly Payment — Which to Choose?
When you make an overpayment on a secured loan, your lender will typically apply it to reduce the outstanding balance and then either reduce the remaining term (keep your monthly payment the same but end the loan sooner) or reduce your monthly payment (keep the term the same but lower your obligation each month). Different lenders have different default approaches, and some give you a choice.
Reducing the term is almost always the better financial outcome if you can afford to keep paying the same monthly amount. By maintaining your regular payment level, the loan is cleared sooner and total interest paid is minimised. Reducing the payment, on the other hand, frees up monthly cash flow but extends the period over which you pay interest, which costs more in total.
If your circumstances change and you need to reduce your monthly payment — for example due to a change in income — it is worth asking your lender whether your payment can be reduced on the basis of overpayments you have already made. This can provide a useful safety net: overpay while your income is strong, build up a buffer, and have the option to reduce payments if needed without having to remortgage or apply for a payment holiday.
