Claude Opus 4.8 vs GPT-5.6 Sol for everyday maths

Early in the mortgage, you owe the most money, so interest piles up fast on that large balance. Overpaying now shrinks that balance immediately, meaning every future month's interest is calculated on a smaller amount. That saving compounds year after year for the rest of the term.

In the final years, you owe very little, so there's barely any interest left to save, and hardly any time for the benefit to build up. Timing matters because interest feeds on both size and duration.

Mortgage interest is charged on the amount you still owe. Overpaying early reduces that balance for many years, so you avoid interest on the overpaid amount throughout most of the mortgage. It also means more of each later payment goes toward clearing the remaining debt rather than interest. In the final five years, the balance is already much lower and there is little time left for interest to build up, so identical overpayments save far less.