The best extra payment is the largest one you can keep up without straining the rest of your budget. Here are four practical ways to land on a number, and how to pressure-test it.
The amount of extra you pay matters far less than how long you keep paying it. A modest overpayment made every month for years beats an ambitious one you abandon after the first tight month, because the early years are when extra principal cancels the most future interest. So the first question is not how much you could throw at the loan in a good month, but how much you can commit to in an average one, including the months with a surprise bill or a slow paycheque.
A useful test is to pick a number, set it aside automatically for two or three months as if it were already going to the loan, and see whether your budget still balances. If it does, that amount is genuinely spare and you can commit it with confidence. If those months feel strained, dial it back. An overpayment you never have to cancel is worth more than a larger one you stop and restart, both for the math and for the habit.
If a blank box is paralysing, anchor on a rule. Rounding up is the gentlest: push a $462 payment to $500, or a $1,140 payment to $1,200, and the rounding goes straight to principal without you noticing. The one-percent rule scales with the loan: add roughly one percent of the balance each month, which is meaningful on a large mortgage and trivial to absorb on a small loan. Both give you a starting figure in seconds.
Two more rules target the calendar instead of the payment. Adding one twelfth of your monthly payment to every payment quietly contributes one extra full payment a year, the same trick that makes biweekly schedules work. Alternatively, make a single extra payment annually from a bonus or tax refund and mark it principal-only. Whichever rule you choose, put the figure into the calculator on the home page to see the actual months saved and interest avoided for your loan, since the right rule is the one whose result you are happy to sustain.
Extra payments are not linear in their payoff. Because interest is charged on the outstanding balance, the principal you retire early avoids interest for every remaining month of the loan, so a dollar paid in year one of a long loan cancels far more interest than the same dollar paid near the end. This front-loading is why even a small, steady overpayment started today usually beats a larger one you plan to begin once things calm down.
It also means you do not need to find a large sum to see a real result. On a typical loan, a small monthly extra can move the payoff date forward by a noticeable margin and save a meaningful amount of interest, and the saving grows quickly as you raise the amount. The practical takeaway is to start with whatever you can sustain now rather than waiting to afford an impressive number later, then raise it whenever your budget allows.
Treat your extra payment as a dial, not a switch. Whenever your income rises or a competing expense ends, such as a paid-off car or a finished subscription, redirect some of that freed-up money to the loan and your payoff date pulls forward again. Reviewing the figure once or twice a year is usually enough to keep it matched to what you can comfortably afford without turning it into a chore.
There is also a sensible ceiling. Overpaying makes the most sense after you hold a small emergency fund and after any higher-rate debt is cleared, because money tied up in a low-rate loan is hard to get back in a pinch. If a loan carries a prepayment penalty, or your rate is very low and you have better uses for the cash, a smaller extra payment may be the right call. The point is to choose deliberately, not to assume more is always better. This is general information, not financial advice.