Why the day you pay can matter

Illustrative loan. Daily interest = $18,000 × 0.069 ÷ 365 = $3.4027, shown rounded to $3.40; running totals use the unrounded figure. Day 0 is the day of the last payment, and the next one is due on day 30.

Read down the third column and it simply climbs by the same amount each day. That's all simple interest is: the balance, times the rate, times the days that passed. When a payment arrives, the lender takes whatever interest has built up since the last one, and the rest of the $420 reduces the balance. Pay on day 24 and $338.33 goes to principal. Pay on day 30 and it's $317.92. Pay on day 35 and it's $300.90.

That looks like a strong argument for paying early. It is a weaker one than it looks, because the story doesn't end at one payment.

Following it into the next month

Paying early shortens this period but lengthens the next one. If the following payment is still due on day 60, a day-24 payment leaves a 36-day gap before it, and the interest on that gap takes back almost all of what you saved. Here are three two-payment sequences on the same loan:

Paying six days early, once, saved fifty cents. Paying five days late cost thirty-eight cents in extra interest. Neither figure is worth a thought on its own.

Shifting the whole schedule is different. If you moved every payment six days earlier for good, day 24, 54, 84 and so on, then after six payments you'd have paid $564.24 in interest instead of $585.24, and the balance would be $16,044.24 instead of $16,065.24. That $21.00 is essentially six days of interest, collected once, when you moved the schedule. After that the two patterns run in parallel, each period 30 days long, and neither pulls further ahead.

Where you'll meet the daily figure again

The same $3.40 shows up, under a different name, on a payoff quote. When you ask a lender what it would take to close the loan, the answer usually comes with a date the figure is good through and a per-diem amount: the interest that gets added for each day past that date. That per diem is the second column of the table above, worked out on your own balance. It's why a payoff quote from last week is already slightly wrong, and why a check that arrives three days after the good-through date can leave a small balance open. Ask for a quote dated to the day your payment will actually arrive, not the day you send it.

Two caveats worth checking in your contract

First, not all loans are simple-interest loans. With precomputed interest, the finance charge was fixed when you signed, and paying a few days early or late generally doesn't change the interest at all, though late fees still apply. Your contract states which method it uses, usually near the prepayment terms.

Second, lenders don't all count days the same way. We used a 365-day year; some loans use a 360-day year or another convention, which nudges the daily figure slightly. The shape of the table stays the same.

If you only take one number from this page, take $3.40. That's what each day cost on this loan. On yours, the figure is your balance times your APR divided by 365, and once you know it, you'll know exactly how little the calendar matters, and how much a missed due date does.