Why does paying earlier sometimes reduce interest?

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Briefing

Two fictional accounts each pay two hundred dollars during the same thirty-day period. One payment posts on day five, the other on day twenty-five. Their interest charges are different: sixteen dollars and thirty-one cents versus eighteen dollars and ninety-four cents. The reason is not a bigger payment. The earlier payment leaves a smaller interest-bearing balance in place for more days.

Both accounts begin at one thousand dollars. We use twenty-four percent nominal annual interest divided by three hundred sixty-five, with no grace period, fees, new purchases, or daily compounding. Interest accumulates unrounded and posts once at the end. Our key timing convention is precise: a payment posts before the balance is measured for that day. Sending a payment is not automatically the same event as having it credited.

For the day-five payment, days one through four still carry one thousand dollars. On day five, the two-hundred-dollar payment reduces the measured balance to eight hundred. Days five through thirty therefore contribute twenty-six days at eight hundred. Four days plus twenty-six days is thirty. This is why we do not count five full days at the original balance: our payment-posting convention includes day five at the reduced balance.

For the day-twenty-five payment, days one through twenty-four carry one thousand dollars. Days twenty-five through thirty contribute six days at eight hundred. The same two hundred dollars has reduced principal in both accounts, but the later account spent twenty extra days at the higher balance. Nothing about this comparison changes either account's required due date. Those obligations would need to be checked separately.

Multiply each day's balance by the daily rate and add the results. The early model is four times one thousand plus twenty-six times eight hundred, all multiplied by zero point two four and divided by three hundred sixty-five. It rounds to sixteen dollars and thirty-one cents. The later model uses twenty-four times one thousand plus six times eight hundred and rounds to eighteen dollars and ninety-four cents. The rounded difference is two dollars and sixty-three cents.

The saving is specific to these assumptions. It comes from two hundred dollars being absent for twenty additional days, not from a universal day-five trick. If no interest is charged because a purchase grace period applies, this particular comparison is not the relevant explanation. Actual posting cutoffs, rates, calculation methods, and other activity can change the result. Earlier payment also needs to be feasible; a model cannot establish available funds.

Payment timing can affect interest where daily balances apply. Check when the payment is credited, which balance bears interest, and how the issuer counts it, while meeting required deadlines. This is general education, not individualized financial advice. The next briefing separates paying an old balance from adding new purchases.