Per Diem Interest on a Car Loan
What is per diem interest on a car loan?
Per diem interest is the interest that accrues daily on a loan's outstanding balance, calculated as the annual rate divided by 365 days. It's why a payoff amount is only accurate for the day it's quoted, and why lenders issue 10-day payoff quotes that lock the figure for a set window instead of letting it drift. On a $16,000 balance at 21.6% APR, per diem interest runs about $9.47 a day.
Key takeaways
- Per diem interest is daily interest: the annual percentage rate divided by 365, applied to whatever principal balance is currently outstanding.
- It's the reason a car loan's payoff amount is higher than the balance printed on your last statement, and why the payoff figure changes every single day.
- A 10-day payoff quote exists specifically to lock a moving per-diem number in place for a set window, so a refinance, trade-in, or private sale can rely on a fixed figure.
- On a simple-interest loan, per diem interest is also the mechanism behind why paying a few days early, or paying extra principal, genuinely reduces the total interest owed.
- On $16,000 outstanding at 21.6% APR, per diem interest is about $9.47 a day; the same balance at 11.43% APR is about $4.38 a day — the rate changes the daily cost directly.
What is per diem interest?
Per diem interest is the interest that accrues on a loan's outstanding balance for a single day, calculated by dividing the annual percentage rate by 365 and applying it to whatever principal is currently owed. "Per diem" simply means "per day" — the term describes the unit the interest is measured in, not a separate kind of loan.
On most auto loans, interest is charged this way: daily, on the declining balance, rather than as one lump finance charge fixed at signing. That structure is what's usually meant by simple-interest amortizing financing, and per diem interest is the mechanism doing the daily work inside it.
Why does my payoff amount keep changing?
Because per diem interest keeps accruing between billing cycles, even on days you don't make a payment.
| What it shows | How current it is | |
|---|---|---|
| Statement balance | What you owed as of your last billing date | Can be several weeks old |
| Payoff amount | Statement balance plus per diem interest accrued since then | Accurate for one specific day |
The gap between those two numbers is entirely per-diem interest. It's a small amount on any single day, but it adds up the longer you wait between your last statement and the day you actually pay the loan off. Full detail on this gap is in payoff amount and 10-day payoff.
What does per diem interest actually look like in dollars?
Smaller than most borrowers expect on any given day, but it moves directly with both the balance and the rate. Two illustrations on a $16,000 outstanding balance, computed at this site's approved reference rates:
| Outstanding balance | APR | Per diem interest |
|---|---|---|
| $16,000 | 21.6% (deep-subprime used average, Q1 2026) | about $9.47/day |
| $14,000 | 11.43% (used-market average, Q1 2026) | about $4.38/day |
Per diem = APR ÷ 365 × outstanding balance. Illustrative balances chosen to show the mechanic.
Over a 10-day window, that first figure adds up to roughly $94.68 in accrued interest alone — which is exactly why a payoff quote issued 10 days ago is no longer the right number to pay today.
Why do lenders quote a 10-day payoff instead of the running total?
Because a per-diem figure is a moving target, and nobody transacting on it wants the number to shift mid-transaction. A 10-day payoff locks the payoff amount for a fixed window so a refinance lender, a dealer handling a trade-in, or a private buyer can rely on one number instead of a figure that changes daily.
Does per diem interest mean paying early actually saves money?
Yes, on a simple-interest loan. Every day you pay off principal earlier is a day the loan stops accruing per-diem interest on that portion of the balance. It's a small amount for any single early payment, but the same mechanism is what makes extra principal payments meaningfully reduce total interest over the life of a loan — see amortization vs. simple interest for the fuller math.
The exception is a precomputed-interest loan, where the finance charge was fixed at signing rather than accruing daily. On that kind of contract, paying early doesn't reduce interest the same way, because there was no per-diem calculation running in the background to begin with.
Common questions
How is per diem interest calculated on a car loan?
Take the annual percentage rate, divide it by 365, and multiply by the outstanding principal balance. That gives the interest accruing for a single day, which is added to what you owe until the next payment is applied.
Why does my payoff amount go up every day I wait?
Because per diem interest keeps accruing on the balance you still owe, even between billing cycles. A payoff quoted today will be a few dollars to a few hundred dollars lower than the same payoff quoted a month from now.
Why do lenders quote a 10-day payoff instead of just the current balance?
Because per diem interest makes the true payoff a moving target. A 10-day payoff locks that number in place for a set window, so whoever is paying it off — a new lender, a dealer, a private buyer — can rely on a figure that won't shift before the check clears.
Does paying a car loan off a few days early actually save money?
On a simple-interest loan, yes, by exactly the per-diem amount for each day saved. It's not a large sum on a single loan, but it's the same mechanism that makes extra principal payments worthwhile over the life of a loan.
Does per diem interest apply to every car loan?
It applies to simple-interest loans, which is most auto financing. Some subprime and buy-here-pay-here contracts use precomputed interest instead, where the finance charge is fixed at signing and doesn't accrue daily the same way.