Auto Loan Per Diem & Payoff Calculator

How auto-loan per diem interest works

Many simple-interest auto loans accrue interest each day on the unpaid principal. A rough estimate is principal × annual rate ÷ day-count basis. Multiplying that daily interest by the number of accrued days gives the added interest between your last posted balance and payoff good-through date.

Worked example

On $18,500 at 7.9% with Actual/365, daily interest is approximately $4.0041. Over 14 accrued days, the additional interest is approximately $56.06. Without fees or other unpaid interest, the estimated amount due is $18,556.06.

Before paying off an auto loan

1. Request the lender's official payoff and ask how long it is valid.
2. Confirm the interest accrual basis and whether the good-through date is included.
3. Check whether recent payments have posted and reduced principal.
4. Include any permitted late charges, deferred interest, or unpaid fees only when they actually apply.
5. Ask how extra payments will be credited and how a lien release is handled.

For a general per-diem estimate, see the existing Payoff Per Diem Calculator. For transactions occurring between dates, use the dated-payment payoff statement instead of assuming the balance stayed constant.

Published November 2026 · Calculations run in your browser; no information is sent to Power of Interest by these tools.