How to Calculate per Diem on Auto Loan

Per diem on an auto loan refers to the daily interest charge on the outstanding loan balance. This calculation helps borrowers understand their daily financial commitment and track interest accumulation over time. In this guide, we'll explain the formula, provide a step-by-step calculation method, and include a practical example.

What Is Per Diem on an Auto Loan?

Per diem (Latin for "by the day") on an auto loan represents the daily interest charge based on the outstanding loan balance. Unlike fixed monthly payments, per diem calculations show how interest accumulates each day, which can be particularly useful for:

Understanding daily interest costs
Tracking interest accumulation over time
Comparing different loan options
Planning budget adjustments

Per diem calculations are most commonly used in short-term loans or when comparing different loan terms. For standard auto loans, monthly payments are typically more relevant.

Per Diem Calculation Formula

The basic formula for calculating per diem on an auto loan is:

Per Diem = (Daily Interest Rate × Outstanding Loan Balance) / 365

Where:

Daily Interest Rate is the annual interest rate divided by 365
Outstanding Loan Balance is the current amount owed on the loan
365 represents the number of days in a year (using 365 for simplicity, though some calculations use 360)

This formula provides the daily interest charge based on the current loan balance. The result is typically expressed in the same currency as the loan amount.

How to Calculate Per Diem

Step-by-Step Calculation Method

Determine the annual interest rate of your auto loan
Find the current outstanding loan balance
Calculate the daily interest rate by dividing the annual rate by 365
Multiply the daily interest rate by the outstanding balance
Round the result to two decimal places for currency

Key Considerations

When calculating per diem, keep these factors in mind:

Per diem calculations are most accurate when using the exact current balance
The calculation assumes a 365-day year (some financial institutions use 360)
For loans with compounding interest, the per diem amount may change daily as interest is added to the principal
Per diem is typically calculated on the unpaid balance, not the original loan amount

Example Calculation

Let's calculate the per diem for a $20,000 auto loan with a 5% annual interest rate and a current balance of $18,500.

Annual interest rate = 5% or 0.05
Outstanding balance = $18,500
Daily interest rate = 0.05 / 365 ≈ 0.000136986
Per diem = (0.000136986 × $18,500) ≈ $2.53

In this example, the per diem is approximately $2.53 per day. This means the borrower is paying about $2.53 in interest each day based on the current loan balance.

Note: The actual per diem amount may vary slightly depending on when in the month the calculation is made and whether the loan compounds interest daily.

Frequently Asked Questions

How often should I calculate per diem on my auto loan?: You should recalculate per diem whenever your loan balance changes significantly, typically after each payment or when the interest rate changes.
Is per diem the same as daily interest?: Yes, per diem refers to the daily interest charge on your auto loan. It's calculated based on the current outstanding balance and the daily interest rate.
Can per diem calculations help me pay off my loan faster?: Yes, understanding your daily interest costs can help you make more informed decisions about loan repayment and budgeting.
What if my loan has compounding interest?: For loans with compounding interest, the per diem amount may change daily as interest is added to the principal. You'll need to recalculate frequently.
Is per diem calculation the same for all auto loans?: The basic formula is similar, but some loans may use different assumptions about the number of days in a year or how interest is calculated.