Auto Loan Per Diem Calculator

What Is Per Diem Interest on an Auto Loan?

“Per diem” simply means per day.

Auto loan per diem interest is the amount of interest your loan balance earns each day. Lenders calculate it using three things:

• Your current loan balance
• Your annual interest rate (APR)
• The number of days in a year (usually 365)

Simple explanation

If your per diem is $4.25, your loan grows by $4.25 every day until your next payment posts.

That means:

• Paying late costs you more
• Paying early saves you money
• Paying extra reduces future daily interest

Why an Auto Loan Per Diem Calculator Is Useful

Most borrowers only look at their monthly payment. That hides what’s really happening between payments.

An auto loan per diem calculator helps you:

• See your daily interest cost
• Understand how interest builds between payments
• Estimate interest for a custom date range
• Test how extra payments change your payoff date
• Avoid surprises when requesting a payoff quote

This is especially helpful if you:

• Plan to pay off your car early
• Are refinancing or selling your vehicle
• Want to reduce total interest paid

How the Auto Loan Per Diem Calculator Works

The calculator you provided goes beyond a basic tool. It handles real-world loan behavior and multiple scenarios.

Here’s how it works step by step.

Step 1: Enter Your Loan Details

You start with the basics:

Loan Amount

The original amount you borrowed, such as $25,000.

Annual Interest Rate (APR)

Your stated yearly interest rate. Example: 4.5%.

Loan Term

The total length of the loan in months, from 12 to 96 months.

Loan Start Date

The day your loan began. This is important for calculating your current balance accurately.

Step 2: Choose a Calculation Type

This calculator offers three different calculation modes, which makes it more powerful than most.

1. Current Per Diem (Daily Interest Today)

This option shows how much interest your loan is adding right now.

What it calculates:

• Current loan balance
• Daily interest rate
• Per diem amount (daily interest in dollars)
• Days until your next payment
• Interest that will accrue before that payment

Why this matters
If your per diem is $6.10 and your payment is 10 days away, you’ll owe about $61 in interest before that payment even hits.

2. Interest for a Specific Period

This option calculates interest between two dates you choose.

You enter:

• Period start date
• Period end date

The calculator then shows:

• Days in the period
• Balance at the start
• Per diem at that time
• Total interest for the period

When this is useful

• Estimating interest between payoff quotes
• Checking lender calculations
• Planning a payoff or refinance window

3. Payoff Date With Extra Payment

This is the most strategic feature.

You enter:

• An extra monthly payment amount
• A target payoff date

The calculator shows:

• New monthly payment
• Original payoff date
• New payoff date
• Interest saved
• Whether your target date is achievable

If your goal is not reachable, it even estimates how much extra you’d need to pay to hit that date.

How Per Diem Is Calculated (Plain Math)

Here’s the simple formula behind the scenes:

Daily Interest Rate = APR ÷ 365
Per Diem = Current Balance × Daily Interest Rate

Example:

• Balance: $18,000
• APR: 5%

Daily interest rate:
5% ÷ 365 ≈ 0.000137

Per diem:
$18,000 × 0.000137 ≈ $2.47 per day

That $2.47 adds up every single day until the loan is paid off.

Payment Frequency and Payment Day Matter

The calculator lets you choose:

• Monthly
• Bi-weekly
• Weekly

It also lets you choose your payment day (1st, 5th, 10th, etc.).

Why this matters:

• Interest accrues daily, not monthly
• Longer gaps between payments mean more interest
• Changing payment timing can reduce total cost

This tool accounts for those timing differences automatically.

What the Results Section Tells You

After you click Calculate Per Diem, the results section shows:

• Loan summary
• Current balance
• Monthly payment
• Daily interest cost
• Accrued interest before next payment
• Clear analysis in plain language

You don’t just get numbers. You get context that explains what they mean.

How Extra Payments Reduce Per Diem

Extra payments help in two ways:

1. They reduce your balance
2. A lower balance means lower daily interest

Even a small extra payment can:

• Lower your per diem
• Shorten your loan term
• Save hundreds or thousands in interest

This calculator shows that impact clearly, without guesswork.

Who Should Use an Auto Loan Per Diem Calculator?

This tool is useful if you:

• Want to pay off your car early
• Are checking a lender’s payoff quote
• Plan to sell or trade in your vehicle
• Are comparing refinance options
• Like knowing where your money actually goes

If you have an auto loan, this calculator gives you clarity you don’t get from a monthly statement.