How to Calculate Per Diem Interest

Written by Anytime EstimateFebruary 9th, 20222 minute read

Let’s begin this article with the meaning of “per diem”. Per diem is a Latin word for “per day”, or daily interest. This term is used in the finance industry to calculate the daily interest owed to the mortgage company at closing.

The bank will calculate interest on the loan from the day of closing until the end of the month. For example, let say you’re closing on the 1st day of October, you will pay the bank interest on the entire loan (mortgage) from the first day of the month until the last day of the month . . . 31 days . . . “per diem interest”.

If your settlement occurred on the 15th day of the month, you would pay the bank 16 days interest (count the day of closing), and if you closed on the last day of the month, you would owe the bank one day of interest on the loan.

Per diem interest calculation

The per diem interest rate is easy to estimate. We’ll use a loan amount of $100,000 for this example. The interest rate is 3% and there are 365 days in the year. Closing is on the 15th day of the month.

Step 1: Take the loan amount ($100,000) and multiply the loan by the interest rate (0.03) = $3,000

Step 2: Divide the total interest of $3,000 by the number of days in the year (365). This is your “per diem interest” = $8.22 per day. (Wow, that's how much the bank earns every day for $100,000 at 3%)

Step 3: Multiply the daily per diem cost of $8.22 by the number of days from the date of closing until the end of the month. The number of days is 16, count the day of closing.

The per diem interest paid at closing is $131.51.

Here's the mathematics for per diem. Too complicated? Use the Per Diem Calculator to estimate the monthly cost

Loan Amount
Interest Rate
Total Interest
Number of Days in a Year
Daily Interest Cost
Number of days owed
Total per diem interest paid
$ 3,000
365 Days
16 Days