How can I calculate the amount of interest on my own?

To determine the amount of interest you will be required to pay on each month, use the following formula called the Simple Daily Interest formula:

Simple Daily Interest Formula
Number of days since last payment
Principal Balance Outstanding
Interest Rate Factor
Interest Amount

Practice Example: Let's say the remaining balance on your loan is $9500.00. You sent in a payment of $160.00, 32 days after your previous month's payment. Your interest rate is 8.25% (interest rate factor is .00022587).

32 (days) x $9500.00 (PBO) x .00022587 (interest rate factor)

You would pay $68.66 toward interest and $91.34 toward the principal balance. This would leave you with a loan balance of $9408.66 after the $160.00 payment was applied.