Get the loan balance
Find the annual interest rate
Convert the annual rate to a monthly rate by dividing by 12
Multiply the loan balance by the monthly interest rate
Use the result as the interest due for that month
For an amortizing loan, subtract the monthly interest from the total payment to find the principal paid
Reduce the loan balance by the principal paid
Repeat the calculation each month using the new balance
For daily interest, divide the annual rate by 365 and multiply by the current balance and number of days
Use the mortgage statement or amortization schedule to verify the calculation