Try the following steps -

- Divide the annual percentage rate (APR) of the mortgage by 365 to find the daily percentage interest rate. Suppose the APR on a mortgage is 4.38 percent. Divide by 365 and you get a daily interest rate of 0.012 percent (0.00012 in decimal form).
- Multiply the daily interest rate by the principal amount of the mortgage. If the mortgage is for $250,000 and the daily interest rate is 0.012 percent this works out to a per diem rate of $30.