Per diem (daily) interest While simple interest is generally simple to calculate over the life of a loan or investment, it can also be useful to know how much interest is accruing on a daily, or per-diem, basis. To calculate per-diem interest, take the interest rate (be sure to express it as a decimal, so 10% becomes 0.10) and divide by 365 to determine the daily interest rate. Multiplying this amount by the principal will result in your per-diem interest. Here it is written as a mathematical formula: