The simple interest expense formula is Interest Expense = Principal x Rate x Time. r = The rate of interest expressed as a decimal. For example, 5% would be written as 0.05.
The formula is SI = P × R × T / 100, where SI is the simple interest, P is the principal, R is the interest rate, and T is the time in years. To find the principal in simple interest, rearrange the formula: P = SI × 100 / (R × T).
If a principal amount P is invested at an interest rate r for t years, then the simple interest earned will be I = Prt.