Simple interest is the most straightforward method of calculating interest on a principal amount. Unlike Compound Interest, where interest is earned on previously accumulated interest, simple interest is calculated only on the original principal. The formula is: I = P x R x T, where I is the interest, P is the principal, R is the annual rate (as a decimal), and T is the time in years.

Simple interest is commonly used for short-term loans, auto loans, some personal loans, and certain types of bonds. It is also used to calculate the interest penalty on late payments and tax liabilities. Because interest does not compound, simple interest results in a lower total cost compared to compound interest over the same period and rate.

For example, if you invest $10,000 at a 5% annual simple interest rate for 3 years, the interest earned is $10,000 x 0.05 x 3 = $1,500, giving you a total of $11,500. The daily interest on this investment would be $10,000 x 0.05 / 365 = $1.37 per day, and the monthly interest would be $10,000 x 0.05 / 12 = $41.67 per month.