Enter the principal amount in dollars and cents. Enter the annual rate as a whole number, for instance, 3 is the equivalent of 3% (while it IS legitimate to enter a decimal in the case of 3.5 is 3 and a half percent). The time is typically in years but you may enter any accumulative combination of years, months and days. The number of days in a year is not a factor unless the days field is greater than zero (0). Click the Calculate button; read the simple interest. You may click on Clear Values and do another. Based on the previous instructions, you MAY enter a decimal for data in any of the fields but do not enter any commas or dollar signs (, or $).