Simple interest is money you can earn by initially investing some money (the principal). A percentage (the interest) of the principal is added to the principal, making your initial investment grow! Simple Interest calculator is an online tool to calculate how much interest will be charged on a principal at certain interest rate for particular period of time. When money is borrowed or lended at simple interest, the interest will be charged on the original amount of money, and not on any interest that it has earned. The interest on principal remains same for every month or every year and it is calculated from the principal amount, simple interest rate and length of the time.