Interest is generally defined as the fee paid for the use of money or funds over time. One thing to keep in mind about the power of interest is the types of interest and the amount of time that savings in an account will double in value. Individuals who put money into a savings account at a local bank or credit union will have interest applied to their funds. Types of Interest Interest on money in a bank savings account will be of two types. Individuals will typically see simple interest or compound interest. The amount of interest a saver sees is based on the percentage rate in effect for a specific program. Interest rates may change from time to time and are calculated based on a percentage of the deposit made by a saver. The saver will receive interest on payments according to a specific schedule. Savers who receive simple interest see their interest payment calculated as an assigned percentage of the original amount that was deposited. If a saver opens a savings account with simple interest calculated at 5%, he or she will earn five dollars in interest each year. A saver who decides to leave 100 dollars in his account for at least ten years will have 50 dollars in interest. Individuals who open a savings account with compound interest will see their money grow. There is a benefit to the saver as each new interest payment is added to the total for future interest payments. Many savers will see a larger account balance over time when compound interest is used. The money in an account will continue to grow each period interest payments are made. If a saver opens a savings account and deposits $100 at 5% interest, he or she will earn five dollars at the end of the first year. The five dollars is added to the account total and the interest payment that is paid the following year is five dollars and twenty-five cents. Rule of 72 This is a calculation used to determine when the money in an account will double. An account's rate of return is used to estimate the amount of time money in a savings account doubles. Money placed in a savings account at 4% interest will double in about 18 years.