Widely available on the Internet, a retirement calculator allows you to input important variables, such as your income, how much money you have in retirement savings, and your expected Social Security income, to see where you would be today if you were to retire, and how much you need to start saving in order to retire and live comfortably some years into the future. The actual results of your savings plan depend on how early you start saving, how much you contribute to your plan, the rate of return of your investment choices, and how long you will live.