Excel journeyman know you can plug in numbers and get formula results from a spreadsheet. Lesser known, however, is the ability to figure out what numbers are need to get a certain result, which works perfectly for retirement planning.
"Chief Excel Officer" Chandoo runs down how to use Excel's Goal Seek feature to create a kind of retirement target advisor. You'll fill out a spreadsheet with the financial status you seek to achieve by the time you're ready to put your hammer down, and leave one cell as the variable that tells you how much you'll have to set aside each week, month, or other time period to hit that big goal down the line. Goal Seek is useful in a number of other ways, of course, but the tutorial at Chadoo's Pointy Haired Dilbert blog uses retirement savings as a relatable framework for understanding how Goal Seek should be used in your own quest to find the Xs in your future equations.
Excel Goal Seek Tutorial - Learn how to use goal seek feature by building a retirement calculator in Excel [Pointy Haired Dilbert: Charting & Excel Tips]