Odds are that you’ll retire some day. When you do, you’ll likely have a couple of income sources (social security, company pension) and some savings (bank account, IRA, 401k, 403b, etc.). Will this combination of resources allow for a comfortable retirement? HOW comfortable? For how long? What if inflation takes off, or the market is stagnant?

Wrestling with these questions myself, I finally put together a spreadsheet that will help to answer them. This spreadsheet will tell you how long your nest egg will last under various scenarios of inflation, investment returns, and the like. Just change the numbers in the shaded cells and immediately see the results.

For flexibility, the horizontal axis of the spreadsheet shows “years in retirement” running from 1 to 40, rather than “age.” So whether you plan to retire at 55 or 75, the spreadsheet will show the size of your nest egg after 1 year, 2 years, etc. The Social Security Administration will tell you what your monthly benefits will be, so you can plug that number in. You can then enter your estimated savings when you retire, your monthly living expenses, and so forth. When you plug the numbers into the spreadsheet, the graph will immediately change to show you when your savings will run out.

Here’s a specific example. You learn from the SSA that your monthly benefit starting at age 62 will be $1,500. You plan to have a nest egg of $500,000. You estimate that your monthly expenses will be $5,000, inflation will run at 3%, and you can earn 6% on your savings. Plug those numbers in and the chart shows that your nest egg will run out in 16 years, at age 78. Oops, that’s a little early. Drop your monthly expenses to $4,000 and the chart now shows you’re good to age 87. Much better.

The spreadsheet is fairly sophisticated, in that it uses the “present value” function to show you future results in today’s dollars. Please let me know if you find it useful, and be sure to report any bugs.