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.

As baby boomers start retiring, more and more financial advice is being presented to them, some of it good, some not so good. One specific piece of advice that’s shown up more and more lately is this: if you retire early, delay drawing your social security payments so you can get a larger check when you do start drawing.

The Social Security Administration itself encourages this by sending out statements showing that if you start social security at 62 you’ll get about $1,500/month, whereas if you wait until you’re 66 (the “full” retirement age for most boomers) you’ll get around $2,100/month (this is a fairly typical example for a professional who has “maxed out” social security pay-ins; the same logic applies to everyone). Six hundred dollars a month extra just for waiting? Sounds tempting, doesn’t it?

Not so fast. Let’s look at two sets of people: those who can afford to retire early, and those who are forced to retire early (illness, layoff, etc.).

If you can afford to retire early, you probably don’t need your social security payments just to get by. What if you collected the payments for four years and invested them in a CD or a Canadian royalty trust? This would put you more than $70,000 ahead of someone who decided to delay social security until the age of 66. How long will it take them to catch up? It depends on factors like inflation, but it will be a minimum of 10 years and could be as many as 16.

You read that right. Someone who delays social security will not catch up to your accumulation (even ignoring the interest you might receive) until they’re 76, maybe 82. In the meantime they could, to put it gently, expire. For specifics on this, check out the retirement calculator.

What if you’re in that second set of people, and get laid off from your job? That extra $600/month could come in real handy. The problem is, to get it you have to go for four years with nothing. Social security was designed as a safety net. If you need it, use it. If you don’t, take it early and invest it. It doesn’t pay to wait.