Well done, ladies and gentlemen.

Yes, I'm talking to you -- you owners of Priceline.com (NASDAQ:PCLN), up 141% over the past 12 months. And you, too, buyers of Juniper Networks (NASDAQ:JNPR), up 104%. And same to Mosaic (NYSE:MOS) shareholders, who have more than tripled their money in a year (a return of 205%, to be precise).

Those stocks have been jaw-dropping investments, doubling or more over the past 12 months. So nice work! But while you're throwing back the bubbly and trying to stay on top of the market, let me ask you:

  • Given the value of your entire portfolio -- not just your market-crushing picks -- when will you be able to retire?
  • How much income will your portfolio produce in 10 years? Twenty? Tomorrow?
  • Given that you'll have to sell investments to pay your retirement bills, how do you know your portfolio will last as long as you do?
  • How will you keep taxes from devouring your gains when you do sell?

Earning great returns on your stocks is just one part of the picture. Unless you're investing for the fun of it, at some point you're going to want to convert those returns into cash -- whether to buy that vacation home, that education for a kid or grandkid, or just to pay the bills in retirement. And you must know now whether enough money will be there, and the smartest way to access it.

Run your numbers
Knowing whether you'll have enough to do all you want isn't something you can figure out by scribbling some numbers on a napkin. You need something that will factor in all the moving parts of any financial plan -- salary increases, bigger contribution limits to retirement accounts, taxes, Social Security (if it'll be there and what will happen if it's not), pension income, and tapping home equity.

No, for all that, you'll need some heavy computing power. You can start by checking out some of the free financial calculators on the Internet. That's a good first step.

Unfortunately, as with many things in life, you get what you pay for; many of these calculators don't factor in everything that goes into determining whether you'll have a retirement of abundance or subsistence. Go ahead and try three calculators; you'll get three different answers (probably very different answers).

That's why the Fool's Rule Your Retirement team is offering the "Plan the Perfect Retirement" online seminar, starting this week. The eight-lesson seminar will help subscribers analyze all their sources of retirement income and determine when they can retire and how to make their money last. All of this will be done with the help of a sophisticated financial-planning tool that is far more useful than the standard retirement calculator. You can give the tool a try, and reserve your cyber-seat at the seminar, with a 30-day free trial of Rule Your Retirement.

You need a plan
Whether you give our tool a try or not, you absolutely must determine if you're on track. Try a bunch of retirement calculators and average out the results. Develop a spreadsheet. Use any tools offered by your broker or financial software, such as Intuit's Quicken or Microsoft Money. That will put you ahead of the vast majority of Americans, since study after study has shown that the average U.S. worker has no clue how much he'll need to achieve his financial goals.

Next, decide what you'll do about those outstanding investments. You're a smart investor, so I don't need to tell you that outstanding investments don't always stay outstanding.

While some stocks have doubled over the past years, others -- such as NutriSystem (NASDAQ:NTRI), US Airways Group (NYSE:LCC), and Christopher & Banks (NYSE:CBK) -- are down by a third or more just one year after doubling. It's a reminder that no investment will double every year -- and what goes up, might come down.

So, while you should revel in your market-spanking picks from the past year, that's just one part of your current net worth, at one point in time. Knowing what your future net worth will look like when you want to spend it is the way to make sure your investments really pay off.

In other words, you must have a plan.

This article was first published Feb. 20, 2007. It has been updated.

Robert Brokamp is the editor of the Motley Fool Rule Your Retirement service. Take a 30-day free trial by clicking here -- and get a plan. Robert does not own any of the companies discussed in this article. Priceline is a Motley Fool Stock Advisor recommendation. Microsoft is an Inside Value selection. The Fool has a disclosure policy.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.