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Garbage in, garbage out.

If you've got some computer power and some curiosity, it's easy to calculate how much you need to save for retirement, thanks to the proliferation of personal-finance software.Trouble is, these calculations are only as good as the numbers you plug in. The danger: You make dubious assumptions and thus end up saving way too much - or far too little.

Imagine, for instance, that you're a 30-year-old who's decided to get serious about retirement. You commit to socking away $4,000 every year and, because you plan to invest heavily in stocks, you reckon your investments will earn 10 percent annually.

According to Managing Your Money, the popular personal-finance program from MECA Software in Fairfield, Conn., you will have almost $1.1 million at age 65. Fat city? Maybe not.

Will you really earn double-digit returns?

It's doubtful. Raw stock-market returns could easily be higher or lower, though I suspect the long-run average won't stray too far from 10 percent. More importantly, however, the index returns don't reflect costs, like mutual-fund annual expenses and brokerage commissions.

In addition, if you use the 10 percent-a-year number, you're presuming that you'll invest everything in stocks. But most investors keep some of their money in bonds and cash, which hurts long-run performance.

"You've got to be realistic," says Jonathan Pond, an author and financial planner in Watertown, Mass.

So maybe 10 percent a year is unlikely. How about 8 percent instead? At that rate, your $4,000-a-year savings habit will yield some $690,000 at age 65, according to Managing Your Money. That's a lot less than $1.1 million, but it's nothing to sniff at.

What about inflation?

Unfortunately, because of the upward creep in consumer prices, $690,000 in 35 years isn't going to buy what it buys today. Suppose inflation runs at 3 percent a year. By the time you reach 65, your $690,000 will have the purchasing power of just $245,000.

Getting nervous? The news isn't all bad. Inflation will also boost your salary - and thus your ability to save. Let's say you increase the amount you invest each year along with the 3 percent inflation rate. At age 65, you'll have the equivalent of $340,000 in today's dollars.

If you manage to retire with that much in your pocket, you'll be doing a lot better than most folks. Sit back and relax? It's not over yet.

What if you live a long time?

At age 65, you can expect to live another 20 years, according to the actuarial tables. On that basis, Managing Your Money suggests you could spend over $23,000 a year, figured in today's dollars and supposing your money earns 6 percent annually after you retire. This also assumes you'll run out of money when the actuarial tables suggest you'll run out of breath.

Bad assumption, says Harold Evensky, an investment adviser in Coral Gables, Fla. "If you plan based on those actuarial tables, you're taking a 50 percent chance that you'll outlive your money."

So what should you do? At a minimum, Mr. Evensky suggests planning as though you'll live longer than 70 percent of all 65-year-olds. That means figuring that you'll live until 89 if you're a man and 93 if you're a woman.

Let's err on the side of caution, and assume you'll live until 93. That means your $340,000 will give you some $18,500 in annual retirement income. Not bad. But it sure isn't fat city.

What about taxes?

You want to stash away money without worrying about Uncle Sam. But once you start pulling cash out, the bills come due. What will the tax rate be? Your guess is as good as mine.

Sound like it's tough to save enough for retirement? You better believe it. But if you're serious about retirement planning, you might as well take your dose of realism now, because the taste sure doesn't get any better.