(FORTUNE Magazine) – Maybe it's denial. Maybe it's fear. Maybe it's just the thought of spending a Saturday afternoon staring at spreadsheets. Whatever the reason, studies show that most Americans haven't even attempted to figure out much money they'll need to maintain a comfortable retirement. But since the alternative--blind faith--is a risky strategy on which to stake one's well-being, now is a good a time to start thinking about your Number. So let's get to it.

We'll begin with my rule of thumb. Take your current income and multiply it by one of the following: 40 if you are 39 or younger; 30 if you're between 40 and 49; and 25 if you're 50 or older. The result is an estimate of the nest egg you'll need in order to keep living on 100% of your pre-retirement income, assuming you stop work at age 65 and live to 90. For instance, a 45-year-old making $100,000 today would need roughly $3 million, from all sources, by the time he hit age 65.

Yes, that's a lot of cash. But consider a few things. First, these are future dollars: Our 45-year-old still has 20 years of compounding returns to go (and presumably a salary that will keep growing by at least the rate of inflation) before he needs to hit $3 million. If, say, he'd already saved up $200,000 in a 401(k) by 45 and socked the maximum allowable each year into his plan ($13,000 for 2004), got a company match, and earned an average annual return of 8%, he'd amass about two-thirds of his target by age 65.

Second, that was just a rough calculation--a little something to get your juices flowing. How much you'll actually need for retirement depends on lots of factors, including the rate of return you receive on your savings after you've retired, your life expectancy, and, of course, the lifestyle you intend to lead in your golden years. To get a more precise handle on your Number, check out an online retirement calculator; you'll find a pretty good one at www.troweprice.com.

Using our example, if our 45-year-old wants to enjoy 100% of his pre-retirement income in retirement, he'll require a nest egg of about $3.3 million by age 65. (That's assuming annual inflation and salary increases of 3%, and an annual return of 6% on his savings in retirement.) But if he needs only 80% of his pre-retirement income, then his savings target gets lowered to $2.6 million. And what if he thinks he can swing retirement on 60%? Then he'd need a mere $1.9 million.

In fact, all it takes is a little tweaking of assumptions to transform a dire situation into a manageable one. So experiment with various scenarios. But remember: For most people, if hitting your Number doesn't involve some serious saving in the here and now, your plan is too good to be true. --Janice Revell