If you know how much money you need in the bank to comfortably retire, you're in the minority: Only one in 10 people makes such a calculation, according to the Transamerica Center for Retirement Studies. That might explain why, on average, Americans are on track to replace 60% or less of their income during retirement. Financial advisers generally agree that retirees need to replace 80% or more.

That means someone who brings home an $80,000 salary at the peak of his working years should save enough before retirement to generate at least $64,000 a year in retirement. An investment, such as an annuity, that generates a 3% annual return would require savings of at least $2.1 million to throw off that sum annually. (Retirees can also supplement their income by continuing to work, as well as with Social Security payments and pensions.)

For those who fail to stash enough away in advance, the consequences can be dire: The federal government estimates that 12% of women and 7% of men over 65 live in poverty. Couples fare better than single seniors; the poverty rate is highest among divorced and widowed women, at 21% at 15%, respectively.

In fact, more than half of Americans report having less than $25,000 in savings and investments, according to the Employee Benefit Research Institute, a nonprofit research organization. Just 13% of workers now say they are "very confident" they will have a comfortable retirement. The first step to joining that more self-assured group is to figure out just how much money you'll need. Here are six easy ways to do just that:

1. Use a calculator. Online retirement calculators can estimate how much you should have in the bank before retirement. Figure out if you're on track, based on current savings rates, or if you need to ramp up. "That first calculation is as frightening as it is a good one to scare you half to death on how much you have to save if you live to 90," says Nobel Prize winner and Stanford professor William Sharpe. (Are you saving enough for retirement? Check MSN Money's calculator.)

2. Take a shortcut to generate a ballpark figure. John Ameriks, head of Vanguard's investment counseling and research group, recommends estimating the amount you need in retirement by multiplying your current salary by 12. "People shouldn't get too comfortable until they have a number that's 12 or more times their current salary, so $600,000 for $50,000," he says.

3. Save 18%. That's the savings rate a medium earner ($43,084 in 2010) would need if he or she starts saving at age 35 and plans to retire at age 68 (assuming a 4% return on investments), according to Boston College's Center for Retirement Research. The center issued a brief that provides savings rates based on a variety of factors, including retirement age, rate of return, income and the age that contributions begin. (The savings rates would allow retirees to replace 80% of their working salaries, and the calculations factor in Social Security income.) The Employee Benefit Research Institute reports that on average, employees contribute just 7.5% of their income to their retirement accounts.

The analysis found that the two most important factors for creating a retirement nest egg are one's savings rate and the age of retirement. "If people could work until they're 70, they would have a much higher chance of having a secure retirement. Social Security is higher if you wait until age 70, and it gives your 401k assets a longer chance to grow, and it reduces the number of years you have to support yourself," says Alicia Munnell, the center's director. Less important was the rate of return earned on investments.

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Magic number for retirement