How much do you need to save for retirement? You can get an idea by using any of the dozens of retirement calculator tools offered free on the Internet.

But a recent study by actuarial experts on retirement forecasting shows that many popular calculators have serious flaws. These problems could lead to serious miscalculations when you’re plotting your retirement.

The report by the Society of Actuaries analyzed 12 retirement calculators created by financial services firms, software companies, nonprofits, and government for consumers and financial planning pros. All but one of the six consumer calculators were free–and they had a host of problems.

“These tools take a project that is fairly complex and boil it down to something simple,” says John Turner, an economist and co-author of the report. “They don’t ask you to consider a lot of important variables.”

So it’s buyer beware when it comes to online retirement calculators. Here’s a rundown of the key things to look out for; you can find a more detailed analysis in an article I wrote recently for CBS MoneyWatch.com.

1. Social Security Projections. Most retirees get a third or more of retirement income from Social Security. Yet many retirement calculators don’t gather the detailed information needed to project these benefits accurately, Turner says. “They often project Social Security income using a bare minimum of information: typically your current earnings, your age, and the year you expect to retire,” he says. The Social Security Administration offers the best projection tool, customized to your actual earnings history.

2. Rate-of-Return Assumptions. Three of the free calculators used pre-set future investment rate-of-return assumptions that you can’t change, and their percentages varied widely. One, created by the U.S. Department of Labor’s Employee Benefits Security Administration, assumed a 5 percent average annual return from 401(k)s; several others assumed 10 percent. If a calculator won’t let you choose your anticipated rate of return, either be sure you’re comfortable with its assumption or walk away.

3. Life Expectancy. It’s impossible to know how long you’ll live, of course. On average, 65-year-old men can expect to live another 17 years, and women another 20 years. Some calculators, the study found, automatically input life expectancy figures. But they fail to account for differences by race, income, and gender. And they also don’t take into consideration that you or your spouse might live longer than the averages.

If a calculator forces you to make a longevity prediction, base it on your family history and your health. If you’re married, use different life expectancy numbers for you and your spouse, since women tend to live several years longer than men.

4. Housing. The calculators make very different assumptions about what you’ll do with your house at retirement. “Some assume you won’t liquidate your home; others assume you will sell and downsize,” Turner says. Very few of the tools analyze the impact on your finances of carrying a mortgage into retirement.

Among the free calculators reviewed, only the U.S. Department of Labor calculator lets you plug in home equity when calculating your retirement assets.

5. Inflation. None of the free calculators — and few of the professional tools — listed inflation as a retirement-planning risk. Some of the tools let you plug in just one percentage forecast, even though inflation can fluctuate widely over time. Others put in their own default inflation rate, ranging from 2.3 to 4.6 percent. That spread can make a huge difference in how much the purchasing power of your assets will shrink over a 25-year retirement.

6. Spouses. Few of the free calculators helped couples forecast retirement income for a surviving spouse. They rarely let users enter separate information for both spouses and run numbers with differing life expectancies for them, for example. When the calculators recommended annuities for retirement income (most didn’t), none suggested buying one with a survivor’s benefit.

Some of the calculators allow for separate entry of data for each spouse, but even these typically assume that both people retire at the same time. Spousal issues regarding Social Security benefit claims can be complex — beyond the capability of any online calculator.

If you’re married, calculate retirement income needs for you and your spouse together and separately, using different life expectancy scenarios. This will help ensure that the one who lives longer won’t run out of cash. “Doing the ‘what-ifs’ can help you see just how differently things can turn out,” says Turner.

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