If you are retired or are nearing retirement, the main questions on your mind are probably "Will I run out of money in retirement?" and "Will I be able to maintain my standard of living?" For answers, people often turn to free online retirement calculators that gauge how much users will need to save to achieve their retirement objectives, based on details about their finances.

But how well do these calculators account for the inherent risks in retirement, such as how long you will live, how your investments will perform, what the inflation rate will be, and health care and long-term care costs? Not very well, according to a 2009 study by the Pension Research Council.

"We conclude," the study's authors write, "that on the whole, the tools do not highlight nor address retirement risk particularly well; rather, they mainly mask risk."

The authors, retirement experts Anna M. Rappaport and John A. Turner, reviewed the available research on five leading Web-based calculators to see how they handle post-retirement risks. The calculators they looked at were Fidelity's Retirement Income Planner, AARP's retirement planning calculator, MetLife's calculator, the U.S. Department of Labor's calculator and T. Rowe Price's Retirement Income Calculator.

In their working paper "How Does Retirement Planning Software Handle Post-Retirement Realities?" Rappaport and Turner conclude that while the calculators "can provide a rough idea of whether the user is on target for retirement," all inadequately assess the risk of running out of money.

For example, one calculator determines income sufficiency based on average life expectancy and overlooks the very real chances of living longer than the average. Another assumes that everyone, even if not married, receives the same Social Security benefits. Several do not permit calculations to take spouses into account. Among the authors' other findings:

• None of the consumer calculators they evaluated treat inflation as a risk, instead assuming that inflation is constant over the retirement period analyzed.

• None treated expected medical and long-term care expenses as a risk factor or alerted users to the potentially huge impact such expenses could have on retirement plans.

• Few have checks on inconsistent or outlandish assumptions. For example, many programs permit the user to specify long-term risk-free rates of return of 10 or even 20 percent.

• Some calculators do not ask users to indicate expected inheritances or other one-time receipts of assets, and some do not include the value of housing as a source of retirement income.

• Several of the programs ignore taxes, leading users to conclude that they have more retirement resources than they actually do.

• The calculators cannot take account of extreme events such as the recent financial crisis, in which housing values have fallen and mortgage rates have risen -- at the same time that people are losing jobs.

The authors note that "consumers or financial professionals working with them could benefit from trying alternative programs and scenarios within each program."

The study also looked at retirement planning software for financial planning professionals. The authors concluded that while these tools are more complex than their consumer counterparts, they still contain flaws.

To read the Pension Research Council's working paper "How Does Retirement Planning Software Handle Post-Retirement Realities?" click here. (Free sign-up required.)