In retirement, employees will need 15.7 times their final pay when factoring in inflation and postretirement medical costs, according to a study by Hewitt Associates.

Though this estimate is on target with the human resources consulting and outsourcing company's prior projection, in 2008, many employees find themselves in a tougher financial situation because their retirement accounts have shrunk over the past two years.

Indeed, four out of five Americans are expected to fall short of meeting all of their financial needs in retirement unless they improve their savings habits or retire at a later age.

Of the 15.7 times final pay, Hewitt estimates that Social Security will provide 4.7 of it, leaving employees responsible for making up the remaining 11 times final pay. This will likely have to come from sources such as company-provided plans and personal savings.

Of the more than 2 million employees at 84 large U.S. companies it examined, however, Hewitt's study found that just 18% of these people who are expected to work a full career will meet this goal.

On average, employees are projected to accumulate 13.3 times their final pay with Social Security, leaving a shortfall of 2.4 times pay.

Byron Beebe, U.S. retirement market leader of Hewitt Associates, said companies that auto-enroll into a retirement plan have a 90% participation rate on average, but one problem is that employee inertia tends to set in.

"If employees are enrolled at 3%, we'll find that three years from today they will still be at 3%," Beebe said. "That's why we have been talking to more and more employers about auto-escalation."

Employees will often stop increasing their contribution once they reach the company's maximum match level, Beebe said. This often means that workers are stopping at 6%, for example. Hewitt recommends a double-digit contribution level.

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