The study analyzed 77 large U.S. employers, representing 2.1 million employees, and projected that the average worker will need to save 11 times their final pay at retirement (age 65) to keep their preretirement lifestyle.

Of course that varies, the firm said, depending on each worker's individual circumstances, including age, income, anticipated retirement age, and Social Security.

That said, only one out of five workers is on track to meet or exceed retirement income needs once they hit age 65.

Another 20 percent will find themselves close to what they need—but their retirement lifestyle will have to accommodate some cutbacks or they'll run out of money.

So that's 40 percent of workers.

The other 60 percent will be stuck. They won't have enough money to retire till they're 68—the median age at which Aon says U.S. workers will be able to retire with sound financial security.

But—and here's a scary thought—16 percent won't even have enough to retire if they keep working till age 75.

Part of the problem is lack of planning, according to the company.

A separate Aon Hewitt survey found that only a little more than half of workers (54 percent) have even estimated their retirement needs, figured out how much they will need to save, or calculated how much income they might need during retirement.

And just 40 percent of workers have created a financial plan to achieve their retirement goals.