Last, the model assumes the saver will start retirement by withdrawing about 5% of savings, a higher drawdown rate than the 4% usually recommended.

In reality, says Beth McHugh, Fidelity's vice president of market insight, the multiple you need to save "will vary."

While the model is perhaps simplistic, Fidelity's attempt to quantify a magic number is a worthy exercise because it gets people thinking about savings and encourages young people to start saving early, says Steve Utkus of the Vanguard Center for Retirement Research, an arm of financial-services giant Vanguard Group. The center takes a different approach, urging savers to put away 12% to 15% of their income every year, including employer matches, to reach their retirement goals.

You're working hard to save for a decent retirement. So wouldn't it be nice to have a simple rule of thumb to measure whether you're saving enough?

In September, Fidelity Investments, the nation's largest provider of 401k plans, tried to do just that, offering up a magic number: eight. Typical wage earners, Fidelity said, should aim to save at least eight times their final annual pay to be sure they can afford basic living expenses in retirement.

The financial-services company also offered benchmarks to measure your progress along the way. By age 35, your goal is to save an amount equal to your annual pay. By 45, you will want to have saved about three times your salary, rising to five times your salary by 55.

There is a catch, of course: Most of us aren't typical. And for above-average earners, those numbers might be far too low.

To come up with its formula, Fidelity had to make numerous assumptions. Its "typical" worker began saving 6% of his earnings at age 25, gradually increased that to 12% after six years and continued saving that amount each year until retiring at 67. (An additional employer match helped, too.)

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Retirement #: Is 8 enough?