So how much should you save? Here's a guide to finding a magic number for your own situation:
The more you make, the more you need to save, not just in dollars also but as a multiple of your final salary. The reason: Social Security payments will provide a smaller percentage of your desired income than they will for Fidelity's typical worker, so you will have to provide more.
Christopher Jones, the chief investment officer of Financial Engines, which provides services to 401k participants, says someone making $150,000 a year at retirement might need 12 times his salary to maintain his standard of living.
The Center for Retirement Research at Boston College took a crack at the problem in 2010. It estimated that people making $150,000 to $200,000 needed to save three times their salary at age 45 and 10.3 times their final pay at age 65.
Underscoring how much assumptions matter, the Boston College center used a higher real rate of return on investments than Fidelity did -- 4.5% versus 3.2%. The center found that retirees needed from four to 10 times their final pay, depending on whether their incomes were below or above average.
If you plan to retire early, you will need to save more -- and you might not have a choice in the matter. In another argument for saving while you are young and sticking with the plan, a survey earlier this year by the Employee Benefit Research Institute, an industry-funded nonprofit, found that half of retirees left work unexpectedly because of health reasons or layoffs.
Your lifestyle matters. The Fidelity model assumes you need to replace 85% of your final pay -- since you will save on payroll taxes, you won't need to add to your 401k, and your work-related expenses will decline. But if you have been saving a lot and your house is paid off, you might be able to live on even less, which will reduce your savings burden.
In addition, if you have been sending children to college and graduate school in your later years, you might find that retirement without those expenses is surprisingly cheap.
Other savings will help. If you have a pension plan, even a small one, it will reduce what you need to save. On the other hand, if you are a conservative investor who is unwilling to take some risks in the stock market, you will need a bigger magic number to compensate for a lower real return.
Retirement experts suggest that once you are in your 50s, you should consult a financial planner or experiment with online calculators to figure out the number you need to support the retirement you want. You will have to do a lot of guesswork, estimating your final pay, inflation and investment returns.
There are too many variables to accurately project the exact number. The best you can hope for is a high probability that you won't outlive your money.
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