How much money you need to retire comfortably is a question all working adults should ask themselves. Unfortunately, far too few ever bother thinking about it.

But calculating how much you'll need to have saved for retirement is pretty easy and doesn't take much time. Here's a quick, simple three-step approach that can help you find your magic retirement number.

The first step, estimating your expenses, is the trickiest.

If you want a quick ballpark estimate, figure 75 to 85 percent of your current gross income.

If you want a more precise estimate, track your expenses on a worksheet and deduct any costs you expect to go away or decline when you retire, and add whatever new ones you anticipate.

Costs you can scratch off include work-related expenses such as commuting or lunches out, as well as the amount you're saving for retirement. You also might be able to deduct your mortgage if you expect to have it paid off by retirement. Your income taxes also should lessen.

On the other hand, some costs probably will go up when you retire, such as health care, and depending on your interests, you might spend a lot more on travel, golf or other hobbies. If you are going to be retired for 20 or 30 years, you also need to factor in the occasional big budget items such as a new roof, furnace or car.

Step two is to calculate your retirement income. If you contribute to Social Security, estimate how much your monthly benefit will be at the age you want to retire. You can get a personalized estimate at www.ssa.gov/estimator. If you're married, remember to count your spouse's benefits, too.

If you have a traditional pension plan from an employer, find out how much you are likely to get when you retire. Figure in any other income from other sources you expect to have, such as rental properties, part-time work and etc.

The final step is to calculate the difference. Subtract your annual expenses from your annual income. If your income alone can cover your bills, you're all set. If not, you'll need to tap your savings, including your 401(k) plans, IRAs or other investments to make up the difference.

Send your senior questions to Savvy Senior, P.O. Box 5443, Norman, OK 73070, or visit SavvySenior.org. Jim Miller is a contributor to the NBC Today show and author of "The Savvy Senior" book.