As you approach retirement, it's easy to become fixated on the magic number--a pot of money large enough to allow you to retire comfortably without outliving your savings. But figuring out whether you can afford to retire requires math, not magic, along with a thoughtful analysis of how you plan to spend your time and money.

Plenty of online calculators will help you figure out whether you can afford to retire based on the amount of money you'll need to replace a specific percentage your current income. A popular rule of thumb suggests that you should plan on replacing 70% of what you currently make, or 80% if you want to live large.

But this guideline is deeply flawed, financial planners say. We'll walk you through the steps to come up with a realistic estimate of how much money you'll need to retire in style.

What's Wrong with the 70% Rule?

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During their early years in retirement, many retirees end up spending as much as or more than they did when they were working, says Jennipher Lommen, a certified financial planner in Santa Cruz, Calif.

However, if you were to move to a lower-cost area, say, or stop supporting adult children, your living expenses could drop in retirement. When you retire matters, too: