Richard Kent Green, a professor of real estate, finance and economics at George Washington University, looked at renting vs. buying when he decided where to live. Green said the active rental market in the Washington area can help you find a rental property to compare with a potential purchase. The key is that the properties are comparable in size, neighborhood, age and all other factors.

For Green, the magic number is six at current interest rates.

Here’s what that means: Find a rental property that’s comparable to the property you’re thinking of buying. Figure out how much it costs to rent annually; if the rent is $2,500 a month, you’re spending $30,000 a year, or 6 percent of $500,000.  So by Green’s math, if the house costs $500,000 or less, it’s a decent bet.

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