To effectively block those UV rays, you need 2 mg of sunscreen per square centimeter of skin, which—based on the concentration of sunscreen in foundation—works out to seven-times as much liquid foundation as you would normally wear and 14-times as much powder. Which isn’t to say that makeup with SPF doesn’t do anything, just that it’s not nearly enough. So wear (and reapply!) sunscreen every day, and never rely on your makeup to do the job on its own.
MORE: The Truth about SPF Makeup