Math might not be everyone's favorite subject, but I recently learned some arithmetic from a colleague that I just had to pass on to you!
When scientists test facial powders to determine SPF (as mandated by the FDA), they typically assume that 2mg of product will be used per cm2 of skin. The average face is about 600cm2 (although that varies from person to person, of course), meaning that a person needs to apply about 1.2g of facial powder to get the SPF stated on the product's label.
So what's the problem? Studies show that most women only apply about 0.085g of powder at a time. In a nutshell, you'd have to apply 14 times the amount of powder you usually do to be sufficiently protected against the sun! Even if you were willing to cake on your makeup, there's no way to pile on that much powder. Bottom line: While it's never bad to use products with SPF, your facial powder is simply not offering enough protection against UV waves to use it in lieu of sunscreen.
And don't ditch that SPF powder entirely -- it might not be enough protection against the sun, but it's a great way to minimize shine. By the way, many powders contain small levels of SPF, but most reputable companies don’t put an SPF on the label because they do not want to mislead you that the powder is adequate sun protection.