SPF ratings are calculated very carefully, and based on a certain amount being applied during testing. If you’re being too skimpy with sunscreen, then the SPF number on the bottle isn’t going to do much for you. Most of us (it’s estimated that between 50-60% of us) are way too skimpy with sunscreen.
How much is enough? For the face, 1/4 to 1/3 of a teaspoon will do the job. For the uncovered parts of the body (think bikini) you need a whole shot glass or about 2 tablespoons to get adequate protection. That’s right, a full shot glass! Remember to apply BEFORE you go out, by about 20-30 minutes, applying once you’re already at the beach is too late.