You're Probably Using a Third of the Sunscreen You Need

Here's the uncomfortable truth about sunscreen: the SPF number on the bottle is measured in a lab using 2 mg of product per square centimeter of skin. Research on how people actually apply it finds most of us use closer to a third of that.

Because the relationship between amount and protection is exponential, applying half the sunscreen doesn't give you half the SPF — it gives you far less. Your 'SPF 50' can quietly perform like an SPF 15. That's not a reason to distrust sunscreen; it's a reason to use enough of it.

The rule of thumb for your face and neck: about two full finger-lengths of product (the 'two-finger rule'). For the body, roughly a shot-glass worth. And reapply every two hours in the sun, or after swimming or heavy sweating — no sunscreen lasts all day.

A daily facial sunscreen you actually like the feel of matters more than the highest SPF you can find, because you'll apply enough of it. For long sun exposure or the beach, a water-resistant body formula is the right tool.