The rules of sun protection are simple and finite. Any FDA-abiding, outdoor-going, yearly mole-checking, skin-haver should know. Please follow these rules, outlined by ITG's routine skin saver Jordana Mattioli, for optimal health and happiness.

How much you apply

There's a science to it. The FDA requires sunscreens to be tested at 2mg/cm², which means two milligrams of sunscreen per square centimeter of skin. "It's pretty hard to measure how much skin you have on your face, which is why we say 1/2 teaspoon for your face and neck," Jordana explains. "I encourage everyone at least once in their life to actually measure out the exact amount one day, just to see how much that actually is." It'll feel like a lot if you're not used to it, so Jordana recommends a two step application process. Apply a little and wait a few minutes...then apply the rest.

What you're doing today

Oh, the reapplication question. Maybe the #1 question ITG editors get. The every-two-hour rule is important, particularly if you're going to be in direct sun for long stretches of time. "If you're inside for most of the day, then morning application—if you apply liberally—is enough," according to Jordana. "But if you leave work at 4PM, and are sitting outside after, then definitely re-apply." If you're going to a nice al fresco happy hour with coworkers, reach for a serum-y sunscreen that spreads easily over your previously applied skincare and makeup. (Sprays and powders that make application easier are hard to get up to that 2mg/cm² rule.)

The way you apply it

"Don't rub too much!" says Jordana. "We want most of the product to end up on your face and not your palms. Apply the sunscreen so that you see it on your skin, then massage it in just a little."

When you bought it

You'll find that most sunscreens list an expiration date unless...the company has conducted stability testing to prove that the formula will stay stable for at least three years. In that case, chuck it three years after purchase. Regardless, check around the package for that date at the start of every season.

Makeup is never enough

Well sort of. If your foundation has sunscreen in it, you still have to abide the 2mg/cm² rule, but it's "close to impossible to wear that much foundation." Your best bet is to find a sunscreen that works as a nice base under makeup. Apply enough, let it set, and then go forth with your makeup while trying not to disturb the sunscreen too much.

Anything else?

Oh there's plenty more. Jordana is nothing if not an exhaustive resource of skin knowledge. Make sure it's broad spectrum to ensure "protection from both UVA and UVB radiation"; if you're consistently getting burned, check the expiration and also your application/reapplication methods against this list; remember that sunscreen never means you're 100% protected—"Seek shade, wear a hat and sunglasses!" Chic!

Photo via Getty.

For more on sun protection, read what these other experts have to say.