You know to apply sunscreen on the reg, so one beach day you slather it all over from head to toe—only to find that you’ve become a full-on lobster several hours later. So, what gives?

Even if you reapply your trusty formula every two hours as you’re supposed to be doing, sunburns can still happen (and that is never fun). The reason that this might be happening? You’re quite simply not applying enough of the good stuff. Because while with your skin-care routine, your philosophy might be less is more, that shouldn’t be your sun protection mantra. There’s a science to how much SPF you’re supposed to be putting on your skin in order for it to be as effective as possible—and it’s probably more than you realize.

What complicates this even more is that the number of different sunscreen formulas have sky-rocketed as of late. It’s a good thing because different people like different things from their SPFs, and the most important factor when looking for any sunscreen is that you’ll like it and actually wear it. The catch? The dermatologist-recommended amount for different formulas varies widely.

For lotions and mousses, Mona Gohara, MD, a Danbury, Connecticut-based dermatologist, says to use an entire one ounce shot glass worth from head to toe to be fully covered.

Then there are sprays, which are a fan fave for those on the go—they’re tough, since they’re usually invisible, but there is a key rule for staying safe regardless. “I always tell my patients to spray [one spot] for at least two seconds and then rub in with your hands,” notes Dr. Gohara. “Don’t treat it like a perfume where you hope the droplets land correctly—actually rub in the spray.”

Other people are into sticks, which are particularly useful when applying SPF to your face or while exercising outside. “Always apply sticks two times on the area being treated,” she says. A once-over isn’t ideal when keeping the burn away.

Of course, besides being sure to apply plenty of sunscreen, it’s also key to apply it everywhere—including those hard-to-remember spots that tend to get easily burned. “The most commonly missed spots are your ears, eyelids, scalp, lips, and top of feet,” says Dr. Gohara. The more you know, the better you can stay protected. Happy applying!

P.S., here’s the 2018 dermatologist guide to sunscreens. Oh, and do you know the difference between sunscreen and sunblock?