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When you buy a sunscreen that says SPF 30 on the packaging, you probably assume it's going to give you SPF 30 protection. And it will — conditionally. Everything you need for skin well-protected from the sun is inside any mineral or chemical sunscreen with a higher sun protection factor, but ultimately, how well your skin is defended from sun damage depends heavily on applying — and reapplying — enough of your chosen formula. And if we're being completely honest with ourselves, very few of us are actually applying enough each and every day. 

"To get the protection labeled on the product, you would need to replicate the application they do in the lab, which is 2 milligrams per square centimeter for the average person. That translates to half a teaspoon on face and neck combined — a quarter teaspoon each — and 1 teaspoon per arm and leg each, front and back," says board-certified dermatologist Ranella Hirsch. "Realistically, people use somewhere around half of that amount."

Needless to say, we could all use a reminder about how much sunscreen to use and why. Here's what the experts have to say about application, reapplication, and all of the variables that come into play, like SPF level, being indoors, and more.

How much sunscreen should you be applying?

Every board-certified dermatologist we asked, including Marisa Garshick, said the same thing: To achieve the SPF labeled on a sunscreen bottle, it is recommended to use 2 milligrams of sunscreen per square centimeter of skin, "which is equivalent to one shot glass for the body or two tablespoons," Garshick clarifies. "For the face, it is recommended to apply a nickel-sized dollop — the equivalent of two finger lengths, which is also equivalent to half a teaspoon, to the face and neck combined." 

Because this is the amount needed to provide the level of protection indicated on the bottle, she says, it is important to do this regardless of if the formula is mineral or chemical, where you're located, or what time of year it may be.

As for sunscreens with which you may not be able to easily eye the amount by squeezing globs onto your fingers or into a shot glass, you'll have to rely on other techniques to ensure you're applying enough. "For stick sunscreens, it's best to apply four passes back and forth and then rub it in after for even coverage," Garshick says. One of our favorites: Cetaphil Sheer Mineral Sunscreen Stick Broad Spectrum SPF 50. And for sunscreen mists and sprays, she recommends spraying the skin until it's shining. "I always tell patients, 'If it's glistening, then you're listening.'" We're big fans of Supergoop! Play Antioxidant Mist SPF 50.

Cetaphil Sheer Mineral Sunscreen Stick Broad Spectrum SPF 50

Supergoop! Play SPF 50 Antioxidant Mist

What about SPF level?

The amount of product you apply is extremely important, but so is the SPF number. And you'll be hard-pressed to find a dermatologist that will tell you anything under SPF 30 is sufficient enough — even with diligent application — to truly protect skin from discoloration, collagen breakdown, and skin cancer. 

"A common inaccuracy that is still widely shared is that because SPF 30 filters 97 percent, SPF 50 blocks 98 percent of rays, and SPF 100 about 99 percent, there is little difference between them. This isn't right," Hirsch explains. "If you put it a different way, by going from SPF 30 to SPF 50, you get 1.5 times the UV getting through the filter, so there is a meaningful difference between SPF 30 and 50." Therefore, she recommends a minimum of SPF 50.

In addition to seeking out a high SPF number, make sure the sunscreen you select is indicated as broad-spectrum. That means it protects against both UVB and UVA rays, the latter of which cannot be addressed by SPF alone. 

These rules for application, SPF, and broad-spectrum aren't just for when you're going to be outside. If you're inside most of the day and anywhere near a window, that window may block UVB rays, but it probably won't block UVA rays. "It's best to use an SPF 30 on a daily basis even if you are not going to be outside," says board-certified dermatologist Azadeh Shirazi, who says to make indoor application a habit. "After all, we all go outside to check the mail, walk the dog, sit next to a window. Over time, this accumulation of UV radiation takes a toll on the health and appearance of our skin."

How important is reapplication?

In a word: very. And for the most part, it should be happening pretty frequently. 

"Reapply every two hours at a minimum but more often if sweating or emersed in water," board-certified dermatologist Roberta Del Campo tells Allure, echoing the recommended reapplication frequency of every expert we spoke to. 

But does that mean you have to strip down and reapply sunscreen all over your body every two hours? Not necessarily. Garshick says clothing may provide some degree of protection — a white cotton T-shirt, for example, offers a UPF of 5 — so how long you'll be outside and the type of exposure would determine the need to reapply sunscreen underneath clothing. "If you will be in direct sun exposure or spending a significant time outdoors or at the beach, it is best to wear UPF clothing to provide an extra degree of protection," she says.

The next best thing, according to Del Campo, is clothing with a high thread count. "Clothing that has tight thread knitting protects best if it's not UPF-enhanced clothing," she says. "I always recommend a layer of sunscreen under clothing with high amounts of sun exposure, but I do not necessarily recommend reapplying under clothing as the sunscreen doesn't break down as quickly without direct sunlight or excessive sweating."

However, since certain body parts other than your face aren't covered by clothing, Garshick says those particular areas definitely need sunscreen reapplication. "It is important to remember to reapply to the scalp, hands, neck, ears, lips, tops of feet, and areas that may be exposed that you may not realize," she says.

And yes, reapplication is important indoors, too — especially if you spend any amount of time in a room with natural light. "If it is light enough for you to see, that is sun," says Hirsch. "If you are indoors all day and not near any direct sunlight, reapplying just doesn't have the same urgency. But if you sit, as many people do, at a desk facing a window with sunlight pouring in, you should really think about that, because UVA goes through glass."

And remember, you're not just protecting yourself against UVA rays when you're indoors. "While the majority of blue light exposure is from the sun, our digital devices emit high energy visible light, and some sunscreens may offer protection against this," Garshick says.

Tips for applying enough sunscreen

The best way to ensure you're applying enough sunscreen every day is to simply force yourself into the habit, like you have with your moisturizer. "Applying an SPF moisturizer daily makes it a habit that pays off in the long run. If you only do it occasionally, then it's not going to be consistent," says Shirazi, who personally loves using tinted moisturizer to serve multiple purposes. But nowadays, sunscreen doesn't even have be called a moisturizer in order to serve as one, she says, because daily sunscreen formulas have become more moisturizing, so there's no need to apply a moisturizer beforehand. 

If you're reluctant to really slather on a lot of sunscreen, at the very least, choose a sunscreen with a higher SPF number. "The increased level of protection compensates, to an extent, for under-application," says Hirsch. However, "this does not negate the need to reapply after two hours for sweating or swimming or being outdoors, which is one issue with the super high SPFs — people tend to think they are 'one and done' for application, which is not correct."

To get an especially perceptible idea of how much sunscreen you're applying, Del Campo recommends looking for a formula that goes on obviously but doesn't leave a cast. Her favorite: Elta UV Clear Broad Spectrum SPF 46. "It goes on white but dries clear so you can visually see that you have covered the entire surface area," she says. "A liberal amount is always best." We also love Farmacy Green Defense Daily Mineral Sunscreen SPF 30, which disappears into all skin tones surprisingly well for a physical formula.

EltaMD UV Clear Broad-Spectrum SPF 46

Farmacy Green Defense Daily Mineral Sunscreen SPF 30

And finally, Garshick says, don't forget that sunscreen is the most essential piece of the sun-protection puzzle, but not the only one. "It is also important to remember other sun-protective strategies," she says, "including seeking shade, wearing a wide-brimmed hat and sun-protective clothing, and avoiding peak hours of UV exposure."

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