How Much SPF Do You Need? Here’s Everything You Need to Know About Sunscreen, According to Dermatologists

How much SPF do you need from your sunscreen? You already know that if you’re going to spend a prolonged amount of time outside during this hotter-than-hell summer, you need to put on sunscreen. But how much is enough? The SPF rating, along with how often you need to reapply—and if it’s okay to forgo sunscreen if you spend the bulk of your time inside—is all a bit murkier.

Women are more than twice as likely to wear sunscreen than men. Not unrelated, men are significantly more likely to die of melanoma than women—a type of cancer that is almost completely avoidable. It starts with knowing the basics about sunscreen, including SPF meaning, SPF 15 vs SPF 30, and how much SPF you need per day.

What Does SPF Mean, and How Much SPF Do You Need?

First things first. What does SPF stand for? Rosanne Paul, DO, an Assistant Professor of Dermatology at Case Western Reserve University, explains that it stands for “Sun Protection Factor.” She explains that the SPF number indicates how long you can be in the sun and avoid burning. For example, slather SPF 15 on your skin and you’re protected 15 times longer than if you weren’t wearing any sunscreen at all.

In theory, this should mean that SPF 30 protects the skin twice as long as SPF 15—and that SPF 60 would protect the skin twice as long as SPF 30. But in actuality, Dr. Paul says this isn’t the case. “Just because you apply SPF 30 instead of SPF 15 does not mean you get double the length of time protected from the sun. It’s more related to the amount of exposure than how long it lasts,” she says.

For example, Dr. Paul says if you slather on SPF 30 sunscreen and go outside at 9 a.m. for one hour, the amount of UV rays you’re exposed to is similar to spending 15 minutes outside mid-afternoon. “The rays are more intense in that midday sun. So it’s not so much the time of exposure as the amount of energy it takes to induce a sunburn on unprotected skin,” she says.

Since the number on your bottle of sunscreen isn’t a great indicator of how long you can stay in the sun without your skin getting fried, it’s more helpful to know what level different SPFs protect against the sun’s UV rays. Here’s the breakdown: SPF 15 gives 93% protection, SPF 30 gives 97% protection, and SPF 50 gives 98% protection. As you can see, there isn’t much of a difference in protection between SPF 30 and SPF 50, which Dr. Paul says SPF 30 or SPF 40 is the standard recommendation from dermatologists.

Is there such a thing as too much SPF? Jeremy Brauer, MD, a board-certified dermatologist, dermatologic surgeon, and founder of Spectrum Skin and Laser, doesn’t think so. “The higher the SPF, the better,” he says. That said, Zaineb Makhzoumi, MD, MPH, a dermatologic surgeon at the University of Maryland School of Medicine, says there’s not really a big benefit to going over SPF 45. “Studies have shown that once you get above an SPF of 40 to 45 you are not really getting much added protection from the blistering rays of the sun,” she says. So while SPF 70 or SPF 100 isn’t a scam, it’s more than you need.

When and How Often You Should Be Applying Sunscreen

Okay, now you know what SPF means. But it’s still important to know how often you need to reapply sunscreen to your face and body. Maybe your facial moisturizer already has SPF 15. Is SPF 15 enough for daily protection? Dr. Brauer says no, emphasizing that everyone should be applying SPF 30 or higher to their face every day.