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.

Dr. Makhzoumi has even stricter advice. “Despite its convenience, I do not usually recommend the combination sunscreen/moisturizer products, mostly because they usually only contain an SPF of 15 to 30 which is just not enough for daily protection.” For your face, she recommends SPF 40. For other parts of your body exposed to the skin, all three derms recommend applying SPF 30 or SPF 40 as part of your morning skincare routine, including to your head if you’re bald, behind the ears, and hands—places Dr. Brauer says people tend to forget about.

What if you’ll be spending minimal time outside. Can you skip applying sunscreen? Sorry, but no. “UVA and UVB rays can go through windows, so you’re still exposed inside or when driving in your car,” Dr. Paul says.

In terms of how often to apply sunscreen, this is where if you actually are outside or not does play a role. If you’re inside all day, Dr. Paul says applying sunscreen in the morning is enough. Otherwise, she says it’s important to apply every two hours and immediately after swimming or sweating. When you do apply (and reapply your sunscreen), Dr. Paul says it takes about a shot glass’s amount of sunscreen to cover the body.

Dr. Makhzoumi says sunscreen with a minimum SPF of 30 should be applied every day—even when it’s cloudy or in the winter. “The risk for skin cancer is associated with what we call ‘cumulative ultraviolet exposure’. It is really important to understand this concept because it means that every minute counts,” she says. “Even when you are not planning on spending the day on the golf course or on the water, the little bit of sun exposure you get each day adds up. Remember, little minutes here and there actually amount to a lot and it is your cumulative amount of sun exposure that leads to your risk of skin cancer. So if you skip the sunscreen on the days you don't plan on being outside, you are actually putting yourself at higher risk for skin cancer.”

By the way, literally everyone needs to be applying sunscreen. That means people of all skin tones and even people who “don’t burn, only tan.” If you have light colored skin or have had skin cancer in the past, you need to be especially diligent. “Once you've had one skin cancer, you're at an increased risk of having another one in your lifetime. Not only is skin cancer a risk, but the UV radiation can damage the collagen and connective tissue in your skin, resulting in a loss of elasticity. The damage and loss of collagen in your skin presents as an increase in fine lines, wrinkles, blemishes, spots, and sagging of the skin,” Dr. Brauer says.

What All the Different Types of Sunscreen Mean

When you’re shopping for sunscreen, Dr. Brauer says it’s important to make sure the one you’re eying protects against both UVA and UVB rays, which can both damage skin. UVA rays have a longer wavelength, which means they can go deeper into the skin. Not protecting yourself from UVA rays can cause skin aging. UVB rays have a shorter wavelength, so they primarily impact the outer layer of the skin. These are the rays that can cause sunburns and skin cancer. To make sure you’re protected from both, Dr. Brauer says to look for sunscreen labeled “broad spectrum” or “UVA/UVB protection.”

Then there’s the whole mineral versus chemical debate. Dr. Brauer explains that both mineral and chemical sunscreens protect the skin from the sun’s UV rays. They just do it using different mechanisms. He says that mineral sunscreens act as a physical barrier, preventing penetration. Usually, they contain zinc oxide or titanium dioxide. In contrast, he says that chemical sunscreens protect the skin from UV rays by converting UV rays to heat energy and dissipate the heat, once they have already penetrated the skin. “Since chemical sunscreens work by ultimately generating heat and may produce irritation, mineral sunscreens may be a better choice for some individuals,” he says.

Mineral sunscreens (also called natural or physical sunscreens) are likely safer, but they can leave a matte, chalky residue on the skin, which is why some people aren’t into them. But if you have acne-prone or sensitive skin, mineral sunscreen may be a better option for you.

Some sunscreens are labeled “water resistant.” “Sport sunscreens” can be thought of as “water resistant” because the logic behind the label is that sports equals sweating. Considering that being in the sun goes hand-in-hand with sweating (and swimming), Dr. Brauer says a water resistant sunscreen is worth seeking out. But it’s important to know that no sunscreen—even water resistant ones—are completely waterproof. It’s still important to reapply your sunscreen after getting out of the water or every two hours, whichever happens first. Additionally, some water resistant sunscreens are only water resistant for a certain amount of time (such as 40 minutes). This will be clearly indicated on the label—just be sure to check.

How To Apply Sunscreen, Step By Step

Here are the dermatologist-approved steps to applying sunscreen correctly, whether you’re spending the day inside or directly in the sun.

1. Apply sunscreen with a minimum SPF of 30 to your face and any exposed body parts in the morning. Don’t forget about your head, forehead, ears, and hands. Protect your lips by applying lip balm with SPF. Do this even on cloudy days and in the winter.

2. If you are staying inside all day, you’re good to go. If you are going to be outside in the sun, reapply sunscreen with a minimum of SPF 30 roughly 30 minutes before going outside. It takes about a shot glass’s worth of sunscreen to appropriately protect the body.

3. Reapply sunscreen every two hours maximum or after swimming or sweating.