You need to use about one ounce, or an entire shot glass worth of sunscreen, to cover your entire body 15 minutes before going out in the sun. This should be applied in a layer that is 2 mg per cm square of skin thick and you should be applying it to clean, dry skin as well. A sunscreen lip balm is also recommended. You may be surprised to learn that the vast majority of people do not apply sunscreen correctly and do not know how much sunscreen they need. As a result, they are not getting the full benefits of using it to prevent skin cancer. Let’s take a look at the step by step instructions you need to apply sunscreen properly.

Before Sun Exposure

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If you read the label of most sunscreens they recommend that you apply the sunscreen a full 15 minutes before you head outside. This is often true for a chemical sunscreen as well as a mineral sunscreen. This is because the sunscreen needs to dry to provide uniform protection. It’s a common belief that it takes 15 minutes to absorb, but that’s not true. Instead what happens is that, when it’s wet, the sunscreen is very messy at a molecular level. It’s an emulsion of ingredients that don’t naturally go together so in its wet form there are molecules of UV absorbing materials and molecules of emulsifiers and moisturizers, etc. If you go out in the sun with sunscreen still damp on your body, it’s like holding up a screen in a literal sense. Some sun will be blocked but some gets through in all those places where the moisturizers or emulsifiers are.

If you allow the sunscreen to dry then it creates a thin, uniform layer of protective elements all over your skin. That’s why you need to wait 15 minutes, it’s just to allow the active ingredients to form a complete layer over your skin. If you have oily skin this may take longer.

Make sure that, if you are using a lotion type of sunscreen, you give the bottle a good shake to ensure that the product is well mixed before you begin to apply it as well. Sometimes sunscreen can separate if it has been sitting for a while and the products used as moisturizers and emulsifiers may rise to the top. If you don’t shake the bottle you may be applying those and getting little of the chemicals or minerals that actually protect you from the sun.

Apply Liberally

This is a strange direction many sunscreen brands use in their instructions. Apply liberally is very vague and does not offer a lot of guidance. As we said above, an ounce is usually the amount of sunscreen you should be using to cover the exposed skin of a typical adult for a day out in the sun.

When you travel, airplanes limit the amount of sunscreen you can bring on board in carryon to a three ounce bottle. So the amount of sunscreen you can bring on a plane is equal to about 3 applications, if you need a little context.

In lab testing, when manufacturers are determining the SPF value of their sunscreen they test it by applying a 2mg/cm2 thick layer to the skin of subjects who will be using it. So, in lab conditions, 2mg/cm2 is the liberal amount they are assuming you will use when you get the sunscreen home. That works out to about one ounce for the average person’s whole body. But this is never actually listed on any labels so it’s no surprise to learn that, based on research, most people are not using enough sunscreen.

In tests, the average person is only using about 40% of the sunscreen they need to get optimal protection. That means if you think you have an hour out in the sun, you actually only have about 24 minutes if you apply sunscreen like most people do.

Sun Protection Factor

The sun protection factor of any given sunscreen affects how you apply it because it dictates how long it provides protection from uva and uvb rays. The lower SPF the more or more often you need to apply the sunscreen. Sun protection factor, or SPF, is a measure of the relative amount of time you can endure exposure to the sun’s UV rays before they begin to cause your skin to redden and therefore do damage.

Some people do not realize that SPF is a relative measure and believe SPF indicates a static time that they have protection from the sun, like 30 minutes or two hours. It’s a common misconception based on the idea that SPF 30 means you have sun protection for 30 minutes or because many sunscreens recommend you reapply after two hours indicating they must offer two hours of protection. Neither of these reflect how SPF works.

SPF is relative to both you as the individual user and the intensity of the sun on the day you are exposed. So if you have darker skin and it takes you a long time to tan, and it’s an overcast, cloudy day with little sun, you will be absorbing much less UV radiation overall. An SPF 30 sunscreen may last you for two hours. But if you have very fair skin and burn easily, and it’s the sunniest day of July at high noon, you may start to burn even with sunscreen applied far sooner.

You need to apply a high SPF sunscreen and you need to apply it more often if you are sensitive to the sun and have fairer skin that burns very easily.

An Example of How SPF Affects How You Apply Sunscreen

Let’s say you have very fair skin and the UV Index, which tells you how bad your exposure risk to UV rays are on any given day, is above 11. Above 11, the average person is at risk of burning in less than 10 minutes. A fair skinned person can burn in as little as five minutes.

If you put on SPF 30 sunscreen, you would get 30 times the amount of protection, so 150 minutes. But don’t forget that most people only use 40% of the sunscreen recommended which means you now have about 60 minutes instead of 150 based on this. And if you don’t let it dry first or you get wet or start sweating, that will lower the amount of time again.

That means if you apply a higher SPF, you can help ameliorate some of these factors. At SPF 50 the same person on the same day would theoretically have 250 minutes dropped down to 100 after the 40% reduction. That’s an extra 40 minutes, which isn’t bad at all. And, of course, if you actually do apply the sunscreen properly you’d have that full 250 minutes. Except for the rules about reapplying.

Reapply Sunscreen

All sunscreen on the market recommends you reapply after about two hours. Chemical sunscreens and physical sunscreens have similar recommendations here. If you’re sweating or swimming they recommend around 80 minutes for a reapplication. So if the SPF is supposed to give you up to 250 minutes and maybe much more on a less sunny day, why do they recommend reapplying after 120 minutes?

You need to reapply sunscreen regularly for two main reasons.

  • Perspiration, swimming, every time you touch your face or rub against clothing, etc. runs the risk of rubbing away more of that protective layer. The sunscreen could stay on you for 250 minutes if you stayed perfectly still in a temperature controlled, dry room, but it’s very unlikely. So you’re just hedging your bets and using more to be cautious.
  • The other reason is maybe a bit unexpected and has to do with the proper method of applying sunscreen. Like I said earlier, most people don’t know how to apply sunscreen properly with that 2 mg per cm squared of skin rule, or to use a whole shot glass to cover their whole body. So why don’t sunscreen manufacturers make that clear on the bottle? Because it’s easier to simply tell you to apply it more often. The sheer number of people who will be using sunscreen, including children, makes it hard to get everyone on board with rules that don’t necessarily seem intuitive. Your instinct is to put on sunscreen and rub it in. That’s what almost everyone does and that’s how we all end up with too little. So to manage that, rather than convince people to use more than they’d naturally be comfortable with, companies instead just recommend you apply it more often.

The Bottom Line

There are a few simple steps to ensure the best and easiest sunscreen application to prevent sun damage.

  • Make sure your skin is clean and dry before applying sunscreen.
  • Sunscreen needs to be applied thoroughly to all exposed areas of skin 15 minutes before going out in the sun to allow it time to dry on your skin and form a protective layer.
  • Ensure places like ears and back of the neck get well covered.
  • The average adult requires about one ounce, or one shot glass, worth of sunscreen to cover their exposed skin. This is recommended to be applied at a thickness of 2 mg per cm squared of skin.
  • Since it’s likely you may be applying as little as 40% of the recommended amount of sunscreen you need to make sure you also reapply sunscreen on a regular basis. This is typically after about 80 minutes of sweating or toweling off after swimming, and after about 2 hours in general. This applies to water resistant sunscreen and spray sunscreen, too.

Always defer to the label of the sunscreen that you’re using. If it has specific directions that are different than what you may be used to, follow those as they will be the most accurate.