You’ll want to hold on to this handy trick.

Meghan Overdeep
July 20, 2018
boophotography/Getty Images

When it comes to applying sunscreen, it can be hard to know when enough is, well… enough.

If you continually find yourself either over or under applying, you’re in luck: the “shot glass” rule is here to help.

Sunscreens are tested at two milligrams per square centimeter of skin to determine their SPF, but it’s nearly impossible for the average person to figure out what that looks like come slather time.

“Nobody is going to take their home kitchen scale and measure out grams of sunscreen,” David J. Leffell, chief of dermatologic surgery and cutaneous oncology at the Yale School of Medicine, told The Wall Street Journal.

To adequately cover the body, Leffell and other experts agree that a good rule of thumb is to apply a shot glass full of sunscreen. However, keep in mind that the amount should be adjusted based on a person’s size. As Leffell went on to explain, the same amount of sunscreen for someone who’s 5-feet-2 wouldn’t be appropriate for someone who’s 6-feet-3.

Sandy Walsh, a spokeswoman for the FDA was able to elaborate a bit more. “The example of a shot glass is based on an average human and is given as an example only,” she told the WSJ. “The amount of sunscreen applied for each consumer will be based on their specific attributes including body size, the amount of body hair and sensitivity to the sun.”

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It seems easy enough, but studies have shown that most people apply only 20% to 50% of the amount needed to obtain the labeled SPF. If you don’t have a shot glass handy, or can’t seem visualize one when the time comes, another good rule of thumb is simply to reapply after every hour or two in the sun. And if you see yourself getting pink: keep slathering.