Your sunscreen might have a dark secret: Unless you’re slathering liberal amounts of the lotion on your face, arms, and neck this summer, you could be leaving your skin more exposed than you think.

Both the Centers for Disease Control and Britain’s National Institute for Health and Clinical Excellence recommend using a sunscreen with an SPF of at least 15.

But unless you’re spreading 2 milligrams of lotion onto every square centimeter of exposed skin, you’re likely getting just a fraction of the sun protection listed on the label, according to a report from the Drug and Therapeutics Bulletin, a company that conducts independent evaluations of medical treatments.

In fact, most people apply between 25 percent and 75 percent of the proper amount of sunscreen, greatly reducing the lotion’s protective factor, according to the American Academy of Dermatology.

So what should you do to protect yourself? Use the guide below to make sure you have maximum coverage.

And make sure to get an annual skin screening, too. You can find free screenings in your area through the American Academy of Dermatology. And if you’re in the Philadelphia area, you can see our friends at Penn Dermatology, where a free screening is offered on May 30, 2015.

Take A Shot

When trying to determine how much lotion to use, follow this simple rule: “You should apply the equivalent of a full shot glass,” says Elizabeth Hale, M.D., a dermatologist and clinical professor at NYU’s Langone Medical Center.

That’s if you’re going to the beach and your arms, legs, chest, back, and face will be exposed.

Squirt, Rub, Repeat

Hale says a lotion with an SPF of 15 could provide adequate protection from the sun if applied properly—that is, a shot glass-worth every two hours. But you’ll need to reapply even more frequently if you’re swimming or sweating, and most people don’t reapply as often as they should.

She recommends you use an SPF of at least 30 to ensure your skin stays protected even if you forget to reapply, and especially if you’re an outdoor athlete.

Look Beyond UVB

”SPF only refers to the ability to block UVB rays,” Hale says. “But we know now that UVA rays also contribute to premature aging of the skin as well as cancer.”

So look for a lotion with both. Hale says sunscreen that also includes “antioxidants” adds an additional layer of protection. Antioxidants target “free radicals,” which can cause mutations to your DNA that lead to cancer, Hale explains.

Double Up

“I tell people to apply two coats of sunscreen,” says Adnan Nasir, M.D., a clinical professor of dermatology at the University of North Carolina. “If you only apply once you may have thin spots, or spots you missed.”

Dr. Nasir says it’s fine to apply the second coat immediately after the first.

(Does it all seem like too much trouble? Find out what happens When Skin Cancer Attacks Your Face. It could happen to you, too.)

Don’t Forget Your Lips

“The lower lip is like a solar panel in that it faces the sun directly, and skin cancer of the lip is very serious,” Nasir explains, adding that most sunscreen won’t stick to your lips.

Look for lip balm with at least an SPF of 15, and reapply every two hours.

Layer Up

Nasir also recommends wearing sun protective clothing if you’re going to be out in the sun for several hours.

“If you hold a regular t-shirt up to the sun, you can see the sun’s rays through the clothing. That means the sun can see you, too,” Nasir says. “Sun protective clothing has a tighter weave, denser fabric, and it also has a dye in the fabric that absorbs ultraviolet light.”