I wore SPF 50 every day last summer and still got burned. Twice. I thought the sunscreen was defective until I saw a video where a UV camera showed how much sunscreen people actually miss. I was applying about half of what I needed, and completely skipping three areas that burn the fastest.

You are using way too little

The SPF rating on the bottle is tested at a thickness of 2 milligrams per square centimeter of skin. In real life, most people apply about half that — which does not give you SPF 25 instead of SPF 50. The protection drops off exponentially, so half the amount might give you SPF 10 or less.

For your face and neck, you need about a quarter teaspoon of sunscreen. For your whole body in a swimsuit, you need about a shot glass worth — roughly one ounce. If a bottle of sunscreen lasts you all summer, you are not using enough.

The spots everyone misses

• Eyelids. The skin is thin and gets a lot of sun. Use a mineral sunscreen stick here — it stays put and does not sting your eyes.
• Ears. The tops and backs of ears get fried constantly. They are also one of the most common places for skin cancer. Do not forget them.
• Back of the neck. Especially if you have short hair or wear it up.
• Tops of feet. Sandals plus sun equals burned feet. Every time.
• Part in your hair. Your scalp burns. A spray sunscreen or a hat solves this.

Apply before you go outside

Chemical sunscreens need about 15 to 20 minutes to bind to your skin and become effective. If you apply when you are already at the beach or on a walk, those first 20 minutes are unprotected. Apply at home, before you put on clothes. Mineral sunscreens (zinc oxide, titanium dioxide) work immediately, so they are better for last-minute application or reapplication.

Reapply. Actually do it.

Sunscreen breaks down after about two hours in direct sun — faster if you are sweating or swimming. No sunscreen is truly “all day” despite what the label says. Two hours is the rule. I set a timer on my phone now because otherwise I forget, and forgetting is how I got burned both times last summer.