I spent a day at the lake last summer, reapplied sunscreen every two hours like the bottle said, and still got burned. The problem was not the sunscreen. It was that I was using half the amount I needed and forgetting my ears. A dermatologist friend explained what I was doing wrong. It was basically everything.

You Are Using Too Little

To get the SPF on the label, you need about a shot glass worth of sunscreen for your whole body. Most people use a quarter of that. Using half the recommended amount reduces SPF 30 to about SPF 7. This is not intuitive but it is how the testing works — SPF ratings assume a thick, even layer.

You Are Missing Key Spots

Tops of ears, back of neck, tops of feet, and the part in your hair. These are the places I burn every single time. Also: lips. Get a lip balm with SPF. Lip skin is thin and burns fast. I once had a sunburned lower lip and could not eat anything with salt for three days. Do not be me.

SPF Numbers Are Misleading

SPF 30 blocks about 97% of UVB rays. SPF 50 blocks about 98%. SPF 100 blocks about 99%. The difference above 50 is tiny. You are better off using SPF 30 correctly than SPF 100 sparingly. Do not pay extra for numbers that sound impressive but add almost nothing.

Water Resistant Does Not Mean Waterproof

Water resistant sunscreen protects for forty to eighty minutes in water. After that, you are unprotected. If you towel off after swimming, you have wiped the sunscreen off — reapply immediately. I learned this the hard way swimming in a lake and toweling off between dips. The sunscreen stayed in the towel, not on my skin.

Reapply More Often Than You Think

Every two hours is the baseline. If you are sweating, swimming, or wiping your face, reapply sooner. Chemical sunscreens degrade in sunlight whether you are in water or not. I set a timer on my phone now. It is annoying but less annoying than a sunburn.