Every SPF number you've ever seen on a bottle was measured with sunscreen applied at 2 milligrams per square centimeter of skin. Independent studies consistently show real-world application is closer to 0.5–1.0 mg/cm² — between one-quarter and one-half of the test density. The math is brutal: SPF protection drops roughly with the square root of the dose. Half the dose ≈ √SPF, so SPF 30 applied at half-density behaves like SPF 5–6.