Sun Protection Calculator

This calculator estimates how long skin can stay in the sun before burning, using a baseline unprotected burn time by Fitzpatrick skin type -- from about 67 minutes for the fairest skin type up to 300 minutes for the darkest, at a reference UV Index of 1. Effective UV Index adjusts the actual UV Index upward for two amplifying conditions: altitude adds roughly 10% more UV exposure per 1,000 meters of elevation (the commonly-cited WHO figure), since thinner atmosphere filters less radiation, and proximity to a reflective surface adds a further bump that depends on the surface -- roughly 10% near water, 15% off sand, and as much as 80% off fresh snow, since UV bounces back up onto skin at very different rates depending on what it reflects off of. Unprotected Burn Time divides the skin-type baseline by that Effective UV Index -- a higher effective UV shortens safe exposure proportionally. Applying SPF extends that window, but not by the full labeled factor, because the SPF number on a bottle is measured in a lab applying a thick, even 2 mg per square centimeter layer, and most people apply far less in practice. Per Faurschou & Wulf's real-world application-thickness research (British Journal of Dermatology, 2007), protection scales with how thin the actual layer is relative to that 2 mg/cm2 standard: at a commonly-observed 1 mg/cm2 application (half the lab standard, this calculator's default), real-world protection is close to the square root of the labeled SPF rather than the full number, and at an even thinner 0.5 mg/cm2 application it drops to roughly the fourth root.

Applying the full 2 mg/cm2 lab-standard thickness gives the full labeled SPF. Reapply Every is set to 75% of the protected burn time, capped at 120 minutes, matching common sunscreen guidance to reapply at least every two hours regardless of SPF. UVB Blocked is a simple 1 minus 1/SPF conversion -- SPF 30 blocks about 97% of UVB, not 100%. Treat this as general sun-safety planning, not a substitute for checking a live UV forecast or consulting a dermatologist about photosensitivity.