SPF protection time calculator
SPF multiplies the time your skin can take before it reddens, but only if you apply the amount tested in the lab. Most people apply less than half, and the real protection drops far faster than the number on the tube suggests.
The estimate starts from each phototype's average time to redness at UV 1 and ignores altitude, reflection off snow or water, and medication that increases sun sensitivity. Reapply every two hours whatever the result says.
Frequently asked questions
Why does SPF 50 not last fifty times longer?
It does, in the lab, at 2 mg/cm². The amount people actually apply is usually under half, and protection falls off non-linearly — half the amount is far less than half the SPF.
SPF 30 versus SPF 50 — does it matter?
SPF 30 blocks roughly 96.7% of UVB, SPF 50 roughly 98%. The gap looks small, but the radiation getting through is nearly halved.
Does reapplying reset the clock?
Partly. Reapplying restores the film, but the day's cumulative exposure stays. At a high UV index, shade and clothing matter more than another layer.