How it's calculated
safe time = (base minutes ÷ UV index) × SPF
base minutes = unprotected burn time for skin type I–VI (≈10/15/25/40/60/90 min at UV 1), UV index = current UV level, SPF = sunscreen factor. Block = 1 − 1/SPF. Result is capped at 8 hours.
Common questions
How often should I reapply sunscreen?
Reapply at least every two hours, and sooner after swimming, sweating, or toweling off.
Does a higher SPF let me stay out much longer?
It helps, but the gain levels off. SPF 30 blocks about 97% of UVB and SPF 50 about 98%, so the extra time is smaller than the number suggests.