See how much UV an SPF blocks and how often to reapply.
An SPF rating tells you how much UV is blocked: SPF 30 stops about 97 percent of UVB. No sunscreen fully blocks, so thumb-size amounts must be reapplied every couple of hours. Higher SPF is not a license to stay out longer, because protection fades with sweat and water, making reapplication timing just as important as the rating.
Sunscreen SPF Coverage
UV blocked = (1 - 1/SPF) x 100; reapply roughly every 2 hours
UV blocked = (1 - 1/SPF) x 100; reapply roughly every 2 hours An SPF rating tells you how much UV is blocked: SPF 30 stops about 97 percent of UVB. No sunscreen fully blocks, so thumb-size amounts must be reapplied every couple of hours.
Higher SPF is not a license to stay out longer, because protection fades with sweat and water, making reapplication timing just as important as the rating.
This calculator takes 3 inputs: SPF rating, Daily exposure hours, Skin type. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.