Have you ever wondered what the SPF rating means?
The SPF rating tells you how long you can stay in the sun without burning from UVB (burning) light. It's like a sunburn meter. SPF is a measure of a sunscreen's ability to prevent UVB from damaging the skin.
Here's how it works:
If it takes 20 minutes for your unprotected skin to start turning red, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer — about five hours.
with an SPF 15 x 20 minutes of sun time = 300 … is how many minutes you can stay in the sun without burning. 300 minutes divided by 1 hour of 60 minutes = 5 hours of sun protection without a sunburn.
Don't forget to re-apply every 2 hours, or apply immediately if you've been perspiring or after swimming.