Sunscreen is rated by sun protection factor (SPF), which indicates how long you can be in the sun before getting burned. For example, if an unprotected person begins burning in 10 minutes, a 15-SPF sunscreen would shield that individual's skin for 150 minutes.
An SPF of 2 blocks 50 percent of UVB rays, and an SPF of 15 blocks 93 percent. Be aware, though, there are diminishing returns the higher you go. An SPF of 30 blocks 97 percent of UVB, but SPF 50 or 60 won't provide much more. The American Academy of Dermatology (AAD) recommends that you should use at least SPF 30, preferably in a waterproof or water-resistant product.