SPF stands for sun protection factor which measures how long a sunscreen will protect you from ultraviolet (UV) rays. For reference SPF 15 sunscreen blocks 93% of UVB rays, SPF 30 sunscreen protects 97% of UVB rays and SPF 50 blocks 98% of UVB rays. We suggest using an SPF 30 or SPF 50 as this will provide adequate protection and coverage.