Knowing your SPF before you buy the product is significant. You need to understand what the Spf does and how much of it do you need. So the general SPF powers are SPF 15, SPF 30 and SPF 50. So no sunscreen is known to block all the UV rays, there is a certain percentage of coverage that each of these providers. While SPF 15 blocks 93% UV rays, SPF 30 blocks 97% and SPF 50 blocks 98%. Such an approximate difference of 1% between SPF 30 and SPF 50.