Ever wonder what SPF and its corresponding number on sunscreen bottles mean? SPF stands for Sun Protection Factor (not too complex) and the number is there to give you an indication of how long the sunscreen will be in effect. You multiply how many minutes you can stay out in the sun unprotected before getting sunburned by the number on the bottle. For example, if you can stay out in the sun unprotected for approximately 10 minutes before burning and you use sunscreen with SPF 15, then the sunscreen will be effective for 150 minutes (assuming you don’t go in the water or sweat profusely). Of course its accuracy is limited because you probably don’t know how long it takes for your body to get sunburned. Nevertheless, now you know what that number means and yes, the higher, the better.
Sources
http://ezinearticles.com/?What-Does-SPF-Mean-With-Sun-Block?&id=390632