SPF 30 blocks 97 per cent of UVB rays; SPF 50 blocks 98 per cent of UVB rays; and SPF 100 blocks 99 per cent of UVB rays. You should also pay attention to the PA+ and the terms broad spectrum.  A lower SPF broad spectrum sunscreen gives better protection than a higher SPF non-broad-spectrum sunscreen. The broad spectrum is the key here.