

The SPF (Sun Protection Factor) scale is not linear:

SPF 15 blocks 93% of UVB rays



SPF 30 blocks 97% of UVB rays



SPF 50 blocks 98% of UVB rays

So, one way of looking at this is that SPF 30 sunscreen only gives you 4% more protection than SPF 15 sunscreen.

Or, another way of looking at it is:

SPF 15 (93% protection) allows 7 out of 100 photons through

SPF 30 (97% protection) allows 3 out of 100 photons through.



So, while you may not be doubling your level of protection, an SPF 30 will block half the radiation that an SPF 15 would let through to your skin.