Is SPF 50 twice as protective as SPF 30?

No. SPF 30 blocks about 97% of UVB rays, and SPF 50 blocks about 98% — only a 1% difference in a lab. In real life, people don’t apply enough, don’t reapply, and sweat it off, so that edge gets swallowed by human error. Focus on using enough (a shot glass for your body) and reapplying every two hours — that matters more than the number.