Why does SPF 50 beat SPF 30 by only about 1 percentage point?

Answer: It lowers the transmitted UV fraction from about 1/30 to about 1/50, which makes the blocked-percentage gain small.

It raises the UV-blocking rate from about 30% to about 50%, which makes the 1-point gap misleading. — This sounds plausible because SPF numbers look like percentages: SPF 30 blocks 30%, SPF 50 blocks 50%. But SPF is a ratio, not a percent-blocked score: SPF 30 lets about 1/30 through and blocks about 96.7% to 97%, while SPF 50 lets about 1/50 through and blocks about 98%. So the blocked-percentage difference is only about 1 point, while the leak drops from 3.3% to 2%.

It doubles the safe exposure time from two hours to four hours, which makes the 1-point gap irrelevant. — This gets the reapplication advice half right: a higher SPF can give more margin if the sunscreen film stays put. But dermatology guidance still says to reapply about every two hours outdoors and after swimming or sweating, regardless of SPF. A higher number is not a time-extension coupon; the film moves, thins, breaks down, and washes off. The 1-point gap is about UV transmission, not about how long the product lasts.

It lowers the transmitted UV fraction from about 1/30 to about 1/50, which makes the blocked-percentage gain small. ✓ — Correct: SPF is a ratio of how much UV reaches skin, not a simple percent-blocked score. SPF 30 lets about 1/30, or 3.3%, through and blocks about 96.7% to 97%; SPF 50 lets about 1/50, or 2%, through and blocks about 98%. So the blocked percentage rises only about 1 point, but the remaining leak falls by roughly two-fifths. That is why SPF 50 can look barely better on the percentage-point scale yet still cut the transmitted UV meaningfully.