SPF 50 vs SPF 30

On paper the gap is unimpressive. SPF 30 filters about 96.7% of UVB and SPF 50 about 98%, so the step up buys roughly one extra percentage point of blocked radiation. That statistic is repeatedly used to argue the higher number is marketing, and on the lab numbers alone it almost is.

The argument falls apart on how sunscreen is actually used. Those figures come from applying 2 mg/cm², a thick layer almost nobody replicates — real-world application is commonly a quarter to a half of it. Protection does not fall proportionally with the amount applied; it falls faster. Applied at half the tested thickness, an SPF 50 lands nearer the label value of an SPF 25, and an SPF 30 lands somewhere well below 10. The higher rating is best understood as headroom against your own application.

The other thing the number does not tell you is UVA. SPF measures UVB only. A European PPD rating or the UVA-in-a-circle mark, or a Japanese PA+++ grading, is what tells you about the band responsible for photoaging — and a broad-spectrum SPF 30 beats an SPF 50 with weak UVA protection.

Practically: use SPF 50 if you apply thinly or burn easily, use whichever texture you will actually reapply, and check for a UVA mark before comparing the big number.