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SPF numbers are not linear, and the jump from 30 to 50 buys far less than people expect. Enter an SPF to see how much UVB it filters and how much still gets through.
Blocked percentage is 1 minus 1 divided by the SPF, the definition behind the label: SPF 15 filters about 93 percent of UVB, SPF 30 about 97 percent and SPF 50 about 98 percent. The FDA requires reapplication at least every 2 hours, and immediately after swimming or towel drying; water resistance claims are capped at 40 or 80 minutes. SPF measures UVB only, so look for broad spectrum for UVA.
SPF numbers look linear and are not. The curve flattens hard above SPF 30, which is why doubling the number buys about one extra percent of protection.
Blocked share is 1 minus 1 divided by the SPF, the definition behind the label. SPF measures UVB only, so look for broad spectrum to cover UVA, and reapply at least every 2 hours.
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