SPF stands for Sun Protection Factor. It is a laboratory ratio: how much UVB it takes to redden protected skin versus bare skin. It does not measure UVA, water resistance, or how many hours you can sit outside.

The number is a ratio, not a clock

The FDA tests SPF with a thick, even coat (2 mg per cm²). SPF 30 means it took about 30 times more UVB to burn the protected patch. In that setup SPF 30 blocks about 97% of UVB; SPF 50 about 98%; SPF 100 about 99%.

Jumping from 30 to 50 adds one percentage point, not double the protection. Several countries cap consumer labels at “SPF 50+” for that reason.

SPF is silent on UVA

Tanning, aging, and a large share of long-term skin damage ride on UVA. U.S. bottles that cover both wavelengths say broad-spectrum. European and Japanese labels often add PA or PPD for UVA. A high SPF that is not broad-spectrum is a sunburn product, not a full UV product.

Why the “30 times longer” story fails outdoors

That math assumes the test coat, no sweat, no swim, no towel. Real skin gets maybe half the labeled SPF. The AAD still says reapply every two hours regardless of the number on the bottle. Higher SPF does not buy a longer interval.

Amount: about one ounce for the body, half a teaspoon for the face. If you skimp, an SPF 30 bottle can behave like SPF 10–15. See how long sunscreen lasts and the SPF calculator for today’s index.