Skinpu — Skin Barrier and SPF Basics | skinpu.com
SPF is a fraction: 1 - 1/SPF. For SPF 30 that is about 96.7% of UVB blocked; for SPF 50, about 98%.
The arithmetic behind an SPF number is short. The fraction of UVB blocked equals 1 - 1/SPF. SPF 30 gives 1 - 1/30, about 0.967, or roughly 96.7%. SPF 50 gives 1 - 1/50, about 98%.
The gap between the two labels is smaller than the numbers suggest: about 1.3 percentage points of UVB. Higher SPF values compress toward 100%, which is why the curve flattens above SPF 30.
On narrow screens, swipe or scroll the plate sideways.
SPF describes UVB only, covering roughly 280-320 nm. UVA covers about 320-400 nm and is not described by the SPF number at all. A broad spectrum claim is what addresses UVA; the SPF value on its own says nothing about it.
All of these percentages assume the tested dose of 2 mg of product per square centimetre of skin. Applied thinner, the effective SPF falls well below the printed number, and the label figure no longer describes what reaches the skin.
Reading an SPF number therefore means reading two things at once: the fraction of UVB blocked, and the dose the fraction assumes. Both are part of the same definition.
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