Kmiskincare — Skin Care Routine and SPF | kmiskincare.com
SPF is a fraction of burning radiation that gets through the film at 2 mg/cm², and the same dose and reapply interval apply to every labelled number.
SPF 15 means 1/15 of the burning radiation reaches the skin when the product is applied evenly at 2 milligrams per square centimetre. Read the label as a throughput figure: about 93% of the burning radiation is kept out at that dose, and about 7% passes the film.
SPF 30 works the same way at 1/30, and SPF 50 at 1/50. The arithmetic narrows quickly: going from 15 to 30 halves the fraction that gets through, while going from 30 to 50 removes only another percentage point or so of throughput.
On narrow screens, swipe or scroll the plate sideways.
All three figures are measured at the same dose of 2 mg/cm². The number on the bottle describes the film at that thickness; a thinner film raises the fraction that reaches the skin above the labelled figure, and a thicker film lowers it.
Coverage converts the dose into a volume: a tablespoon, 25 ml, of sunscreen should cover each limb, the face, the chest and the back for thorough coverage. Seven areas, each with its own tablespoon, is the amount the labelled SPF numbers were measured against.
The number does not carry a clock. Higher-SPF products do not stay effective longer and must be reapplied, usually every two hours, so SPF 50 and SPF 15 share the same renewal schedule.
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