SPF 50 is not twice as good as SPF 25
The number on a sunscreen is a ratio, not a percentage, and the curve it sits on flattens hard. What actually decides your protection is the two grams nobody applies.
Sun protection factor is the ratio of ultraviolet needed to redden protected skin against the amount needed to redden unprotected skin. SPF 25 means twenty-five times as much. It is not a percentage, and treating it as one is where the confusion starts.
The curve, in three numbers
- SPF 15 blocks about 93% of UVB.
- SPF 30 blocks about 97%.
- SPF 50 blocks about 98%.
Doubling from 15 to 30 buys you four percentage points. Doubling again buys you one. The returns flatten because the relationship is reciprocal — you are removing a fraction of what is left, not adding a fixed amount.
That single percentage point is still worth having if you have melasma, a history of skin cancer, or you are recovering from a laser course. For most people on most days, the gap between 30 and 50 is smaller than the gap between wearing it and not.
The variable that dwarfs all of this
SPF is measured at 2mg of product per square centimetre of skin. For a face and neck, that is roughly a third of a teaspoon — noticeably more than almost anyone puts on. Applied at half that thickness, protection does not halve; it falls closer to the square root. An SPF 50 applied at 1mg/cm² performs somewhere near an SPF 7.
What the number does not cover at all
SPF measures UVB, because it is measured by redness and redness is a UVB phenomenon. UVA penetrates deeper, drives photoageing and contributes substantially to pigmentary disorders, and is not described by the SPF figure at all.
In Europe look for the UVA in a circle mark, which certifies UVA protection at at least a third of the labelled SPF. In the UK you may also see the Boots star rating. Either is the information you actually want; neither is on the front of the bottle in large print.
Which one to buy
The one you will reapply. A £40 sunscreen used once in the morning protects a face less well than a £9 one reapplied at two o’clock. Mineral or chemical is, for the overwhelming majority of skins, a texture preference rather than a clinical one — the exception being reactive and post-procedure skin, where we will hand you a zinc-based one and tell you why.