What SPF Actually Measures — and the Math Behind 15 vs 30 vs 50
SPF 100 isn't twice as protective as SPF 50 — here's the real math behind the number on the bottle.
SPF measures how much UV energy is needed to sunburn protected skin versus unprotected skin. SPF 15 blocks about 93% of UVB, SPF 30 about 97%, and SPF 50 about 98% — the jumps shrink dramatically as the number climbs.
The FDA explains that SPF is a relative measure of sunburn protection, not a linear timer, and that broad-spectrum testing is what tells you a product also guards against UVA, which drives aging and skin cancer.
Questions we get a lot
What does SPF actually measure?
SPF, or Sun Protection Factor, measures how much UV energy is required to produce sunburn on protected skin compared with unprotected skin. It is primarily a measure of UVB protection. To also gauge UVA protection, look for the words 'broad spectrum' on the label.
How much more does SPF 50 block than SPF 30?
Very little. SPF 30 blocks roughly 97% of UVB and SPF 50 about 98%, so the difference is only around one percentage point. The bigger gains happen at the low end — SPF 15 to SPF 30 — after which returns diminish.
If higher SPF barely helps, why use SPF 50?
Because almost everyone underapplies sunscreen, a higher labeled SPF provides a buffer against the thin coverage most people actually achieve. Still, no SPF blocks 100% of UV, so generous application and reapplication matter more than chasing the highest number.
r/SkincareAddiction: 'Is SPF 50 really worth it over SPF 30, or is it just marketing?'