SPF (Sun Protection Factor) measures a sunscreen's ability to filter UVB rays, which cause sunburn. But what does "SPF 50" actually mean? And is it really twice as protective as "SPF 25"? Spoiler: no.
What do SPF numbers mean?
SPF 15 blocks about 93% of UVB. SPF 30 blocks 97%. SPF 50 blocks 98%. The difference is real but much smaller than the numbers suggest. Above 50, the gain is marginal — that's why labels read "SPF 50+".
UVA vs UVB: don't confuse them
UVB burns; UVA ages and drives melanoma risk. SPF only measures UVB. For UVA, look for the "UVA" logo inside a circle on the package — it guarantees protection of at least one third of the SPF.
How much, how often?
The "2 mg/cm²" rule means roughly one teaspoon for face and neck and six teaspoons for the whole body of an adult. Reapply every two hours and after every swim.
Why pair Sun Day with sunscreen
Sunscreen doesn't eliminate the risk, it delays it. Sun Day calculates in real time the UV dose received despite protection and warns you when it's time to reapply or seek shade. It's the assistant that completes your protection routine.
Mineral or chemical filters?
Mineral filters (zinc oxide, titanium dioxide) reflect UV and suit sensitive skin, kids and coral reefs. Chemical filters are lighter but some are controversial. Sun Day doesn't recommend brands but helps you measure real-world effectiveness.
Complete your sunscreen with Sun Day
The free, ad-free UV app that watches over your skin.