SPF

The short answer

SPF is a sunburn-protection number. The FDA explains that SPF values are determined from testing that measures protection against sunburn, so the number mainly indicates UVB protection^[1].

That is useful, but incomplete. For daily skincare, I would read SPF together with broad spectrum, water resistance when relevant, and whether you will apply enough product.

What SPF does not tell you

SPF does not automatically mean strong UVA protection. UVA matters for pigmentation, visible ageing, and long-term UV damage, so the label should say broad spectrum if you want coverage across both UVA and UVB.

The AAD recommends choosing a sunscreen that is broad spectrum, water resistant, and SPF 30 or higher for exposed skin^[2]. That is a better checklist than chasing a number alone.

Mads's practical read

For most people, SPF is not the place to get clever. Pick a sunscreen you can wear generously and consistently. If it pills, stings, or looks terrible on you, the theoretical SPF number will not help much sitting unused in a bathroom drawer.

That is the very unglamorous sunscreen rule I keep coming back to: the best SPF is the one you apply properly, then keep using without turning the morning routine into a negotiation.