The Two-Finger Rule, Demonstrated
Exactly how much sunscreen goes on your face, measured.
SPF 15 blocks ~93% of UVB, SPF 30 ~97%, SPF 50 ~98%, SPF 100 ~99%.
The number on the bottle assumes a full dose, so under-applying quietly downgrades an SPF 50 toward SPF 15 protection.
Frequently asked questions
How often do I need to reapply sunscreen?
Reapply about every two hours of sun exposure, and immediately after swimming, heavy sweating or toweling off. Indoors and away from windows you generally don't need to reapply, but a single morning application also wears off — so reapply if you're near windows or heading back outside.
Does a higher SPF mean more protection?
Only marginally. SPF 30 blocks about 97% of UVB, SPF 50 about 98%, and SPF 100 about 99% — and no sunscreen blocks 100%. Applying enough and reapplying matters far more than chasing a high number, and SPF says nothing about UVA protection.
What is the two-finger rule for sunscreen?
The two-finger rule is a simple guide: squeeze sunscreen in two lines along your index and middle fingers, base to tip, to cover the face and neck. It approximates the research-backed amount (about a quarter to half teaspoon) that most people otherwise under-apply.
r/SkincareScience: 'Is SPF 100 a marketing gimmick or worth it?'
Sources & citations
- 'Teaspoon rule revisited: Proper amount of sunscreen application,' ResearchGate 234041524
- ewg.org ↗