SPF 15 blocks ~93% of UVB, SPF 30 ~97%, SPF 50 ~98%, SPF 100 ~99%.
Apply too little and an SPF 50 can perform more like an SPF 15 in real life.
Frequently asked questions
How much sunscreen should I actually apply?
Most people apply far too little. Sunscreen is tested at 2 mg/cm², but typical use is a quarter to half that — which can drop an SPF 50 to real-world SPF 15 or lower. Aim for about a quarter to half teaspoon for the face and roughly a shot glass (1 oz) for the whole body.
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.
Does sunscreen fully prevent tanning?
No. A tan is the skin's response to DNA damage, and because no sunscreen blocks 100% of UV — and most people under-apply — some tanning can still occur. Sunscreen reduces the damage, but there is no safe tan from the sun.
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 ↗