The Teaspoon Rule for Your Whole Body
One ounce, about a shot glass — and almost nobody uses that much.
A full adult body needs roughly one ounce — about a shot glass — of sunscreen.
SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98% — a smaller gap than the numbers suggest.
Frequently asked questions
Does SPF 100 matter?
Only marginally. SPF 100 blocks about 99% of UVB versus 98% for SPF 50 — a tiny difference that can create false confidence. A very high SPF may help heavy under-appliers, but it doesn't extend protection indefinitely or cover UVA, so reapplication and broad-spectrum coverage still matter.
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/SkincareAddiction: 'Does sunscreen completely stop you from tanning?'
Sources & citations
- EWG, 'What's Wrong With High SPF?' report
- chemistconfessions.com ↗