When a sunscreen label says SPF 30, what does that number actually tell you about the UV protection it gives your skin?

Answer: It lets about one-thirtieth of the burn-causing UV dose reach your skin.

It adds about 30 minutes of burn-free time in the sun. — The number is not a fixed extra-time promise. SPF is tested by comparing the UV exposure needed to cause redness on protected skin with that needed on unprotected skin, so it is a dose ratio rather than a timer. That is why the label is about less UV getting through, not guaranteed extra time outside.

It blocks 30 percent of the burn-causing UV dose before it reaches your skin. — This sounds tempting because the label contains the number 30, but SPF is not a percent-blocked score. Applied as directed, SPF 30 lets about 1/30 of the test UV dose through, so roughly 96.7 percent is filtered. The percent filtered is the result of that ratio, not the number written on the bottle.

It lets about one-thirtieth of the burn-causing UV dose reach your skin. ✓ — SPF 30 is closer to a denominator than a strength score. In testing, it means about 1/30 of the UV dose that would burn unprotected skin reaches skin when the product is applied correctly, so about 96.7 percent is filtered. The last few percentage points are harder to gain: moving from 15 to 30 helps more visibly than moving from 30 to 50.