SPF percentage.

Worked example

SPF 30: one thirtieth of the sunburn-causing UV dose gets through, 100 / 30 = 3.33%, so 96.67% is held back. SPF 50: 2.00% gets through, 98.00% is held back. The gap between them is 1.33 percentage points, but 1.67 times as much UV reaches the skin at SPF 30 as at SPF 50 (3.33 / 2.00). In reverse, 98% held back is 100 / (100 - 98) = SPF 50.

How it works

The US FDA test defines SPF as the minimal erythema dose (MED) of sunscreen-protected skin divided by the MED of unprotected skin, measured with 2 mg/cm² applied. MED is the smallest UV dose that produces perceptible redness. So SPF 30 means it takes 30 times the UV dose to redden protected skin in the test.

Turned into shares: share reaching = 100 / SPF % and share held back = 100 - 100 / SPF %. From a percentage: SPF = 100 / (100 - percent held back).

What this is not. It is arithmetic on the lab definition, for sunburn-causing (erythemal) UV at the 2 mg/cm² test dose. SPF is not the same as time in the sun, and real use is usually thinner, wears off with sweat, water and rubbing, and varies across the skin, so protection in practice is lower. SPF does not cover UVA by itself: the label's broad spectrum statement comes from a separate test. SPF values do not add when you layer products.

Sources and basis (checked 2026-10-04): US FDA, 21 CFR 201.327 (SPF test, 2 mg/cm², MED definition); US FDA, Sunscreen: How to Help Protect Your Skin from the Sun (SPF is related to amount of exposure, not time).

FAQ

What percent of UV does SPF 30 block?

At the lab test dose, SPF 30 lets 1/30, about 3.3%, of sunburn-causing UV through, so about 96.7% is held back. That is an arithmetic reading of the SPF definition, not a measure of protection in everyday use.

How much better is SPF 50 than SPF 30?

On the percentages, 98% held back versus 96.7%, a gap of 1.3 points. Another way to say it: SPF 30 lets about 1.7 times as much UV through as SPF 50. Applying enough, evenly and reapplying matter more than the difference between the two numbers.

Does SPF 100 give twice the protection of SPF 50?

No. SPF 50 holds back 98% and SPF 100 holds back 99%, so SPF 100 lets half as much through at the test dose, but in practice thin application and wear-off shrink the difference.

Do SPF values add up if I layer two sunscreens?

No. SPF 30 over SPF 15 is not SPF 45. Use the product at the labelled amount and reapply as the label directs.

Does SPF tell me how long I can stay in the sun?

No. The FDA says SPF relates to the amount of solar exposure, not time. Sun strength varies with time of day, place and season.