SPF 30 vs SPF 50

SPF 30 filters about 97% of UVB; SPF 50 about 98%. Written as blocked percentages the gap looks trivial. Written as transmitted light — 3% versus 2% getting through — it is a third less. Neither framing settles it alone. What does settle it in practice is that a higher SPF buffers against the thin application everybody does, so SPF 30 is a sensible floor and SPF 50 is the right default for long exposure, children and anyone in strong sun.

What SPF actually measures

SPF is the ratio of how long skin takes to redden with the product on versus without it, measured in a laboratory using 2 milligrams of product per square centimeter of skin. It is a sunburn measurement, and sunburn is predominantly a UVB effect — which is why SPF tells you nothing about UVA on its own and why the separate broad-spectrum claim exists.

It is often described as a time multiplier — SPF 30 means thirty times longer before burning. That framing is technically derived from the test but badly misleading in practice, because it depends on UV intensity, your skin, and having applied the full tested dose. Nobody should be planning their day around it.

Why the two framings disagree

97% versus 98% sounds like a rounding error. 3% versus 2% transmitted is a third less UV reaching skin. Both describe the same numbers, and which one feels more meaningful depends on whether you are thinking about the light stopped or the light that gets through.

The honest reading is that the difference is real but modest, and that it is smaller than the difference between applying the full amount and applying half of it. Which brings us to the argument that actually matters.

The argument that decides it: nobody applies enough

Studies of real-world use consistently find people apply between a quarter and a half of the tested 2 mg/cm². And protection does not fall proportionally — it falls faster. Applying roughly half the tested dose of an SPF 50 does not give you SPF 25; the effective protection is substantially lower still.

So a higher SPF functions as a buffer against the way sunscreen is actually used. That is the strongest case for SPF 50, and it is a case about human behavior rather than about the chemistry. The better fix is to apply the full amount — but choosing 50 costs nothing and helps when you do not.

What about SPF 70, 100 and above?

Diminishing to the point of irrelevance. SPF 100 filters about 99% against SPF 50's 98%. The FDA has proposed capping label claims at SPF 60+, on the reasoning that higher numbers imply a meaningful difference that does not exist and encourage people to stay out longer.

There is also a practical cost: very high SPF formulas tend to be heavier and less pleasant, which makes people apply less of them. A pleasant SPF 50 applied generously beats a thick SPF 100 applied thinly.

What to buy

• Daily incidental exposure: SPF 30 is fine, and the formula you enjoy wearing matters more than the number.
• A beach or pool day: SPF 50, with 80 minutes of water resistance.
• Children: SPF 50, for the buffer against hurried application. Best sunscreen for kids.
• High altitude or snow: SPF 50, and reapply more often — UV intensity rises with altitude and snow reflects a large share of it back up.
• Skin with a history of cancer or on photosensitizing medication: SPF 50, plus clothing and shade, and a conversation with your clinician.