SPF 50 does not block twice what SPF 30 blocks. Doubling the factor halves the leak — the share of burning UV that still gets through — and 50 is not double 30. The percent a bottle seems to "block" barely moves. Neither figure is permission to stay out longer.

The factor is a dose ratio

The European Commission’s 2006 recommendation defines sun protection factor as the minimum erythemal dose on protected skin divided by the dose on the same bare skin. That quotient is the digit.

What still arrives is the reciprocal. Cancer Council Australia prints both directions. SPF 30 filters 96.7% of UV, so 1/30 — 3.3% — reaches the skin. SPF 50 filters 98%, so 1/50 — 2% — reaches the skin. The Council calls that filtered gap marginal: 1.3 points. TGA rounds the pair to about 97% and 98% of UVB. Australian labels may not say "sunblock," because no film stops all of it.

The Commission’s point is the shape of that curve. Protection rises in a straight line only for sunburn dose: an SPF 30 "protects twice as well from sunburn" as an SPF 15. The absorbed shares in that sentence are 93% and 97% of UVB. Twice the sunburn protection, four points on the filtered scale. Factors above 50, it says, do not substantially increase protection from UV.

ARPANSA’s table adds the step below 30. SPF 15 blocks 93.3% of UVR, so about 6.7% still gets through — twice the leak at 30. From 30 to 50, it does not halve.

What 30 to 50 actually changes

2 ÷ 3.3 ≈ 0.6. About three-fifths of the UV still getting through SPF 30 still gets through SPF 50. Roughly two-fifths less arriving — not half, and not double. Same step as those 1.3 filtered points.

The same reciprocal puts SPF 100 at 1% through. Against SPF 50’s 2%, the leak halves again and the filtered share moves one point, 98 to 99 — the stretch the Commission called not a substantial increase.

Where the printed number stops

Europe’s recommendation keeps the label list short. "High" is a labelled 30 (measured 30–49.9) or 50 (measured 50–59.9). "Very high" is 50+, which means the test cleared 60. Above 50, the recital says, UV protection does not substantially increase, and a run of nearby integers does not help anyone compare bottles.

Australia stops the claim at SPF 50+ too. TGA is stricter than a rounded 50: a 50+ label must test at least SPF 60. The same reciprocal puts a measured 60 at about 1.7% through (1/60), against 2% at 50. A smaller leak. Not a new kind of product.

The United States does not currently cap the printed number. The deemed final order has no maximum SPF. A proposed order would print no higher than SPF 60+, while allowing a formula up to 80, on evidence of added benefit for broad-spectrum products up to 60. The 10 September 2026 final order did not adopt that cap. A US 70 or 100 is still legal. It is not the number Europe or Australia print.

The curve, read wrong

1. "50 is almost double 30, so I can stay almost twice as long." The double in the Commission’s recital is sunburn dose, and it is 15 versus 30. FDA rejects the clock: burn in one hour, and SPF 15 does not buy fifteen hours. Intensity changes through the day, so the same minutes are not the same dose. ARPANSA states the theoretical multiple — ten bare minutes, five hundred at a perfect SPF 50 — and then says not to spend it. The product cuts exposure. It does not lengthen the outing. Reapply every two hours anyway.
2. "98 and 96.7 are the same tube." Close on the filtered scale, not the same leak. Stepping down to 15 reopens it: that leak is about twice the leak at 30.

This arithmetic is the test coat. The bottle SPF assumes a coat you rarely put on: 2 mg/cm², nearer a shot glass than a swipe. A thin coat of 50 is not a thick coat of 30. The digit is also silent on UVA. Broad spectrum is a pass, not a second SPF.

Buy the step, then leave

Use the live UV index for the rate.

1. Treat 15 to 30 as the halving. That is the doubling in the Commission’s recital. What SPF stands for is the ratio, not a timetable.
2. Treat 30 to 50 as a smaller leak, not a longer stay. About 2% through versus 3.3%, and only on the test coat.
3. Past 50, the filtered percent is already flat. SPF 100 leaves 1% through on this identity. Europe prints 50+. Australia’s 50+ has to clear 60.
4. Reapply every two hours, and keep the shirt. Sweat and a towel take film off. WHO’s practical guide says sunscreen should never be used to prolong sun exposure. Protection still starts at index 3.

Limits

This is a reading of the label scale, not advice for a skin condition. The percents assume the laboratory coat and a steady test lamp.

Sources

SPF as protected MED over unprotected MED; SPF 30 protects twice as well from sunburn as SPF 15 while absorbing 97% versus 93% of UVB; factors above 50 do not substantially increase UV protection; labelled 50+ means measured ≥ 60: Commission Recommendation 2006/647/EC, recital 20 and the labelling table. SPF 30 filters 96.7% (3.3% through) and SPF 50 filters 98% (2% through), a 1.3 point gap: Cancer Council Australia, sunscreen fact sheet. About 97% and 98% of UVB, no "sunblock," and SPF 50+ must test at least 60: TGA, SPF testing for consumers. Blocked shares 93.3 / 96.7 / 98% at SPF 15 / 30 / 50, label cap 50+, theoretical minutes are not a longer outing, reapply every two hours: ARPANSA. SPF is exposure, not time in the sun: FDA, Sun Protection Factor. No maximum on the deemed final order; proposed label cap 60+ with formulas up to 80; the 10 September 2026 order left that cap for later: FDA sunscreen Q&A. Do not use sunscreen to prolong exposure: WHO practical guide. Protect from index 3: WHO UV index Q&A. The 1%, 1.7%, and 0.6 figures are that same 1/SPF identity applied to 100, 60, and 2/3.3. They are not a separate measurement, and they are not a personal timer.