How skincare works
SPF 30 vs SPF 50: 1.3 percentage points apart
At the dose both numbers were measured at, SPF 30 blocks about 96.7 percent of burning UV and SPF 50 about 98. That gap is real, and it is smaller than the one almost everyone opens by under-applying. Both framings of it are drawn below.
The short answer
Pick the one you will actually wear generously every day. The amount moves more than the number.
- Is SPF 50 better than SPF 30?
- At the tested dose it blocks more: 98 percent of burning UV against 96.7. Framed as what gets through, 1 in 50 against 1 in 30, which is a third less UV reaching skin. Neither framing is wrong; they are the same arithmetic wearing different clothes.
- Should I use SPF 30 or 50?
- Whichever you will wear generously every day, because the gap between them is 1.3 percentage points at a dose almost nobody applies. A typical application is about 52 percent of that dose, and one in vivo study found the effective SPF falls exponentially as the layer thins, which moves protection far more than the choice between these two numbers.
- Does SPF 50 last longer?
- No. The number is not a clock, and reapplication advice is the same for both: reapply at least every two hours, and after swimming, towelling or heavy sweating.
- What does the SPF number actually mean?
- It is a burn-time ratio measured in a lab at a fixed layer of product. The full story, with the curve every SPF sits on, is in what does SPF mean.
The difference, drawn both ways
The difference between SPF 30 and SPF 50 depends entirely on which way you frame the same arithmetic. Blocked, the two look like twins. Let through, the gap is a third. Honest shelves show both, because both are the same numbers:
Past 50 the curve flattens further: SPF 100 blocks 99 percent at the same dose. Each doubling of the number halves what gets through, and each halving matters less in absolute terms:
What moves more than the number
Both numbers were earned at 2 mg of product per square centimetre. Measured on real faces, an unguided application delivers about 1.04, which is 52 percent of the tested layer; with written instructions it rose to 1.45 and still fell short.
One in vivo study measured what happens to SPF when less is applied and found exponential growth: at half the tested amount the effective SPF fell towards its square root, and at a quarter towards its fourth root. By that study's own arithmetic, an SPF 50 worn at half the tested amount behaves closer to 7.1 than to 50. The choice between 30 and 50 cannot survive that kind of shortfall, which is why the honest advice is boring: apply generously, and work out what generously means for your face with the sunscreen calculator.
An illustration of one in vivo study, not a measurement of your product.
How to actually choose
At the tested dose the two are 1.3 points apart, and neither is worn at the tested dose. So choose on the things that decide whether you wear enough of it, daily: texture, finish, how it sits under whatever else you wear, and whether the pack makes a generous layer feel normal. A sunscreen you enjoy applying at the right amount beats a higher number applied thinly.
For the record: SPF 30 blocks approximately 97% of UVB rays. Above that, the difference is marginal; SPF 50 blocks about 98%. That sentence, from our routine guide citing the American Academy of Dermatology, is the whole numbers case. The rest is amount and reapplication. If you are going to be out for hours, the model says reapply 15 to 30 minutes after you go out, not only at the two-hour mark.
Sources
The percentages are arithmetic on the SPF definition (an SPF lets through 1/SPF of erythemal UV); the measured claims below were resolved through PubMed on 3 September 2026.
- SPF 30 blocks approximately 97% of UVB rays. Above that, the difference is marginal; SPF 50 blocks about 98%.Our own guide, How to build a simple skincare routine, citing the American Academy of Dermatology.
- One in vivo study measured what happens to SPF when less is applied and found exponential growth: at half the tested amount the effective SPF fell towards its square root, and at a quarter towards its fourth root.Faurschou A, Wulf HC. The relation between sun protection factor and amount of sunscreen applied in vivo. Br J Dermatol 2007;156(4):716-9. PMID 17493070. 20 volunteers, one SPF 4 product at 0.5, 1, 2 and 4 mg/cm2; exponential fit r2 = 0.903.
- What people actually apply, without instructions: 1.04 mg/cm².Le Digabel J, Questel E, Lauze C, Carballido F, Josse G. In vivo evaluation of sunscreen application by multispectral imaging. Skin Res Technol 2023;29(8):e13320. PMID 37632173. 26 healthy volunteers, single unguided application to the face.
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