The arithmetic, in one paragraph
Sun protection factor describes the fraction of ultraviolet B that gets through a film of sunscreen applied at two milligrams per square centimetre. The transmitted fraction is one divided by the factor. Under SPF 15, one fifteenth gets through, which is 6.7 per cent. Under SPF 30, one thirtieth, or 3.3 per cent. Under SPF 50, one fiftieth, or 2 per cent. Expressed the way packs express it, that is 93.3, 96.7 and 98 per cent filtered.
The presentation matters. A jump from 96.7 to 98 per cent sounds like a rounding error, and it is the same jump as a fall from 3.3 to 2 per cent transmitted, which is a reduction of about 40 per cent in the ultraviolet reaching skin. Put the other way round, SPF 30 lets through roughly two thirds more UVB than SPF 50. Neither framing is wrong. The second is the one that describes what your skin receives.
Then the dose problem arrives
Every figure above assumes two milligrams per square centimetre, the dose used in laboratory testing. Published measurements of ordinary use land consistently below that, commonly somewhere between a quarter and a half of the test dose. The film people actually apply is thinner than the film the number describes.
Protection does not fall in proportion to the missing product. A widely used approximation in the sunscreen literature treats the relationship as exponential: the protection actually delivered is roughly the labelled factor raised to the power of the fraction of the dose applied. Applied to half the test dose, that model turns a labelled SPF 50 into something near 7 and a labelled SPF 30 into something near 5.5. A more forgiving linear reading gives higher numbers. The models disagree about the size of the loss and agree completely about its direction, which is the part that matters at the shelf.
| Labelled factor | UVB transmitted at test dose | UVB filtered | Relative UVB reaching skin, against SPF 50 |
|---|---|---|---|
| SPF 15 | 6.7 per cent | 93.3 per cent | Over three times as much |
| SPF 30 | 3.3 per cent | 96.7 per cent | About two thirds more |
| SPF 50 | 2.0 per cent | 98.0 per cent | Baseline |
| SPF 50+ | Under 2 per cent | Over 98 per cent | Slightly less, unquantified by the label |
This is why we treat a high factor as insurance rather than as permission. The margin between 30 and 50 is small when you apply the tested dose and it is the difference between adequate and inadequate when you apply a third of it.
What the tested dose looks like on a body
Two milligrams per square centimetre is an abstraction until it is converted. United Kingdom guidance for the public translates it as roughly six to eight teaspoons of product to cover the body of an average adult: rather more than half a teaspoon for the face, head and neck, one teaspoon for each arm, one for each leg, one for the front of the body and one for the back. Most people apply a fraction of that and top it up rarely.
For a face alone, the practical rule that survives contact with a bathroom is two fingers: a line of product squeezed along the length of the index and middle fingers, applied to face and neck. It looks like too much until you have done it three times. That, rather than the number on the front, is the single change that most improves the protection people actually get.
What the label categories mean in the United Kingdom
Labelling here uses four bands rather than treating every number as distinct. Low protection covers factors 6 and 10. Medium covers 15, 20 and 25. High covers 30 and 50. Very high is reserved for 50+, which is the highest claim permitted, because above that level the measurement uncertainty exceeds the difference being claimed. There is no such thing as a meaningful SPF 100 claim on a United Kingdom pack.
The factor also says nothing about ultraviolet A, which drives most photoageing and contributes to skin cancer risk. For that you need the UVA in a circle mark, which certifies that UVA protection is at least a third of the labelled SPF. A high factor with no UVA mark is an unbalanced product, and the ratio is covered in our explainer on UVA ratings.
When 50 is worth it and when 30 is enough
Choose 50 or 50+ when the exposure is sustained or the skin is vulnerable: a full day outdoors, water, snow, altitude, a history of skin cancer, photosensitising medication, a condition such as melasma, or any situation where reapplication will be difficult. Choose 50 for children as a default, because application is rushed and uneven at the best of times.
SPF 30 is a reasonable everyday choice for incidental exposure in the United Kingdom when it is applied generously and reapplied, and a generous 30 beats a rationed 50 every time. What we would not do is treat a higher factor as a reason to stay out longer. The relationship between factor and safe time outdoors is not one the number supports, and using it that way is how people burn while wearing sunscreen.
Limits
Everything here concerns UVB, because that is what SPF measures. It does not describe visible light, which matters for pigmentation, or photostability, which decides whether the film still performs by mid afternoon. The exponential dose model is a model, not a measurement of your skin, and it is presented as a way to reason about the risk rather than as a prediction. This is a consumer guide and not medical advice.
Sources
- NHS, sunscreen and sun safety, on minimum factor, the UVA mark and reapplication. Accessed 15 August 2026.
- British Association of Dermatologists, patient information on how much sunscreen to apply and how to choose a factor. Accessed 15 August 2026.
- Cancer Research UK, on ultraviolet exposure, sunburn and cancer risk. Accessed 15 August 2026.
- ISO 24444, the international in vivo method for sun protection factor testing, which defines the two milligrams per square centimetre application used throughout this page.
Reviewed 15 August 2026. The arithmetic here is fixed. The application guidance is the part to check against current public health advice.