The SPF number is the most recognised figure in skincare and one of the most widely misread. It is not a percentage, it is not a duration in hours, and it does not describe protection against the wavelengths that do most of the ageing. Understanding what it actually measures changes how you choose and, more importantly, how much you use.
What SPF measures
SPF stands for sun protection factor, and it is a ratio. It compares the ultraviolet dose needed to produce a just perceptible reddening of protected skin against the dose needed to produce the same reddening on unprotected skin on the same person.
Two implications follow immediately.
It measures erythema, meaning burning. Burning is overwhelmingly a UVB effect. So SPF is essentially a UVB measurement, and it says nothing directly about UVA, which is the band that penetrates to the dermis and degrades collagen and elastin.
It is measured at a defined application density of two milligrams per square centimetre of skin. That is a thick, visible layer. Study after study finds real world application at somewhere between a quarter and a half of it.
What the numbers translate to
| SPF | Approximate UVB filtered at test dose | Fraction reaching skin |
|---|---|---|
| 15 | About 93 per cent | Roughly 1 in 15 |
| 30 | About 97 per cent | Roughly 1 in 30 |
| 50 | About 98 per cent | Roughly 1 in 50 |
| 100 | About 99 per cent | Roughly 1 in 100 |
This table is why people say the difference between SPF 30 and SPF 50 is small. On the percentage filtered, it is. On the fraction getting through, SPF 50 lets through around forty per cent less than SPF 30, which over decades of daily exposure is not nothing.
The more important point is what happens when you under-apply. Protection does not scale linearly with the amount you use. Applying half the test dose does not give you half the SPF, it gives you considerably less than half, because the relationship between film thickness and attenuation is not linear. A generously applied SPF 30 outperforms a thinly smeared SPF 50 in practice.
The number on the bottle is a laboratory result. The number on your face is whatever you actually put there.
The UVA rating, which matters more than men think
Because SPF only describes UVB, a separate indicator is needed for UVA. In the UK you will see one or both of the following.
- The star rating. A five star system indicating the ratio of UVA protection to UVB protection. Crucially it is a ratio, not an absolute. Five stars on an SPF 15 product means the UVA protection is proportionally high relative to a low UVB protection, so the absolute UVA protection is still modest. Read the stars together with the SPF, never alone.
- The circled UVA mark. A logo indicating the product meets the European recommendation that UVA protection is at least one third of the labelled SPF. It is a threshold, not a scale, so it tells you the product clears a bar rather than how far it clears it.
For daily use, a practical target is SPF 30 or higher with a high star rating or the circled UVA mark. That combination gives meaningful protection across both bands, which is the point, since UVA is present all year and passes through window glass. The behavioural side of this is covered in sunscreen for men, why the resistance.
The dose, translated into something usable
Two milligrams per square centimetre is not a usable instruction while standing in a bathroom. The practical translations are these.
- Face, neck and ears: roughly two full finger lengths of product squeezed along the index and middle fingers, or a generous half teaspoon.
- Whole body in swimwear: roughly six to eight teaspoons, which is far more than most people use and is the reason beach burns happen through sunscreen.
- Reapply every two hours during sustained outdoor exposure, and immediately after swimming, heavy sweating or towelling regardless of what the packaging claims about water resistance.
Water resistant does not mean waterproof. UK and EU labelling permits water resistance claims based on retained protection after defined immersion periods, and the retained protection is a proportion, not the whole.
Expiry, heat and the glovebox problem
Sunscreen filters degrade. Products carry either a date of minimum durability or a period after opening symbol, shown as an open jar with a number of months. Both assume storage at normal temperatures.
A bottle left in a car through a summer has been held at temperatures that accelerate degradation of both filters and emulsion, and it may separate or lose potency without looking any different. Last year's half used bottle from the boot is the least reliable product you own.
Common misreadings, corrected
| The belief | The correction |
|---|---|
| SPF 30 means thirty times longer in the sun | A laboratory ratio, not a timer. Reapplication intervals are unaffected by the number |
| SPF 50 is nearly twice SPF 30 | About one percentage point more filtration, but around forty per cent less ultraviolet through |
| Makeup or moisturiser with SPF is enough | Almost never applied at anything near the test density |
| Cloud means no protection needed | A substantial proportion of ultraviolet passes through light cloud, and UVA more so |
| A dark base tan is protection | Equivalent to a very low SPF at best, and it is itself evidence of damage |
| Sunscreen blocks vitamin D entirely | Not demonstrated at real world application levels |
Where to check current guidance
Product labelling rules and public guidance change. Current UK advice on sunscreen selection and use is published by the NHS and by Cancer Research UK, and the British Association of Dermatologists publishes a sunscreen fact sheet aimed at the public.
How the filter names themselves appear on the ingredient list, and how to tell a light formulation from a heavy one before you buy it, is covered in how to read an ingredient label. Where sunscreen fits into a routine that men actually keep is set out in the minimum effective routine.