The sun protection factor is the most familiar number in personal care and one of the most widely misread. This note sets out what it is, how it is arrived at, and the single assumption inside it that changes how the number should be used.
What the number is
The sun protection factor is a ratio. It compares the ultraviolet dose required to produce a defined degree of reddening on skin that has been treated with the product, against the dose required to produce the same reddening on untreated skin. If protected skin needs a substantially larger dose to reach the same endpoint, the ratio is correspondingly larger.
Two things follow immediately. First, it is a measure against a specific biological endpoint, which is reddening, and reddening is driven principally by ultraviolet B. So the sun protection factor is largely a statement about ultraviolet B, which is why ultraviolet A has to be indicated separately. See our note on the UVA circle.
Second, it is not a duration. The persistent belief that a factor multiplies the time you can stay out is the most damaging misconception in the category, because it converts a laboratory ratio into a permission slip. Real exposure varies by hour, by latitude, by altitude, by surface and by how much of the film is still on you.
The method
ISO 24444 sets out the in vivo determination of the sun protection factor. The essential features are that it is carried out on human skin, that a specified quantity of product is applied to a defined area, that a controlled ultraviolet source with a defined spectrum is used, and that the endpoint is assessed under specified conditions. Standardisation of every one of those variables is what makes results from different laboratories comparable.
There are also in vitro methods in the family, notably ISO 24443 for the determination of ultraviolet A protection, and work has continued for years on in vitro alternatives for the sun protection factor itself. The in vivo method remains the reference for the number on the front of the pack.
The assumption inside the number
The method specifies how much product goes onto the skin per unit area. That quantity is materially more than people apply in ordinary use, which has been documented repeatedly in the published literature on application behaviour.
The consequence is not a small rounding error. Protection does not fall in simple proportion to the quantity applied, and the shortfall runs against the user. A person applying a thin layer of a high factor product is receiving materially less protection than the label states, and has no way of knowing by how much.
This is why we treat quantity as the dominant variable on this site, ahead of the number on the pack, ahead of the water resistance claim, ahead of formulation type. Our note on how much to apply sets out what the tested quantity means in practice and why the usual advice to apply generously and to apply twice exists.
Why the numbers stop being interesting at the top
The relationship between the sun protection factor and the proportion of ultraviolet B absorbed is not linear. Moving from a low factor to a moderate one closes a large part of the gap. Moving from a high factor to a very high one closes a much smaller remaining part. This is why UK and EU labelling groups products into categories and caps the top of the scale with a plus rather than continuing to print ever larger numbers: the differences at the top are small relative to the precision of the test and to the variability introduced by how the product is applied.
The practical reading is that the difference between applying a moderate factor properly and applying a very high factor thinly runs in favour of the moderate factor applied properly.
What the number on the pack tells you
SPF 50
What it certifies
That the product achieved a sun protection factor in the band that carries this label, determined by a published in vivo method in which a specified quantity of product per unit area is applied to human skin and the ultraviolet dose required to produce a defined reddening is compared with untreated skin. It is a ratio against a specific biological endpoint.
What it does not
It is not a number of hours, not a percentage, and not a description of ultraviolet A protection. It does not scale linearly, so a higher number does not deliver a proportionally larger benefit. And it describes performance at the tested application quantity, which almost nobody replicates.
What UK guidance recommends
The NHS publishes practical guidance on sunscreen, including a minimum factor and a minimum ultraviolet A indication, applying twice rather than once, and reapplying regularly and after swimming or towelling. The British Association of Dermatologists publishes a public fact sheet along similar lines. We point at them rather than restating numbers, because their guidance is maintained and ours would not be.
What changes the number in the field
- Quantity applied. The largest factor by a distance.
- Evenness. A film with gaps has no factor at the gaps.
- Time and removal. Water, sweat, towels, clothing and hands.
- Photostability of the filter system over hours. Covered in our note on what happens to filters over a long day.
- Reflected ultraviolet from your surroundings, which increases the dose without changing the film. See reflected ultraviolet.
In vitro methods and where the field is going
The in vivo method has an obvious drawback: it requires exposing human volunteers to ultraviolet until a defined degree of reddening appears. There has been sustained work for years on developing in vitro alternatives for the sun protection factor, in the same family as the existing in vitro ultraviolet A method in ISO 24443.
The challenge is that a laboratory measurement through a film on a substrate has to reproduce the behaviour of a film on living skin, including how it spreads, how it sits in the texture of the skin surface and how it behaves as it dries. Those are hard things to model, which is why the in vivo method has remained the reference for so long.
For a reader, the practical implication is limited today and worth knowing about anyway: the numbers on packs are the output of a specific method, methods change, and a figure produced under one method is not automatically identical to a figure produced under another. This is a further reason to treat the factor as a category rather than as a precise quantity.
Choosing a factor for a condition
We do not tell people what to buy, but the reasoning available from the standards is straightforward. For a long day outdoors under load, the relevant considerations are a factor at the upper end of what you will actually apply generously, the presence of an ultraviolet A indication, and a water resistance claim if water or heavy sweat is involved. Beyond that, the differences between products matter less than the quantity you apply and the number of times you reapply, which is where our reapplication note takes over.
What is not verifiable
We publish no sun protection factor measurements, no comparisons and no product results. We have tested nothing. Anyone claiming to tell you the real world factor you are receiving on a beach is offering a guess dressed as a figure. The verifiable things are the method number, the assumption inside it, and what you personally did with the tube.
How to read the front of a pack in ten seconds
Look for the factor. Look for the ultraviolet A indication. Look for the water resistance claim if you are going near water. Ignore everything else on the front, because the rest is presentation. Then apply considerably more than feels necessary, and reapply. That sequence is worth more than any amount of comparison between numbers.