Water resistance is the only claim on a sunscreen pack that speaks directly to what people actually do with it, and it is the claim most widely misunderstood. This note sets out what the procedure is, what it establishes, and where the boundary of its usefulness lies.
The shape of the procedure
The method is standardised. ISO 16217 covers the water immersion procedure for determining water resistance, and ISO 18861 covers how the percentage of water resistance is expressed. The underlying sun protection factor determination that both rely on is ISO 24444, the in vivo method.
The logic is a before and after comparison. Protection is established on treated skin. The treated skin is then immersed for a defined period, allowed to dry without towelling for a defined interval, immersed again, and so on for the number of cycles the claim requires. Protection is then established again, and the proportion retained determines whether the product may carry the claim.
The important structural fact is that a stronger claim means more immersion cycles, not different water. Very water resistant is the same procedure run for longer.
Reapplication clock: How the immersion procedure is structured
A schematic of the shape of the procedure, not of any product's performance. No retention figure is implied.
What the procedure genuinely establishes
It is worth being clear that this is a real test producing real information. It establishes that a formulation adheres to skin under immersion well enough to retain a defined proportion of its protection through a defined pattern of exposure to water. That is a meaningful property, it differs enormously between formulations, and a product that passes it behaves differently in water from one that does not.
It is also repeatable and comparable, which is the entire point of standardisation. Two products carrying the same claim have been through the same shape of procedure. That is not nothing, and the framework deserves credit for it.
The six things it does not simulate
1. Moving water
The procedure uses controlled conditions. The sea has current, chop and breaking water, all of which apply shear force to a film that still water does not.
2. Salt water
Sea water is not the same medium as the water used in a controlled procedure, and formulations do not necessarily behave identically in both.
3. Cold water
A British lake in June and a controlled test bath are different thermal environments, and temperature affects how films behave on skin.
4. Abrasion
The largest gap. Nothing in the procedure involves rubbing. In surfing, the board removes the film. In swimming, the stroke and the wetsuit do. In cycling, the hand does. Rub resistance is a separate property that the water resistance claim simply does not address. See water, sweat and rub resistance.
5. Towelling
The procedure specifically allows drying without towelling, because towelling would remove the film mechanically and make the test measure something else. In real life, everyone towels.
6. Your duration
The cycles are defined. Your swim is not. Beyond the tested pattern, the claim has nothing to say, which is why our conditions files place the reapplication clock where they do.
The quantity assumption sits underneath all of it
Every one of these procedures rests on the sun protection factor determination, and that determination applies a specified quantity of product to a measured area of skin. It is considerably more product than people apply in ordinary use. That assumption propagates through the water resistance result too: the retained proportion is a proportion of a protection level established at the tested quantity, not of whatever you actually put on. Our note on the tested quantity is the most important thing on this site to read alongside this one.
What the words on the pack mean
Water resistant, tested to retain protection after immersion
What it certifies
That a published, repeatable procedure was carried out, that protection was measured before and after a defined pattern of immersion, and that the proportion retained met the threshold the claim requires. Very water resistant reflects a longer pattern of immersion. These are genuine formulation differences and the claim is meaningful.
What it does not
It does not certify performance in moving water, in salt water, in cold water, under abrasion, after towel drying, or beyond the immersion periods tested. It is not a guarantee that any particular proportion of protection is present on your skin at any particular moment.
The United States expresses the same idea differently
Under the United States labelling rules, sunscreen is regulated as an over the counter drug and water resistance is stated on the label as a duration in minutes, following the Food and Drug Administration's procedure. The claim is more explicit about its own boundary, which has advantages for the reader. It is not directly interchangeable with the UK and EU categories, because the procedures differ. Our note on the United States position covers it.
How to use the claim well
- Treat it as a comparative signal between products, not as a duration for your session.
- Assume the claim expires at the end of its tested pattern, and reapply on that basis.
- Treat every towel dry as a full reset.
- In any activity with rubbing in it, do not rely on the claim at all, because it was never about that.
- Where you can put fabric between yourself and the sun, do, because fabric does not have any of these problems.
Reading the claim in five seconds
At a shelf, the useful sequence is short.
- Is there a water resistance claim at all? If not, assume none has been substantiated, and treat immersion as removing the film.
- Which claim is it? Very water resistant reflects a longer pattern of immersion than water resistant, and that is a genuine formulation difference worth having if you are going in the water.
- Is your activity about water or about rubbing? If it involves a board, a strap, a seal or a towel, the claim is not addressing your dominant loss mechanism.
- Stop. Nothing else on the front of the pack tells you anything further about water.
That sequence takes longer to read than to perform, and it is more useful than comparing any two products in detail, because the differences that matter most are between the claim categories rather than within them.
Why standardising this at all was worth doing
It is easy to write critically about the limits of a test method and lose sight of the alternative. Before water resistance was standardised, a manufacturer could describe a product's behaviour in water in whatever terms they chose, and nothing about that description was comparable with anything else on the shelf. A defined procedure with a defined threshold replaced an unbounded marketing claim with a bounded technical one. That is real consumer protection, and the correct response to its limits is to understand them rather than to dismiss the whole thing.
What is not verifiable
We do not publish retention percentages, thresholds or product results, because we run no tests and we will not repeat figures we cannot stand behind. What we will tell you is the number of the standard, so that you can look it up yourself, and the shape of the procedure, so that you can judge how far it travels towards the thing you are actually doing.