Water resistance testing and the 50% retention threshold

The point of the water-resistance result

Water resistance is not a separate protection number printed beside SPF. It is a comparison between two SPF measurements: one taken before the water-immersion procedure and one taken after it. ISO 18861:2020 specifies calculation of the percentage of water resistance as the ratio of SPF measured after the bath to SPF measured before it.

That structure matters because the result records retention, not preservation. A product may begin with one measured SPF and have a lower measured SPF after immersion, while still meeting the threshold for a water-resistance label. The label term therefore records that the product met a defined test convention. It does not record that its pre-immersion SPF was still present at the end of the procedure.

The second interpretation is not what the percentage calculation says. The calculation asks how the post-immersion SPF relates to the pre-immersion SPF. It does not turn the original SPF figure into an assurance of unchanged performance after immersion.

For context on the underlying SPF measurement, see the site’s related reading, What an in-vivo SPF test involves. For the standards named around these measurements, see The ISO standards behind the numbers.

The standards named in the method

Three standards identify different parts of the testing picture. ISO 24444 is the in vivo SPF determination. It is the standard named for determining SPF itself. ISO 16217:2020 specifies the water-immersion procedure for determining water resistance. ISO 18861:2020 specifies calculation of the percentage of water resistance.

Read together, those names establish an order. First, there is an in vivo SPF determination. There is also a water-immersion procedure. The percentage of water resistance is then calculated as the ratio of the SPF measured after the bath to the SPF measured before it. The result is not simply a report of whether a product has entered water. It depends on the relationship between the two SPF measurements.

This distinction is useful when a pack appears to compress several matters into a short phrase. A standard can describe a determination, a procedure or a calculation. The label convention decides which tested result supports a particular wording. None of these document titles, on their own, says that the printed SPF has not changed after immersion.

The details of the standards beyond these identified functions are not set out here. The relevant standards are paywalled, and this reference does not infer unverified bathing-period or drying details from their titles.

The 50% threshold in the labelling convention

Cosmetics Europe recommendation No 16 on water resistance labelling, published in 2005, sets out the convention described here. Under that recommendation, water resistant is substantiated over forty minutes of immersion and very water resistant over eighty minutes of immersion. For either claim, the product must retain at least fifty per cent of its pre-immersion SPF.

The threshold supplies a clear decision rule. It is a threshold for carrying either of those label terms. It is not wording that tells the reader that all of the original SPF remains after immersion.

Screenshot rule: a water-resistance claim under this convention indicates retention of at least half of the pre-immersion SPF over its stated immersion period. It does not indicate full retention of the pre-immersion SPF.

The word at least is important. Fifty per cent is the minimum retention threshold. A tested product may retain more, but the claim itself does not state how much more. A reader cannot derive a post-immersion SPF number from the words water resistant or very water resistant alone.

Why SPF 50 can become SPF 25 in this test

The arithmetic behind the threshold is direct. Fifty per cent of SPF 50 is SPF 25. Therefore, an SPF 50 product can meet the stated fifty per cent retention threshold while performing at SPF 25 at the end of the forty-minute immersion period used for the water-resistant claim. The pack does not say that in the water-resistance wording.

This is not a different category of SPF printed on the label. It is an illustration of what the retention threshold permits. The pre-immersion SPF is the reference point. The post-immersion SPF is the measured result after the bath. When the latter is half the former, the calculated retention is fifty per cent.

What the label says is a water-resistance term. What a reader may hear is that SPF 50 remains SPF 50 throughout the relevant period. The calculation does not support that second statement. It permits a reduction to half of the pre-immersion figure and still reaches the minimum threshold.

This example is about the stated method and threshold, not about any named product. It also does not establish the post-immersion SPF of every product carrying the term. The label convention sets a minimum retention condition; it does not disclose the individual product’s measured post-immersion SPF on the pack.

Why reapplication follows from the method

Reapplication follows from the logic of the water-resistance method rather than from a claim that water resistance has failed. The method compares a pre-immersion SPF with a post-immersion SPF and permits a water-resistance label where at least fifty per cent is retained over the relevant period. It therefore recognises that the measured SPF after immersion may be lower than the measurement before immersion.

A label term does not provide a continuing record of the post-immersion SPF. It tells the reader that the product met the applicable retention threshold for the specified immersion period. It does not say that the pre-immersion SPF value continues unchanged beyond that comparison.

Decision rule: where a claim is based on a comparison that allows retention of half the original SPF, do not read the claim as a statement that the original SPF remains unchanged after immersion.

This is a reading point, not an instruction about selecting a product or managing time outdoors. The relevant distinction is between the labelled starting SPF and the minimum percentage retained in the test convention. Reapplication is the practical implication of a method that permits a lower post-immersion SPF, rather than an extra promise supplied by the label term.

The related reading Waterproof and sweatproof on a sunscreen considers the separate question of those claim words. It should not be treated as an extension of the water-resistance result described here. This page concerns the specified immersion procedure, the percentage calculation and the label convention attached to them.

Industry guidance, UK cosmetics and the evidence file

Cosmetics Europe recommendation No 16 is industry guidance, not law. It provides the labelling convention for the water-resistant and very water-resistant terms described here. Its role is distinct from the legal classification of the product and from the location of the supporting evidence.

In the UK, the product is a cosmetic under the UK Cosmetics Regulation. The substantiation sits in the product information file. These points answer different questions. Classification answers what sort of regulated product it is. The product information file is where substantiation sits. The recommendation describes the industry convention for the relevant water-resistance wording.

Calling the recommendation guidance is not a reason to overlook its importance in reading the label. It explains the stated forty- and eighty-minute conventions and the fifty per cent minimum retention condition. But it should not be described as law, and the pack wording should not be expanded beyond what that condition records.

For the broader regulatory background, see the site’s related reading, The European recommendation behind the label.

What this page does not establish

This page does not compare products, identify which product to buy, or assess whether a particular product’s evidence is sufficient. It does not reproduce unverified procedural details from the paywalled standards. In particular, it does not state the number or length of individual bathing periods, drying rules, or any other figure beyond the forty- and eighty-minute immersion periods and the fifty per cent threshold supplied by the labelling convention.

It also does not say that every product carrying a water-resistance term has a post-immersion SPF of exactly half its starting SPF. Fifty per cent is the stated minimum retained proportion for either claim. The SPF 50 to SPF 25 example shows what the threshold permits; it is not a measurement attributed to a particular product.

The explanation applies to the specific UK framing given here: cosmetics under the UK Cosmetics Regulation, substantiation in the product information file, and the Cosmetics Europe 2005 recommendation as industry guidance. It does not set out classifications or label rules in other jurisdictions.

• It does not turn water resistance into a promise that SPF is unchanged.
• It does not add procedural details that have not been verified.
• It does not determine whether an individual pack’s claim is compliant.
• It does not cover waterproof or sweatproof claims beyond noting that they are separate wording questions.

The narrow conclusion is sufficient: the test compares before and after immersion, the calculation is a percentage ratio, and the label convention allows either water-resistance term when at least half of the pre-immersion SPF is retained over its stated period.