Hydrostatic Head Explained

Hydrostatic head expresses resistance to water penetration as a pressure, using the height of a water column as the unit. 1,000 mm of water column is the pressure exerted at the base of a one-metre column of water; 20,000 mm is a twenty-metre column.

Laboratories report results in centimetres or millibars of water. ISO 811 notes that the SI unit, the pascal, is unusual for this test. Consumer marketing uses millimetres because larger numbers read better, but the measurement is identical: 2,000 mm H₂O is the same as 200 cm H₂O.

Where standards state requirements in pascals, the conversion published alongside the EN ISO 24232 class table applies: 1,000 Pa is approximately 102 mm H₂O.

A conditioned fabric specimen is clamped horizontally so that an area of 10,000 mm² — 100 cm² — is exposed to water pressure applied from below or from above, with the face of the fabric against the water unless another side is specified. Pressure then rises steadily while the specimen is watched continuously.

The permitted rates of increase are 10 cm H₂O per minute or 60 cm H₂O per minute. The standard states plainly that results obtained at different rates may not be the same, and that the higher rate is more applicable to fabrics with high water resistance. Water is grade 3 distilled water at either 20 °C or 27 °C, and the standard notes that the higher temperature yields lower hydrostatic head values, with the size of that effect varying between fabrics.

The result is the pressure at which water first appears at the third place in the specimen. Very fine droplets that form and do not grow are disregarded. At least five specimens are taken from different places in the fabric, avoiding creases and fold marks, and individual results plus the mean are reported.

This is the part of the standard most worth knowing, because it shows how much context a single marketing figure omits. A compliant report states the conditioning atmosphere, the water temperature used, whether pressure was applied from below or above, the rate of pressure increase, which side of the fabric was tested if not the face, any specimen deviations, the individual and mean results, the accuracy of the manometer, whether first-drop rather than third-place penetration was reported by agreement, and the date of test.

The standard also specifies the precision with which pressure is recorded: 0.5 cm H₂O below 1 m of water, 1 cm between 1 and 2 m, and 2 cm above 2 m. Its guidance on manometer ranges for manual apparatus tops out at 2 m of water column for closely constructed fabrics — an order of magnitude below the 20 m implied by a 20,000 mm claim, which is one reason such figures come from other equipment and are hard to place against the standard's own examples.

ISO 811:2018 defines the fabric-level hydrostatic pressure test. Its scope is explicitly fabric, and it limits its own relevance to articles subjected to water pressure for short or moderate periods.

The regulated garment standard changed identity in 2025, and this is worth stating precisely because much of the outdoor web still cites the old one. ISO published ISO 24232:2024 in October 2024 through ISO/TC 94/SC 13; its European adoption, EN ISO 24232:2025, states on its cover page that it supersedes EN 343:2019, and the CEN register records EN 343:2019 as withdrawn with effect from 22 October 2025. Reading ISO 24232's requirement clauses directly, the four waterproof classes are unchanged from the old standard — at least 8,000 Pa, then 8,000, 13,000 and 20,000 Pa after pre-treatment, with class 4 seams also tested after cleaning — and ISO 811:2018 is the normative method it uses. It also states that where specimens of one garment fall into different classes, the lowest result classifies the garment, and it carries an explicit warning that all classes can result in a restricted wearing time.

Beyond those, two further method families produce figures you will encounter. AATCC TM127 is the North American hydrostatic pressure method, performed without a restraint, with a restrained version published separately as TM208; AATCC notes its results may differ from those of its own rain and spray methods. ASTM D751 covers coated fabrics including rainwear, and its hydrostatic resistance section permits two distinct pieces of apparatus, a Mullen type tester and a rising water column tester. JIS L 1092 splits the test into a low-pressure method for materials such as waterproofed canvas and tent fabric and a high-pressure method for specimens testable at not less than 10 kPa; modern shell fabric figures come from the high-pressure route.

We should be candid about how much of this we could read. ISO 811 we have in full. For ISO 24232 the publisher's preview covers the scope, definitions and requirement clauses but stops before the test-method clauses and annexes, so the pre-treatment procedures are described here only as far as the requirement tables state them. AATCC TM127 and ASTM D751 are paywalled, and what appears above is drawn from the publishers' own scope statements and section lists — not from the procedures themselves.

Not on the evidence available to us. None of the documents above authorises a conversion or an equivalence between method families, and AATCC states explicitly that its hydrostatic result may differ from results of its own related methods. We therefore publish no cross-method ranking, and we will not describe a figure from one method as better than a lower figure from another.

The obstacle is not only which standard was used. Within ISO 811 alone, two pressure rates are permitted and the standard says results at different rates may not be the same; two water temperatures are permitted and the higher one gives lower values; the side tested and whether the specimen was pre-treated change the number again. Even the apparatus varies — ASTM D751 alone allows two different testers within a single procedure family.

The practical consequence for a reader comparing two hangtags is uncomfortable but honest: unless both brands state the method and the conditions, the only defensible comparison is that both fabrics passed a test their maker chose. That is why we record a test method alongside every performance figure on this site, and print "Not publicly specified by the manufacturer" when there is none.

The test measures one variable under controlled conditions on a flat, new, clamped patch of fabric. A jacket in rain is none of those things. It has needle holes at every seam, openings at the hood, cuffs, hem and pockets, a main zip, and fabric that flexes, abrades and ages. Water arrives as droplets with almost no pressure, and the pressure that does matter is mechanical and localised — pack straps, hip belt, knees, seat.

Two other real-world factors sit entirely outside the test. The first is the outer face fabric's repellency: once it saturates, the jacket feels cold and clammy and breathes worse, which wearers routinely mistake for leaking. The second is time — most consumer figures describe new fabric, and EN ISO 24232's requirement that higher classes hold their level after pre-treatment exists precisely because performance falls with use.

This is also why some manufacturers publish no figure at all. GORE-TEX certifies finished garments against its own wet-weather requirement rather than publishing a fabric millimetre rating, and Patagonia's H2No standard is a testing programme with before-and-after thresholds — 20,000 mm initially, then a requirement to still hold 10,000 mm after simulated long-term use. Neither absence tells you a jacket is less waterproof; both tell you the brand is describing garments rather than fabric patches.