Hydrostatic head: why 30,000 mm is not always better

Hydrostatic head measures pressure, or more precisely, the water pressure a material can withstand before droplets penetrate it. The test is simple: a piece of fabric is clamped beneath a cylinder that is slowly filled with water. Depending on the standard, the pressure rises by 100 to 600 millimetres per minute. The test ends as soon as the third droplet becomes visible on the inside. The height of water reached is the hydrostatic head.

In figures: a hydrostatic head of 10,000 mm means a 10-metre-high cylinder of water could stand on the fabric before anything penetrates it. That corresponds to around 1 bar of pressure. Sounds impressive. But what does it mean in practice?

The biggest misconception of all. The Austrian Camping Club puts it well: if you thought the higher the value, the better, read on. Excessively high hydrostatic-head ratings for tents are often marketing, as real-world use rarely demands such extreme values.

Here is the interesting part: a silicone coating rated at 1,200 mm is often more waterproof than a PU coating rated at 4,000 mm. The reason: silicone is smoother and structurally denser than polyurethane. With PU, the hydrostatic head can halve after intensive use and continue to fall. With silicone, it remains almost constant.

PU therefore needs a much greater safety margin than silicone to deliver the same long-term performance. A high number on paper can quickly diminish in practice.

Under EN 343:2003, clothing is considered waterproof from 800 mm upwards (Class 2), and from 1,300 mm upwards as Class 3. That may sound sufficient, but for outdoor use it is virtually meaningless. Swiss EMPA assumes that functional material is only truly waterproof from 4,000 mm upwards.

Another stumbling block: the USA and Europe measure differently. European manufacturers test new products, while American brands such as Big Agnes and MSR simulate five years of use. This means that 3,000 mm under the US standard performs significantly better in practice than 3,000 mm measured using the European method.

Wrong. Even Gore-Tex acknowledges that this test is not relevant to real-world use. When worn, there is no static pressure as there is in the lab. EMPA tested clothing with a hydrostatic head below 120 mm: under simulated rainfall, it remained completely dry. Hydrostatic head is a theoretical maximum value that does not reflect real conditions.

Hydrostatic head tells you about the fabric. The critical points lie elsewhere, in the construction: seams, zips and hood design. Every needle creates tiny holes in the membrane. Uncovered zips are entry points for water. And the hood’s cut determines whether water runs into the collar.

What many people underestimate: everyday activities generate surprisingly high pressure. A person weighing 80 kg produces around 1,000 mm of pressure when lying down. When kneeling, this rises to roughly 14,000 mm. This explains why waterproof trousers need a higher hydrostatic head than jackets.

A common misunderstanding: the jacket seems to be leaking, but often only the DWR coating has worn out. DWR (Durable Water Repellent) is a surface treatment that does not affect the membrane. Gore-Tex makes it clear: a waterproof jacket remains waterproof even when the DWR wears off.

But without effective DWR, the outer fabric becomes saturated, known as wetting out. The result: the jacket becomes heavy and clammy. The film of water blocks breathability. Water vapour condenses on the cold outer layer. You get wet from the inside, even though the membrane is still waterproof.

Even the best membrane has limits, regardless of its hydrostatic head. Moisture transport requires a temperature difference of at least 15 degrees between the inside and outside. At outdoor temperatures above 20 degrees, or with too many layers, transport slows down or stops altogether.

The RET value (Resistance to Evaporating Heat Transfer) is a better indicator of breathability than the hydrostatic head. The lower the RET, the better the moisture transport. For sports, we recommend a RET below 13.

High-quality rain jackets start at 10,000 mm, well above the standard. For 95% of all outdoor activities, a jacket with 10,000–20,000 mm from an established manufacturer is sufficient, provided the seams are taped and the DWR is high quality.

For tent groundsheets, the hydrostatic head matters more than for the flysheet, as kneeling and lying down create concentrated pressure. For ultralight tents with lower ratings, look for silicone coating: it remains waterproof for longer than PU.

The hydrostatic head alone is not a reliable buying criterion. These points matter just as much, and often even more:

Hydrostatic head is a useful guide, but it is not a reliable buying criterion on its own. At first glance, it seems simple: the more waterproof something is, the less breathable it is. In reality, it is more complex.

30,000 mm is not automatically better than 10,000 mm: construction and coating type are decisive. The EU standard of 1,300 mm is not enough for outdoor use. Silicone with 1,200 mm can perform better over the long term than PU with 4,000 mm. DWR is often more important for how dry you feel than the membrane. And the real weak points are seams and construction rather than the material.