Hydrostatic Head
Hydrostatic head is the height of a water column a fabric holds back before water forces through, measured in millimetres under a standard such as ISO 811.
A 20,000 mm rating means the fabric resisted the pressure of a twenty metre column of water. It is the standard measure of waterproofing, and it is only comparable between fabrics when the same test standard and rate of pressure increase were used.
How the test works
A piece of fabric is clamped, water is pushed against one side at a controlled rate, and the pressure at which water penetrates is recorded. ISO 811 specifies the rate of increase and the water temperature. The American equivalent, AATCC 127, uses a different rate, and a faster ramp produces a higher number from the same fabric.
What the numbers mean in practice
| Rating | Copes with |
|---|---|
| Under 5,000 mm | Light showers. Water resistant rather than waterproof |
| 5,000 to 10,000 mm | Moderate rain, short exposure |
| 10,000 to 20,000 mm | Sustained rain, plus pressure from pack straps and sitting |
| Above 20,000 mm | Prolonged heavy rain, kneeling, heavy loads |
Rain itself never approaches these pressures. What does is a rucksack strap on a wet shoulder, a knee on wet ground, or sitting on a damp bench. That is why the difference between 5,000 and 20,000 shows up in real use.
Censored results
When a test is run to a fixed ceiling and the sample passes it, the report reads "exceeds 20,000 mm". That is a floor, not a measurement. The true value is unknown. Our own shell falls into this category and we say so rather than inventing a headline number.
What it does not tell you
The figure describes flat fabric in a clamp. A finished jacket leaks at untaped seams, standard zips, cuffs and hoods long before its fabric gives way. Ask about seam construction before you compare millimetres.
Related terms
- Waterproof and breathability ratings explained: the full picture, including what brands publish
- VEAR lab test results: our ISO 811 result and its limits
- Taped seams: why fabric ratings do not describe garments
- Glossary: all terms