Hydrostatic head is the one number that gets quoted and the one number most people misread. Here is what it actually measures.
What the number means
A fabric is put under a sealed column of water and the height of that column is raised until water forces through. A 20,000mm rating means it held back a twenty-metre column. It is a laboratory measure of pressure resistance, nothing more.
For context, a fabric is generally described as waterproof from about 1,500mm. Most of the shells we stock are between 10,000mm and 30,000mm.
Why a bigger number is not automatically better
Above roughly 20,000mm the rating stops being the thing that decides whether you stay dry. What decides it instead is breathability, taped seams, and whether the water is getting in through the fabric at all or through the cuffs, the hem and the zip.
The number that matters as much and gets quoted less
MVTR — moisture vapour transmission rate — is how much sweat the fabric lets out, in grams per square metre per 24 hours. A very waterproof, poorly breathable shell will leave you just as wet as a leaky one, from the inside. Where a manufacturer publishes both, we show both.
What we do when it is not published
We leave it out. A windproof jacket or a fleece has no hydrostatic head because it is not a waterproof, and quoting a number for it would be inventing one. If a product page here shows no rating, that is why.