What the number measures
A waterproof rating expressed in millimetres comes from a hydrostatic head test. A column of water is held against a fixed area of fabric and the height of that column is increased until water is forced through. The height at the point of failure, in millimetres, becomes the rating.
So a 20,000 mm fabric withstood twice the water pressure of a 10,000 mm fabric in that particular test. That is the entire claim. It says nothing about how the garment is stitched, how well the hood seals, or how the fabric behaves after two years of washing.
Why the same number can mean different things
Several test standards exist, and they do not all apply pressure at the same rate or against the same sample area. A manufacturer may test a new fabric sample, a washed sample, or a sample after abrasion — and the results differ substantially. Brands are not obliged to say which method produced the figure on the hangtag.
This is why we treat a single millimetre figure as one input among several rather than as a verdict. When two sources disagree about a fabric's rating, we say so and explain the disagreement instead of picking the more flattering number.
- 5,000 mm — light, brief rain; common in inexpensive rainwear and some softshells
- 10,000 mm — the general-purpose range for hiking and everyday rainwear
- 20,000 mm — sustained heavy rain and pressure from pack straps or sitting
- 30,000 mm and above — marketing headroom more often than a practical difference for most users
Why many premium jackets have no rating at all
GORE-TEX garments carry no published hydrostatic-head figure. Gore certifies the finished garment against its own wet-weather requirement instead. Patagonia's H2No works similarly: it is a testing standard applied to garments, not a number attached to a fabric.
The practical consequence is that you cannot rank a GORE-TEX jacket against a 20,000 mm-rated jacket numerically. Anyone who does so is inventing a comparison. Where a figure does not exist, our database says "Not publicly specified by manufacturer" and we assess construction instead.
What to look at instead
Construction is more informative and you can verify it yourself. Fully taped seams, a storm flap or water-resistant main zip, an adjustable hood with a stiffened brim, and a three-layer laminate rather than a printed inner surface all have predictable effects.
Pressure points matter too. Water is forced through fabric under load, so the shoulders under pack straps and the seat when you sit on wet ground fail before the panels that carry no pressure. A high rating helps most exactly there.