Water Column Rating Explained
In brief: A water column rating is a number in millimetres that measures how much water pressure a fabric can hold back before it leaks. A higher number means more resistance to water being forced through. It is a useful comparison tool, but the test method matters and the numbers are not the whole story; real-world waterproofing also depends on seams, zips and the repellent finish.
What the number measures
Picture a sealed tube standing upright on a piece of fabric. You fill the tube with water and keep adding until the pressure forces the first drops through the cloth. The height of the water column at that moment, measured in millimetres, is the rating. A fabric that holds 10,000mm of water column resists more pressure than one that holds 5,000mm. The figure is sometimes written as the hydrostatic head.
The test methods are not interchangeable
This is the part most buyers never hear. There is more than one way to run the test. The ISO 811 method measures the actual height in millimetres at which water penetrates, giving you that familiar number. The AATCC 127 method is often reported as a pass or fail against a set pressure rather than a precise breakthrough height. Because the methods differ, a number from one test cannot be directly compared with a result from another. Treat a water column figure as a guide within a brand, not as an exact league table across brands.
How much is enough
There is a persistent myth that 1,300mm is a legal minimum for a garment to be called waterproof in Europe. There is no such universal legal threshold. What does exist is industry convention. As a rough rule of thumb, many in the trade treat around 1,500mm as the entry point for genuine waterproofing, with serious outdoor shells sitting far higher.
Rough guide to water column figures
- Below ~1,500mm water-resistant at best; fine for a light shower, not for sustained rain.
- ~1,500mm to 5,000mm waterproof for light to moderate rain and everyday use.
- ~5,000mm to 10,000mm solidly waterproof for general outdoor wear.
- ~10,000mm to 20,000mm serious mountain and hill protection.
- 20,000mm and above expedition-grade, holding out under pressure from pack straps and hard weather.
Why pressure points matter
The reason high numbers exist is that water pressure is not just about rainfall; it is about where the fabric gets squeezed. Kneeling, sitting on wet ground, or a backpack strap pressing a wet shoulder all multiply the local pressure on the cloth. A higher water column rating buys you margin exactly at those pressure points, which is why mountain shells carry such high figures even though no rain falls at 20,000mm of pressure.
Where the number stops mattering
A fabric can hold a huge water column and a jacket can still leak, because water finds the easy way in: needle holes at the seams and the teeth of a zip. That is why taped seams and water-resistant or storm-flapped zips matter as much as the headline figure. A 10,000mm jacket with fully taped seams will out-perform a 20,000mm fabric that has been sewn up without sealing. Pair the rating with the breathability figure too, since waterproofing and breathing together decide comfort.
Water Column Rating at OD's Designer Clothing
We use water column figures the way they are meant to be used: as a guide to where a jacket sits, from everyday water-resistance to expedition shells. Just as important, we look at the seams, the zips and the repellent finish, because those are what decide whether you actually stay dry.