mm rating
Also called hydrostatic head, waterproof rating
A waterproof rating measured by how tall a column of water a fabric holds back before leaking, in millimetres.
The test clamps fabric under a sealed tube and fills the tube with water until three drops push through. The height of the column in millimetres is the rating. 5,000mm is the usual threshold for calling something waterproof, 10,000mm handles sustained rain, and 20,000mm and above is serious foul-weather territory.
It measures pressure resistance, not durability or comfort, and a high number tells you nothing about breathability. A cheap PVC-coated suit can rate enormously high and still be miserable to wear.
Pressure comes from more than rain. Kneeling in a kayak, sitting on a wet seat and a pack strap pressing on a shoulder all push water through fabric that would shrug off a downpour.
The number also degrades. Ratings are measured on new fabric, and abrasion, folding and dirt all reduce real-world performance over a few seasons. That is how an old jacket that used to keep you dry stops doing so.
For all-day freshwater fishing, look for 10,000mm or better paired with a breathability figure, not one number on its own.
Related terms
Where this comes up
The guides that use mm rating and show it on real gear.
mm rating is one of 206 terms in the glossary.
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