Waterproof Rating Explained
What do the rating numbers actually mean when classifying a jackets waterproofing?
Manufacturers typically describe the waterproof breathability of fabrics using two numbers. The first is in millimetres (mm) and is a measure of how waterproof a fabric is. In the case of a 10k or 10,000 mm fabric, if you put a square tube with inner dimensions of 1" x 1" over a piece of said fabric, you could fill it with water to a height of 10,000 mm (32.8 feet) before water would begin to leak through. The higher the number, the more waterproof the fabric.
The second number is a measure of how breathable the fabric is, and is normally expressed in terms of how many grams (g) of water vapour can pass through a square meter (m2) of the fabric from the inside to the outside in a 24 hour period. In the case of a 20k (20,000 g) fabric, this would be 20,000 grams. The larger the number, the more breathable the fabric.