Waterproof ratings are a bit of a mystery to most of us, especially since they are often displayed in a range of different ways. During production and testing, a 1" x 1" tube is placed over the fabric and filled with water. Eventually the fabric will begin to leak, at which point researchers measure the height of the water to calculate the fabrics waterproof rating.
Some manufacturers also display waterproof ratings in grams, indicating how much water vapour can escape a fabric over the course of 24 hours. The waterproof rating rule of thumb is simple: the higher the number, the more waterproof the material.