We measure a fabric's resistance to a water column, in mm (now based on the JIS 1092 B standard). The higher the water pressure resistance, the more waterproof the fabric. A component with a waterproof rating of 20,000 mm can withstand up to 20,000 mm of water pressure (which is roughly equivalent to a rain shower lasting 3 hours).
With waterproofing greater than 10,000 mm, these trousers offer a very high level of waterproofing.