Waterproof ratings are given in mm, based on the amount of water the fabric can withstand during a test with a tube of water with a 1 inch diameter. Rain jackets with a rating between 0mm and 1,500mm are water resistant, 1,500mm to 5,000mm is enough to designate a jacket waterproof, while 10,000mm to 20,000mm are referred to as highly waterproof.