Manufactures can measure how waterproof a fabric is by placing a column, filled with water over a piece of taut fabric, to see how many millilitres the fabric can withstand before leaking. The greater the hydrostatic head, the more waterproof the fabric.
To be classified as waterproof garments have to be given a minimum measurement of 1,500mm in the UK. Many manufactures also develop their own rigorous methods to test how waterproof and breathable a fabric is.
The waterproof rating can be reduced by increasing pressure on the garment such as sitting or kneeling on wet ground or wearing a rucksack.