All quality manufacturers scientifically rate their jackets using the hydrostatic head test, the jacket fabric is pulled tight under a 1 inch diameter sealed tube of water. This is then observed over 24 hours to ascertain how many millimetres of water the fabric can withstand before it starts to leak through. Some manufacturers do additional testing, for example, Patagonia repeats the hydrostatic head test after putting their jackets through a 'Killer Wash' to ensure durability and long-term performance.
The waterproof grade is given as a measurement of how much water a jacket can withstand before leaking occurs. Common waterproof grades include: 5k (5,000mm), 10k (10,000mm), 15k (15,000mm) and 20k (20,000mm). The higher the number, the more waterproof the jacket. In the case of a 5k or 5,000 mm fabric, you could fill the 1 inch diameter sealed tube with water to a height of 5,000 mm (16.4 feet) before water would begin to leak through.
What does this mean in reality? We have a general guide for the conditions a jacket will be suited to, but keep in mind that other factors will also affect a jackets suitability to conditions such as, jacket age, when it was last re-waterproofed (which should really be called re-water-repellented), damage to the jacket and the manufacturer quality.