This is how resistant the fabric of the jacket is to water, and is obviously the main reason you’re looking at a waterproof jacket in the first place. There is always a big trade-off here with breathability – but more of that in the next section.
Waterproofing is measured in two ways and is set to a British Standard. You’ll see a garments level of water proofing or water resistance measured either in millimetres (for example, 1500mm) or in PSI (pounds per square inch).
The first measurement, in millimetres, is the most commonly used one and refers to the fabrics ‘hydrostatic head’ – how many millimetres of water can one square inch of the fabric withstand before the water starts to leak through. Waterproof fabric will generally range between 500 to 2,000mm.