Last updated: 16-Nov-16
You will have seen waterproof ratings, on jackets for example, of 10,000/10,000 and 20,000/20,000. But what do the rating numbers mean?
Manufacturers usually describe the waterproof and breathability of fabrics using two numbers. The first is in millimetres (mm) and is a measure of how waterproof a fabric is.
Waterproof rating
In the case of 20,000 mm fabric this refers how waterproof the fabric is when this test is carried out: If you put a square tube with inner dimensions of 1-inch over a piece of fabric, you could fill it with water to a height of 20,000mm (before water would begin to leak through. The higher the number, therefore the more waterproof the fabric. HH is short for Hydrostatic Head.
Breathability rating
The second number is a measure of how breathable the fabric is. It is rated in grams (g). This is the amount of water vapour that can pass through a square meter (m2) of the fabric from the inside to the outside in 24 hours.
So a 20,000g rated fabric allows 20,000 grams of vapour out. The higher the figure the more breathable the fabric is.
The balance of waterproof and breathable
If you want full waterproofing, you need a jacket that is made of rubber! However, this would make you sweat and the inside of the jacket would end up wet. At the other end of the scale is a jacket made of fabric that is very breathable but this would not keep you dry in a big downpour.
Most people want jackets that offer both waterproofing and breathability and there is a balance to be struck. It also depends on how hard you will be exercising in the jacket. Runners for example will want a jacket to be more breathable than, say, sailors.
So there is a balance to be found and in the end the chances are you will have jackets with different levels of waterproof/breathability for different activities.
Chart of Waterproof Rating
| 0-5,000 mm | No resistance to some resistance to moisture. |
| Ideal for: Light rain, dry snow. | |
| 6,000-10,000 mm | Rainproof and waterproof under light pressure. |
| Ideal for: Light rain, average snow. | |
| 11,000-15,000 mm | Rainproof and waterproof except under high pressure. |
| Ideal for: Moderate rain, average snow. | |
| 16,000-20,000 mm | Rainproof and waterproof under high pressure. |
| Ideal for: Heavy rain, wet snow. | |
| 20,000 mm+ | Rainproof and waterproof under very high pressure. |
| Ideal for: Heavy rain, wet snow. |
