Waterproof Ratings Explained

Rain jackets are sold on two numbers: a waterproof rating in millimetres and a breathability rating in grams. Both measure real things under laboratory conditions, and neither predicts how a jacket performs on a wet ridge as well as buyers assume.

1. 1

   What the mm rating measures

   Hydrostatic head: the height of a column of water the fabric holds before leaking. 10,000mm handles sustained rain; 20,000mm handles heavy rain under pressure such as pack straps or kneeling. Above roughly 20,000mm the differences stop mattering for hiking, because rain does not exert that pressure.

2. 2

   What the g/m² rating measures

   Moisture vapour transmission rate — how much water vapour passes through a square metre in 24 hours. Higher is more breathable. The catch is that manufacturers use several different test methods that produce very different numbers, so ratings are not comparable across brands.

3. 3

   Why the numbers mislead

   Both are measured on clean, new fabric in a lab. In the field, the surface treatment wears, the face fabric wets out, and breathability collapses regardless of the membrane's rating. A well-maintained 10,000g jacket outperforms a neglected 20,000g one every time.

   Reproofing guideWhy maintenance beats specifications

   Read Guide →
4. 4

   Layer count matters more than ratings

   A 2-layer jacket has a loose hanging liner — cheapest and least durable. A 2.5-layer has a printed inner coating — lighter, packs small, feels clammy. A 3-layer bonds a fabric backer to the membrane — most durable, most breathable, most expensive. This tells you more about real performance than any number.

5. 5

   What to check instead of ratings

   Pit zips, hood adjustment that works with a hat, a hem drawcord, and seams that are fully taped. These determine whether you are comfortable in rain far more reliably than the spec sheet.

What the Numbers Mean