Waterproof and Breathability Ratings Explained

Waterproof, breathable outerwear has come a long way in recent years. In order to help customers determine what type of weather conditions and activities a particular garment will perform best in, manufacturers attach a lot of different numerical ratings to their products. This waterproof and breathability ratings guide will help you make sense of all those numbers and determine which piece of outerwear is right for you.

How Are Waterproof Ratings Measured?

The outdoor industry determines waterproof ratings based on a water column test that measures water permeability under pressure. Pressure is an important aspect of this measurement because the same ski pants that are keeping snow out when you're standing still may become damp when you apply pressure by sitting on a chairlift or ground.

In the water column test, a one-inch-diameter column of water is placed over the fabric in question for a period of 24 hours. The millimeter rating comes from how high you can fill the column before any water gets through the fabric in the allotted 24-hour time period.

A waterproof jacket should prevent water from soaking into the fabric. Image courtesy of GORE-TEX.

Waterproofing Ratings Explained

  • Less than 5,000 mm/24 hours - lightly water-resistant. It is misleading when a manufacturer advertises a waterproof rating this low. This means this fabric will protect you from mist, or a one-time light splash, but little more. Almost all outerwear and midlayers will provide better protection.
  • 5,000 mm (often written as '5K waterproof') - water-resistant; stands up to light rain and dry snow, under no pressure.
  • 10,000 mm (10 K) - somewhat waterproof; stands up to moderate rain and average snow, under light pressure.
  • 15,000 mm (15K) - waterproof; stands up to moderate rain and snow under moderate pressure.
  • 20,000 mm (20K) - very waterproof; stands up to heavy rain and wet snow under heavy pressure; this is what you want for storm skiing and serious rainwear.
  • More than 20,000 mm - super waterproof under very heavy pressure; this is like what garbage bags and rain boots are rated; great for water protection, but this comes at the cost of breathability.

How Is Breathability Measured?

In order for high-performance fabrics to keep external water from entering and letting internal water vapor (aka perspiration) escape, manufacturers utilize a variety of different technologies. They all basically involve some kind of membrane with pores large enough to let water vapor molecules out, but not large enough to let liquid water seep in.

Breathable membranes allow water vapor to escape but don’t let precipitation through the membrane. Image courtesy of GORE-TEX.

Unfortunately, there is not an industry standard to test the transfer rate of water vapor, or what we refer to as breathability. And even if there was an industry standard, things would still be tricky because of how temperature and humidity affect water vapor transfer rate.

Breathability is often reported in terms of the amount of water vapor, in grams, that can pass through a square meter of the material in question during a 24-hour time period (g/m2).

If we momentarily assume that all tests measure moisture vapor transfer rate exactly the same, and that weather conditions don't change this performance, only then could you fully trust this general breakdown of breathability ratings across the industry.

Breathability Ratings Explained

  • 5,000 g/m2 or less - slightly breathable; not much perspiration can get through this material.
  • 10,000 g/mm2 - breathable enough for moderate activity.
  • 20,000 g/mm2 - breathable enough for highly aerobic activity; it lets lots of water vapor out so you don't get disgustingly sticky when you're working up a sweat.