Whether you're looking for a lightweight waterproof jacket for your spring and summer hikes or a waterproof ski jacket, all major brands like The North Face, Marmot, Mountain Hardwear and Columbia Sportswear will list the waterproof rating in millimeters. But how is that measured and what does it mean?
How Waterproof Rating Are Measured
To find the waterproof rating of a jacket the manufacturer will take a piece of the fabric and stretch it out flat. A one square inch tube is then placed on top of the fabric and it is filled with water. Almost any fabric will allow water to penetrate it after a prolonged exposure and under increased pressure. The number in the waterproof rating is the number of millimeters of water it takes in the one square inch tube over a 24 hour period before the fabric allows water to pass through.
How to Compare Waterproof Jackets
Using the waterproof ratings described above, you know that the larger the number the more waterproof a jacket is. A 5,000 mm waterproof jacket will keep water out for a while but a 20,000 mm jacket will be able to withstand a much larger storm. Therefore, I would suggest buying the jacket with the highest rating in your price range but you'll also want to take weight, style and breathability into consideration.
What Do Waterproof Ratings Mean?
We said the higher the waterproof rating the better but if you're wondering what level of waterproof rating you should look for, refer to the chart below.
5,000 mm = You'll stay dry in a light rain or a dry snow.
10,000 mm = You'll stay dry in a moderate rain storm or snow fall.
20,000 mm = You'll stay dry in a heavy downpour or in wet snow conditions.
For more information check out the Waterproof Buying Guide or read this post to learn how to care for your waterproof jacket.