Different Manufacturers Use Different Scales and Ratings
One clothing maker might have a different rating system than another. One truism in the different ratings numbers is simple: the higher the number, the better the water resistance. For information on different manufacruers ratings, go to the websites in the reference section.
U.S. Determined Set Standard
There is a set standard in the United States. The U.S. government uses PSI to determine whether a rain jacket's fabric is waterproof; anything over 25 PSI on the fabric without the wearer getting wet means the jacket is 100 percent waterproof.
The Numbers Game
What does the 5k, 10k, or 20k rating mean? It measures what volume of water over a 24 hour period the fabric can withstand without the wearer getting wet. A 20k jacket can withstand 20,000,000 mm (nearly 66 feet) of rain in 24 hours before it permeates. 5k can withstand 16 feet in a 24 period; and 10k can withstand 32 feet in a 24 hour period.
Fabric, not Jacket
These numbers and ratings are for the fabric only, not the jacket itself. Seams are either sealed or unsealed, and zippers are typically not sealed. These seams and zippers are structural weak points and might lead to water getting in the jacket.
Knowing the record rainfall for 24 hours is around 6 feet, almost any jacket rated 5k to 20k will keep you dry for a day trip or jaunt through Seattle. But if you are going to be in the rain or wet for prolonged periods, stick to jackets rated 10k to 20k to stay dry.
Article Written By Eric Cedric
A former Alaskan of 20 years, Eric Cedric now resides in California. He's published in "Outside" and "Backpacker" and has written a book on life in small-town Alaska, "North by Southeast." Cedric was a professional mountain guide and backcountry expedition leader for 18 years. He worked in Russia, Iceland, Greece, Turkey and Belize. Cedric attended Syracuse University and is a private pilot.