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You could learn the difference between a 500mm and a 5,000mm Hydrostatic Head rating through firsthand experience like this guy is, or just keep reading here and find out the much easier way…

JJ from our CRM team recently had an ACK customer call in wanting to know about the Hydrostatic Head rating for a product. Just like me, she wasn’t familiar with what this system measured. One of the coolest things about working at ACK is getting to research everything and anything about the gear we carry.

Here are the three facts you should know about the Hydrostatic Head rating system:

1.What is it?

Hydrostatic Head is a method of measuring how waterproof an article of fabric is. Manufacturers take a clear cylinder and securely clamp their material on the bottom end. The cylinder is then slowly filled with water and observed to determine the maximum height of the water within the cylinder before water seeps through the material. The process kind of looks like those paper towel absorbency tests you see in commercials where they dump water over the towels to see which holds up longer – just with all the water contained in a tube rather than over a bowl.


2.What do those numbers mean?

The numbers corresponding with a Hydrostatic Head rating let you know how waterproof or water resistant a material is. If something has a Hydrostatic Head rating of 5,000 mm, that means it could hold a cylinder full of 5,000 mm of water before it would leak out through the weave. For any fabric to be considered “fully waterproof” it needs a Hydrostatic Head rating of  at least 1,000 mm.

3.How do I use this new info?

Generally speaking, the bigger the number, the better. For something like a tent, you’ll find a rating of 1,000mm will resist light showers and 2,000mm will stand up to heavier rain, especially if it’s windy and rain is being pushed against the walls of your tent. For clothing, while Hydrostatic Head is an important consideration, keep in mind that a higher rating doesn’t necessarily mean a better garment. For certain activities like whitewater paddling where you’re sure to work up a sweat, you’ll want something with breathability like the NRS H2Core Rashguard. 

Next time you’re looking for a new rain jacket or tent you’ll have a better understanding of what to look for when determining waterproof v. water resistant now that you know what Hydrostatic Head ratings are all about!

Devyn, Freshly minted Hydrostatic Head Expert(ish)


PS Want even more info on Hydrostatic Head ratings? Check out this article from our friends at MSR who go in depth with their explanation!