Hydrostatic Head - what does it mean?

Hydrostatic head is the measurement of the water pressure that a fabric will withstand - in other words, how waterproof it is.

Imagine a vertical tube with a fabric sample clamped at the base so that no water can escape – now add water to the tube. The height of the column of water in millimeters at the point where the water forces its way through the fabric/coating is called the hydrostatic head.

There is no internationally accepted standard for how high this should be for a ‘waterproof’ fabric.

Besides apparel, there are a couple of items of gear where knowing the hydrostatic head is vital* for understanding whether the product will work in the intended application: dry bags and tents/shelters

A dry bag intended for use inside a backpack will only require a modest hydrostatic head; a dry bag intended for paddling/whitewater will need a much greater ability to keep water out.

Ultra-Sil Dry bags are our go-to for storing gear inside your backpack. The fabric tests at over 2000mm (more waterproof than some tent floors), which is perfectly adequate for waterproof storage inside a pack, but not sufficient for use in paddle situations.

A dry sack intended for paddling will need a hydrostatic head of at least 10,000mm - like our Lightweight or Big River Dry Bags.

A tent suitable for moderately rainy environments needs a floor with a hydrostatic head of 1200 - 2000mm, and a rainfly with a hydrostatic head of 1200 - 1500mm

A tent suitable for torrentially rainy conditions / boggy ground needs a floor with a hydrostatic head of at least 4000 - 5000mm, and a rainfly ideally greater than 1500mm

* If the hydrostatic head for the dry bag or tent you are considering is not published, it is worth looking for a product where the data are available.

Read more on our BLOG.