What Is Hydrostatic Head? A Layman’s Guide

If you've ever done any shopping for wet-weather gear, you've no doubt come across this term multiple times. But what does it mean? And what level of waterproofing do you need? Learn more in this guide!

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Written by: | Reviewed by: Brian Conghalie
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Buying gear and clothes for the great outdoors is a touch more complicated and consequential that picking up our “civvy” gear for kicking about town. While getting a gear purchase wrong for the pub on Friday night may result in a spot of light ridicule and an extended stint of singledom, for the outdoors, the payback for a poor purchase could be far more severe.

As such, when it comes to the “biggies” of our backcountry belongings — if you want the best waterproof hiking pants, jackets, tents, and groundsheets — we want to know we’re spending our would-be beer money on something that’s fit to task. But beyond the breezy and bold claims of the marketing departments, how are we to know what’s going to work?

Enter the acronym! In Part One of our Hiking Duds Decoded Series, we uncovered the mysteries and exposed the blessings of “What is DWR?“; in part two, we turn our attention to the equally enigmatic HH — Hydrostatic Head.

What is hydrostatic head?

Below, we aim to demystify the mystified with a thorough, straight-talking lowdown on the workings and wonders of HH, starting off with a few facts and figures before moving onto an explanation of how these can be interpreted practically for the everyday hiker.

Hydrostatic Head: A definition

The acronym HH stands for Hydrostatic Head. In a few words, this refers to the tests and subsequent ratings used to determine and denote the waterproofing capacity of any textile, most commonly jackets, tents, tarps, and groundsheets. (Note: there is a difference between water resistant and waterproof – these are not interchangeable terms).

Manufacturers use these tests and ratings to differentiate between varying levels of waterproofing. Whereas the label “waterproof” was once a one-size-fits-all umbrella term, these days — and thanks to HH testing — we can now distinguish between products and garments that are very waterproof and those that are only marginally waterproof.

How is Hydrostatic Head Tested?

The HH rating of any product is tested by applying water pressure (i.e. hydrostatic head pressure) on the surface of the fabric and measuring how much it can withstand before allowing water to pass through.

An open-ended laboratory test tube is placed on top of the fabric and filled with water. As the water level rises, the pressure on the fabric increases correspondingly. The point at which the fabric begins to allow water to seep through is what is used to determine its HH, which is given in millimeters. For example: if the fabric begins to leak with 10,000mm of water pressure, then its HH is 10,000mm. 

How to Translate HH into Something Practical

For the purposes of the user, HH can be translated using the following guidelines:

Tents

1,000 HH = The minimal legal requirement to call a tent “waterproof” (but, in practice, found only in very basic tents capable of withstanding the very lightest of showers).

1,500mm = A ballpark figure for summer tents when the worst conditions you expect to encounter are light showers. Prone to leakage after extended exposure to moderate rainfall.

2,000mm = The most common rating for three-season tents and capable of withstanding a combination of heavy rain and driving wind.

3,000+ mm = The proverbial “bombproof” of backcountry parlance, tents with this HH rating are commonly of the expedition and alpine variety and are able to withstand heavy downpours, gale-force winds, and the pressure of objects or bodies pressed against the fabric (which makes lower-rated models prone to leakage).

Groundsheets

Groundsheets are subject to more pressure applied upon them by the bodyweight of a tent’s occupants and other equipment inside the tent, both of which make them more liable to leakage. As such, a higher HH (in the region of 3,000mm and upwards) is required to ensure reliable waterproofing.

Jackets 

In the UK, a mere 1,500 HH rating is required for a jacket to be advertised as ‘waterproof’. In all but a few cases, however, jackets marketed as out-and-out shell layers boast far higher figures. The following guidelines offer an idea of how these figures translate into performance in the field:

10,000mm = Suitable for light rain showers but liable to leak at pressure points where the straps of your backpack are in contact with the jacket (the shoulders, back, and belt area)

20,000mm = Adequate for heavy rain showers and is usually a more breathable fabric than those with a higher HH rating

30,000mm = The HH rating used by manufacturers such as eVent (in DV Expedition models), garments with this rating provide solid waterproofing in even the heaviest downpours and are very unlikely to leak even at pressure points or in the most extreme conditions. On the downside, 30,000mm-rated fabrics tend to sacrifice breathability and this degree of waterproofing is considered overkill by many other garment manufacturers.

HH of Various Brands

Different brands use different HH ratings for waterproof garments and camping products.

Goretex

Despite being a market leader in waterproof products, Gore-Tex is not a great fan of HH testing and doesn’t use it to evaluate the waterproofing capacity of its fabrics. Instead, the folks at Gore put their products through a very rigorous series of tests that are designed to simulate a variety of rain conditions — a process they believe offers a more practical and thorough examination of a garment’s waterproof performance. While this may seem like a cop-out, Gore’s GUARANTEED TO KEEP YOU DRY™ promise shows just how much trust they place in their product and testing process.

eVent

eVent currently offers 3 variations of waterproof fabric which each have different Hydrostatic Head ratings:

  • eVent DV Expedition — 30,000mm: All but impregnable in terms of waterproofing but slightly lacking in breathability compared to the Alpine and Storm varieties (below)
  • eVent DV Alpine — 20,000mm: A performance-oriented, 3-layer laminate that strikes a perfect balance between waterproofing and breathability
  • eVent DV Storm — 10,000mm: Places an emphasis on breathability over waterproofing, doing the former exceptionally well and the latter well enough to deal with moderate showers

Pertex

Pertex comes in two forms of waterproof fabric: Pertex Shield and Pertex Shield Pro.

  • Pertex Shield fabrics focus more on breathability than water resistance and use 2, 2.5, and 3-layer constructions with hydrostatic head ratings in the region of 10,000mm
  • Pertex Shield Pro is intended for use in more extreme conditions and uses a highly breathable, 3-layer waterproof construction with hydrostatic head ratings of around 20,000mm

Polartec

Polartec NeoShell products are made with a cutting-edge blend of softshell flexibility and stretch and hardshell waterproofing. While the HH rating of Neoshell garments is a relatively low 10,000mm, this fabric is all about providing maximum breathability without sacrificing waterproof performance and resistance. Ideal for highly aerobic activities such as mountaineering, ski mountaineering, ski touring, and mountain running or fell running.

Last update on 2024-07-12 / Affiliate links / Images from Amazon Product Advertising API

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Kieran James Cunningham is a climber, mountaineer, and author who divides his time between the Italian Alps, the US, and his native Scotland.

He has climbed a handful of 6000ers in the Himalayas, 4000ers in the Alps, 14ers in the US, and loves nothing more than a good long-distance wander in the wilderness. He climbs when he should be writing, writes when he should be sleeping, has fun always.

Kieran has taught mountaineering, ice climbing, and single-pitch and multi-pitch rock climbing in a variety of contexts over the years and has led trekking and mountaineering expeditions in the Alps, Rockies, and UK. He is currently working towards qualifying as a Mountaineering and Climbing Instructor and International Mountain Leader.

Kieran’s book Climbing the Walls—an exploration of the mental health benefits of climbing, mountaineering, and the great outdoors—is scheduled for release by Simon & Schuster in April 2021.

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