At what point will it leak? In the outdoor industry, hydrostatic head (often abbreviated as HH) is a measure of the waterproofness of a particular fabric, most applicable to the fly of a shelter, canopy, and / or floor materials, as well as rain gear. The measurement reflects how high a column of water a secured piece of fabric could support before leakage occurs. This column of water is measured in millimeters, and the test is performed with a special machine or apparatus that applies water pressure against the fabric in question and measures the result. If the fabric is able to support a column of water 1 meter high before the pressure causes water to be forced through the fabric, a hydrostatic head of 1000mm applies. If the fabric supported a column if water 10 meters high, a 10,000mm rating would result.

The needed hydrostatic head rating will depend on the individual gear item and the intended application.

What Determines Hydrostatic Head & Examples

Just how high a fabric will score and how waterproof it will be depends on several factors such as thickness, weight, weave, and the thickness / quality of its waterproof coating or impregnation, and there are tradeoffs to be considered. A tent with a low hydrostatic head rating may experience "misting", where small droplets of water could find their way through the fabric under the pressure of something like a heavy, wind-driven thunderstorm.

Additionally, kneeling on a tent floor over wet ground could exert enough pressure to force water through the fabric on lower rated fabrics. However, the fabric itself may end up being lighter and result in less weight on your back. In the end it's up to you, but most (though not all!) backpackers generally find the hydrostatic head of silnylon (usually around 1200mm+) to be waterproof enough for the types of conditions encountered on a normal backpacking trip, and even lower rated fabrics can be found on some tents. Higher rated options are readily available as well if further waterproofness is desired (see our guide on how to choose a backpacking tent for more tent-related info).

For applications like a dry bag it all depends on the application. Sea to Summit's Ultra-Sil Dry Bags for example are not rated for prolonged submersion and if you were to try filling them with water they might leak under such high pressure, but they've worked quite well for me inside a pack as additional rain protection in a lower pressure application. As you might expect, higher hydrostatic head usually means heavier when it comes to backpacking and hiking fabrics.

Editor's Note: This Jargon installment originally appeared in Issue 10 of TrailGroove Magazine. You can read the original article here.