When buying a waterproof jacket for running or cycling, you may notice a number in the product specs, followed by mm and sometimes the abbreviation of HH. This is a hydrostatic head rating.
Not sure exactly what that is and why it's important? Don't worry, read on to find out...
What is a hydrostatic head?
Hydrostatic head is a measurement that’s used to determine how waterproof a fabric is.
The figure is based on a water column test. Traditionally, a column of water would be placed on top of a fabric and increased in volume (and thus pressure) until it penetrated the fabric.
The height of the water column at that moment is then taken and applied as a measurement in millimetres, which is the hydrostatic head rating.
In modern times, the test is carried out by a machine that will increase the pressure of the water without needing a column of water stretching skyward.
What gear has a hydrostatic head rating?
Hydrostatic head ratings are normally quoted for products that claim to be waterproof.
For triathletes, waterproof jackets (for both running and cycling) and dry bags will be where you’re most likely to come across a hydrostatic head rating, though you could also spot them when buying a waterproof running hat, for example.
Outside of the multisport world, you’d also find hydrostatic head ratings quoted for things like tents.
What's a good hydrostatic head rating?
For something to be classed as waterproof in the UK it has to have a minimum hydrostatic head rating of 1,500mm.
However, most waterproof jackets will exceed this, with figures of over 10,000mm very common, which is a good benchmark.
That being said, a hydrostatic head rating isn’t the only thing you need to take into consideration. To keep you dry a garment also needs to stop water coming through the seams, too. This is normally achieved via taped seams.
Of course, as hydrostatic head ratings go higher the weight and weave of the fabric often increase too, which can negatively affect both breathability and comfort, particularly when running.
It’s also worth noting that garments you’d expect to experience a lot of abrasion will typically have a higher hydrostatic head to deal with the increase in pressure.
Water column tests, which determine hydrostatic head, also don’t take into account how performance wanes over time or how the fabric performs in real-world conditions, so it’s always worth checking out expert and user reviews before investing.
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