Hydrostatic head at a glance
- Showerproof
- Under 5,000 mm
- Genuinely waterproof
- 10,000 mm and up
- Hard use and tent floors
- 20,000 mm and up
How the test works
The test clamps fabric under a sealed column of water and raises the pressure until water forces its way through. The height at which that happens is the rating. It's a laboratory number, not a weather forecast, but it does let you compare two fabrics on the same scale.
Why the numbers look absurd
Real-world pressures are lower than the numbers suggest, which is why the scale looks absurd. Rain alone barely registers. What generates pressure is you: kneeling on wet ground, a pack strap pressing a wet shoulder, sitting down. That's why the shoulders and seat are where cheap shells wet through first.
What hydrostatic head rating does a backpacking tent need?
A backpacking tent needs its floor rated well above its fly: floors commonly run 3,000 mm to 10,000 mm, since kneeling or sitting concentrates body weight onto a small patch of wet ground, while a fly needs only 1,200 mm to 3,000 mm because rain alone barely generates pressure.
Read the two numbers separately. A listing that quotes one figure for the whole tent is quoting the higher one.
What does a 20,000 mm jacket rating mean?
For a rain jacket, a 20,000 mm hydrostatic head rating sits at the high end of what hiking shells carry, built for mountain use where a loaded pack's straps press wet fabric against your shoulders under real pressure.
Most hikers get sustained-rain protection from 10,000 mm already; hiking shells generally run 10,000 mm to 30,000 mm, largely for that shoulder-strap pressure rather than rain falling straight down. Past 20,000 mm the number stops being the limiting factor, because breathability and construction fail you long before the fabric's pressure resistance does.
Why a high rating can still leak
Because the rating tests the fabric and the water comes in somewhere else. An untaped seam leaks at the seam regardless of the number. A floor whose seams sit at ground level leaks at the seams. A fly that does not reach far enough down lets rain blow under it.
Coatings also age. Polyurethane hydrolyses, silicone survives far longer, and a decade-old tent with an impressive original number may be well below it now. What keeps you dry is seam tape, a bathtub floor, adequate fly coverage and a fabric that has not degraded, roughly in that order.
What this means when you're choosing
- A floor rating well above the fly rating, since kneeling concentrates pressure
- The number for the specific panel, because tents quote different figures for fly and floor
- Whether the rating is initial or after a wash cycle, where stated
The common mistake
Shopping on the hydrostatic head number alone
A very high rating tells you nothing about breathability, and the two often trade against each other. Above about 20,000 mm the extra pressure resistance is not what decides whether you stay dry.
Common questions
What hydrostatic head do I need for hiking?
10,000 mm covers sustained rain for most people. Go to 20,000 mm if you hike in mountains, sit and kneel in the wet a lot, or carry a heavy pack that presses the fabric against wet shoulders.
Do tent floors need a higher rating than the fly?
Yes, because your body weight on a wet ground sheet generates far more pressure than rain on a fly ever does. Floors are commonly rated well above the fly for that reason.
Where this is defined
- ISO 811, resistance to water penetration under hydrostatic pressure, International Organization for Standardization