Outdoor clothing and tent labels often carry a number followed by mm, such as 10,000mm or 20,000mm, alongside a breathability figure. These are not arbitrary marketing figures; they come from standardised laboratory tests, and understanding what they measure makes it possible to compare products across brands rather than relying on category names like premium or expedition.
What hydrostatic head measures
The hydrostatic head test places a column of water on the fabric and measures, in millimetres, how high the column can rise before water begins to penetrate. A fabric rated at 10,000mm withstands a column of that height; a fabric rated at 20,000mm withstands roughly double the pressure before failing, all else equal. This test measures resistance to sustained pressure, which matters for kneeling on wet ground or sitting under a heavy pack strap, more than for simple exposure to falling rain.
General guidance, not a technical specification
Manufacturer test conditions vary in fabric age, temperature and exact procedure even where the underlying method is standardised, so treat published figures as broadly comparable rather than precisely equivalent across brands.
Breathability is the other half of the equation
A fabric can be highly waterproof and nearly unwearable during exertion if it does not let perspiration vapour escape. Breathability is typically expressed as moisture vapour transfer rate, measured in grams of water vapour per square metre over 24 hours. Higher figures indicate more vapour can pass through the fabric, which matters most for aerobic activity like hiking uphill or running, where trapped sweat can leave a wearer just as wet as rain would have.
Why the two figures trade off
Increasing waterproofing generally means tightening the fabric's pore structure or adding coating layers, both of which tend to reduce how easily vapour passes through. Manufacturers manage this trade-off through membrane technology and garment design, such as pit zips and mesh linings, rather than eliminating it entirely.
Matching rating to activity
- Everyday and commuter use in moderate rain: lower hydrostatic head figures are usually adequate, since sustained kneeling pressure is uncommon.
- Backpacking and multi-day hiking: mid-to-high hydrostatic head with strong breathability matters more than maximum waterproofing, since sustained exertion generates significant internal moisture.
- Mountaineering and sustained heavy rain exposure: higher hydrostatic head becomes more important, often at some cost to breathability, since the priority shifts to keeping water out under prolonged, harsh conditions.
- Tents and groundsheets: floor fabric benefits from a notably higher hydrostatic head than the fly, because body weight concentrates pressure on the ground contact points.
Why ratings decline over time
Durable water repellent coatings degrade with UV exposure, abrasion, dirt and washing, which is why a jacket's real-world performance can fall well short of its original rating after a season of use. Re-applying a wash-in or spray-on treatment restores surface water repellency, though it does not restore a degraded membrane, which is a separate and less easily reversible failure mode. Manufacturers publish care instructions specifically because washing method affects how long the rated performance is maintained.
Checking claims before buying
- Look for both hydrostatic head and moisture vapour transfer figures rather than a single waterproof claim.
- Check whether the figures come from an independently referenced test method rather than an in-house-only claim with no stated procedure.
- Read manufacturer care instructions, since a garment's rated performance assumes proper washing and re-proofing.
- For tents, check floor and fly ratings separately, since they typically differ.
A hydrostatic head number tells you how much pressure a fabric resists before it lets water through. It says nothing about how comfortable you will be wearing it uphill.
