Hydrostatic Head Explained: Waterproof Ratings for Tents and Jackets
· Updated
Hydrostatic head, usually shortened to HH, is a measure of how waterproof a fabric is. It is given in millimetres, and it tells you the height of a column of water the fabric can hold back before water starts to push through. A flysheet rated 2,000mm holds a two metre column of water; one rated 5,000mm holds five metres. The bigger the number, the more waterproof the fabric.
Under the UK and EU test standard, a fabric counts as officially waterproof from 1,500mm. That is the floor, not the target. For reliable performance in British weather you want more, and how much more depends on whether you are buying a tent flysheet, a groundsheet or a jacket. This guide gives you the numbers that actually matter.
It is part of the complete UK camping kit guide from Camping Sites in Britain, where you can find and compare campsites, caravan parks and glamping across the country.
What hydrostatic head measures
HH measures water pressure resistance, not how "thick" or "tough" a fabric feels. The fabric is clamped under a rising column of water in a lab, and the height at which water first forces through the weave is recorded in millimetres. The test method is set out in ISO 811, the international standard for resistance to water penetration, so the number on a Vango flysheet is measured the same way as the one on a Berghaus jacket.
That standard height is why 1,500mm keeps coming up. To carry a "waterproof" label under UK convention a fabric has to pass 1,500mm or more, according to outdoor brand Craghoppers. Anything below that is "water resistant" at best, and will wet through in a proper downpour.
The numbers campers actually need
Different bits of kit face different amounts of water and pressure, so they need different ratings. Here is what to look for.
Tent flysheets: 2,000mm and up
A flysheet is sold as waterproof from around 1,500mm, but for the mixed, wind-driven rain you get across most of Britain you want 2,000mm or higher. At 3,000mm to 5,000mm you have real headroom for a wet week in the Lake District or a Scottish autumn. Wind matters as much as rainfall here, because it drives water against the fabric and effectively raises the pressure it has to resist.
Groundsheets: 3,000mm minimum
A groundsheet needs a higher rating than the flysheet above it, and 3,000mm is a sensible floor. The reason is pressure. When you kneel, sit or lie on a groundsheet resting on wet ground, your body weight forces water up against the fabric far harder than rain ever does. A knee on a 2,000mm groundsheet over a puddle can push water straight through. Many quality groundsheets run to 5,000mm or 10,000mm for exactly this reason.
Waterproof jackets: 10,000mm and up
For a jacket that keeps you dry through sustained rain, look for 10,000mm or more. A hillwalking or camping shell will typically sit between 10,000mm and 20,000mm. The catch is that a jacket also has to let sweat out, so the HH number is only half the story. Breathability, measured separately, is what stops you getting soaked from the inside on a hard walk. The best waterproofs balance the two rather than chasing the highest possible HH.
Quick reference table
Use this as a rough guide when you are comparing kit. Treat the numbers as minimums for reliable British conditions, not as marketing badges to beat.
| Item | Waterproof from | Aim for (UK weather) | Why |
|---|---|---|---|
| Tent flysheet | 1,500mm | 2,000mm to 5,000mm | Wind-driven rain raises the pressure on the fabric |
| Tent groundsheet | 3,000mm | 3,000mm to 10,000mm | Body weight forces water up from wet ground |
| Waterproof jacket | 1,500mm | 10,000mm or more | Sustained rain, plus breathability to shed sweat |
| Tarps and bivvy bags | 1,500mm | 3,000mm or more | Direct exposure with little shelter |
Why the number is not the whole story
A high HH rating is worthless if the water gets in around it, so three other things matter just as much as the figure on the label.
Taped seams. Every stitch is a needle hole. Without taped or sealed seams, water tracks through the stitching regardless of how waterproof the fabric itself is. Check that the seams are taped before you trust the rating.
The DWR finish. Durable water repellent is a coating on the outer face that makes rain bead and run off. When it wears out the outer fabric "wets out", soaks up water and feels clammy, even though the HH rating underneath is unchanged. A DWR finish is renewable with a spray or wash-in treatment.
Age. HH is not permanent. Flysheet coatings degrade with UV, flexing and packing away damp, so an old tent can leak long before its rated number suggests it should. This is normal wear, and it is fixable. Our guide to how to waterproof a tent walks through reproofing a tired flysheet, and the best waterproof jackets for campers covers keeping a shell performing over the years.
The trade-off with breathability
A very high HH rating can come at the cost of breathability, which is why more is not always better. Push waterproofing hard enough and a fabric can trap warm, moist air inside, leaving you damp with condensation or sweat even in the dry. For a tent that shows up as morning condensation on the inner; for a jacket it shows up as a clammy back on a climb.
This is why manufacturers pair the HH figure with a breathability rating and with features like vents, mesh panels and pit zips. When you are choosing a jacket for active use, a sensible balance of the two beats a huge HH number on its own. When rain is forecast for a whole trip, pack your kit knowing this, and our guide to preparing for a rainy camping trip covers pitching and packing for a wet weekend.
Frequently asked questions
What hydrostatic head is waterproof?
A fabric is officially classed as waterproof from 1,500mm under the UK and EU standard. For dependable performance in British weather, aim higher: 2,000mm or more for a tent flysheet, 3,000mm or more for a groundsheet, and 10,000mm or more for a jacket in sustained rain.
Is a higher hydrostatic head always better?
Not always. A very high rating can reduce breathability, so a jacket may keep rain out but leave you damp with sweat. For active use, a balance of waterproofing and breathability beats the highest possible number. Taped seams and a working DWR finish matter as much as the figure.
What HH rating does a tent need for UK camping?
Look for a flysheet of at least 2,000mm and a groundsheet of at least 3,000mm. Flysheets are sold as waterproof from 1,500mm, but that is a bare minimum. Ratings of 3,000mm to 5,000mm give you comfortable headroom for a wet British week.
Why does a groundsheet need a higher rating than the flysheet?
Because it takes your weight. Kneeling or lying on a groundsheet over wet ground forces water up against the fabric far harder than rain pressing down on a flysheet. That extra pressure is why groundsheets are rated 3,000mm and up, well above the flysheet above them.
Does hydrostatic head decrease over time?
Yes. Flysheet coatings and DWR finishes degrade with UV, flexing and being packed away damp, so an ageing tent leaks sooner than its rated number suggests. This is normal and fixable. Reproofing the fabric and refreshing the DWR restores much of the lost performance.
What is the difference between waterproof and water resistant?
Waterproof means the fabric passes 1,500mm or more and sheds sustained rain. Water resistant means it handles light showers only, usually below 1,500mm, and will wet through in a downpour. Always check the HH figure rather than trusting the label alone.
The short version
Hydrostatic head tells you how much water pressure a fabric holds back, in millimetres, and waterproof starts at 1,500mm. Aim for 2,000mm or more on a flysheet, 3,000mm or more on a groundsheet and 10,000mm or more on a jacket, then keep the seams taped and the DWR fresh. Once your kit is sorted, browse campsites across Britain and go and enjoy the rain.
