What Is Hydrostatic Head?
Hydrostatic head, usually shortened to HH, is a measurement of how much water pressure a fabric can resist before water is forced through it. It is normally expressed in millimetres, so a fabric rated to 3,000 mm has resisted the pressure produced by a 3-metre column of water during the test.
You are obviously never going to have a literal three-metre tube of water sitting on top of your tent. The water-column figure is simply a convenient way of expressing pressure so one waterproof fabric can be compared with another.
Hydrostatic-head testing is commonly carried out using standards such as ISO 811, where water pressure is gradually increased against a fabric sample until penetration occurs. The important thing to understand is that the result applies to the fabric sample being tested, not automatically to the entire finished tent or jacket.
A higher HH rating generally means the material can withstand greater water pressure. It does not automatically mean the whole product is better quality, because seams, zips, coatings, design and manufacturing quality all matter too.
What Does a 3,000 mm Hydrostatic Head Mean?
A 3,000 mm HH rating means the fabric resisted pressure equivalent to a 3,000 mm, or 3-metre, column of water under the particular test method used. Likewise, 10,000 mm corresponds to the pressure generated by a 10-metre column.
The important word here is pressure. HH isn’t really measuring the volume of water the fabric can hold, and it isn’t predicting how many hours of rain you can stand in before getting wet.
That distinction matters because pressure can come from more than rainfall. Sitting on a wet tent floor, kneeling on it, tightening a backpack strap over a jacket shoulder or leaning against wet fabric can all create local pressure that drives water through a fabric more aggressively than rain falling gently on the surface.
What Is a Good Hydrostatic Head?
There isn’t one perfect number because tents, groundsheets, jackets and trousers face very different conditions. A flysheet hanging freely over a tent doesn’t experience the same pressure as a groundsheet with your knees pressing into wet soil, while a jacket under backpack straps has its own set of problems.
As a rough shopping guide, I am comfortable with tent flysheets around 2,000-3,000 mm and above from a reputable manufacturer for general camping. Jackets often use much higher figures such as 10,000, 20,000 or 30,000 mm because clothing experiences more movement and localised pressure.
I would never buy purely by choosing whichever product has the biggest number. A properly designed and seam-sealed 3,000 mm tent can keep you drier than a badly constructed tent claiming 10,000 mm.
Does a Higher Hydrostatic Head Mean Better Quality?
Not necessarily. It means the tested fabric resisted more water pressure under that particular test, which is useful information, but waterproofing is only one part of a product’s performance.
Higher-waterproof fabrics can sometimes be heavier, stiffer or less breathable depending on how the material is made. There are also several test standards and different test conditions, so two bare HH numbers are not always perfectly comparable unless you know how each was measured.
For tents, I would rather have sensible waterproof fabric combined with excellent seams, a good pitch and a properly designed groundsheet than chase the biggest HH number on Amazon.
What Is a Hydrostatic Head Worth if Everything Else Leaks?
Not much. A tent fly can use extremely waterproof fabric and still leak if water works through needle holes, damaged seam tape or poorly protected zips.
The same applies to jackets. Water can get through a hood opening, cuffs, pockets, zippers or damaged seams even when the main fabric itself has an enormous hydrostatic-head rating.
This is why waterproof-product testing needs to look beyond the fabric. Finished-product construction is every bit as important as the figure printed on the specification sheet.
How Is Hydrostatic Head Measured?
The basic principle is surprisingly simple. A piece of fabric is clamped into testing equipment and water pressure is steadily increased against one side until water penetrates the material.
Under ISO 811, the result is determined from penetration through the fabric under controlled hydrostatic pressure. The exact details matter because things such as pressure rate, water temperature, fabric conditioning and which side of the fabric is tested can affect the result.
Here is the original video showing the general idea behind hydrostatic-head testing:
Hydrostatic Head Ratings for Tents
I would treat the following as a rough buying guide rather than a rain-duration chart. The old version of this article said things such as 2,000 mm lasting a certain number of hours and 4,000 mm being impossible for rain to penetrate, which is much more certainty than an HH test actually provides.
- Around 1,000-1,500 mm – Relatively low waterproof pressure resistance. It may work for sheltered or fair-weather designs, but I would want more margin for regular camping in wet conditions.
- Around 2,000 mm – A common useful starting point for general tent flysheets. Good construction and sealed seams matter enormously at this level.
- Around 3,000 mm – A solid all-round rating for many camping tents and more than enough on a well-designed flysheet for a lot of normal wet-weather use.
- Around 4,000-5,000 mm – Plenty of pressure resistance for a flysheet and common on tents intended for sustained bad weather.
- 10,000 mm and above – Often more relevant to tent floors and specialist fabrics than something you necessarily need on an ordinary flysheet.
Groundsheets deserve special attention because your body weight can create much more pressure than rainfall. It is perfectly normal for a tent to use a substantially higher HH material underneath you than on the flysheet above.
Is 2,000 mm Waterproof Enough for a Tent?
It can be. A good-quality 2,000 mm flysheet with properly sealed seams and a sensible design can perform very well in rain.
I would be more cautious with a 2,000 mm groundsheet if I expected saturated ground and lots of pressure from knees, elbows and sleeping equipment. Again, the location of the fabric matters as much as the headline number.
Is 5,000 mm Good for a Tent?
Yes. A 5,000 mm flysheet has a very healthy level of resistance to water pressure and is more than I would worry about for most normal camping.
That does not mean a 5,000 mm tent is automatically better than a 3,000 mm one. If the seams are poor, the coating deteriorates or the tent is badly pitched, the higher number will not save you.
Hydrostatic Head Ratings for Waterproof Jackets
Waterproof jackets commonly use higher HH figures because the fabric experiences backpack straps, movement and pressure against the body. Again, these numbers are best used as broad comparisons rather than hard categories.
- 5,000 mm – Lower-end waterproof protection that can still be perfectly adequate for casual use and short periods outside.
- 10,000 mm – A common rating for practical waterproof outdoor clothing and a sensible starting point for general hiking.
- 15,000-20,000 mm – Strong waterproof pressure resistance suitable for more demanding and sustained outdoor use.
- 20,000-30,000 mm+ – Very high water-pressure resistance commonly associated with technical waterproof fabrics.
There is no meaningful rule that says 20,000 mm suddenly becomes “100% waterproof,” and 30,000 mm certainly isn’t the highest rating physically possible. Those were useful-sounding shortcuts in the old article, but they weren’t technically accurate.
Hydrostatic Head vs Breathability
Waterproofness and breathability are different measurements. HH tells you how much liquid-water pressure the material can resist, while breathability is trying to describe how effectively moisture vapor can move through the fabric.
There can be design trade-offs between waterproofing and breathability, but it is not as simple as saying every increase in HH automatically causes an equal reduction in breathability. Membrane chemistry, construction, face fabric, backing and test method all make a difference.
This is also why you need to be careful comparing breathability numbers from different manufacturers. Different test methods can produce figures that look directly comparable when they really aren’t.
What Is the Hydrostatic Head Rating of GORE-TEX?
GORE-TEX does not normally market its current waterproof products using one simple consumer hydrostatic-head figure. Instead, Gore tests materials and finished products against its own requirements for waterproofness, durability, wind protection and breathability.
The company’s testing programme includes laboratory testing as well as finished-garment rain-room testing. Gore’s rain room is designed to simulate conditions ranging from light rain to wind-driven downpours, which lets the company look at weak points such as garment construction rather than simply testing a flat square of fabric.
It is also worth noting that not every product carrying GORE branding is necessarily waterproof. Current black-diamond GORE-TEX products are the ones associated with the company’s “GUARANTEED TO KEEP YOU DRY” waterproof promise, while WINDSTOPPER products have a different purpose.
So if you can’t find a neat “28,000 mm” figure for a GORE-TEX jacket, that doesn’t mean the material hasn’t been waterproof tested. It simply means Gore isn’t presenting the product to consumers in exactly the same way as brands that publish a water-column number.
We have more detail in our guide to whether GORE-TEX is worth it.
What Is the Hydrostatic Head Rating of eVent?
eVent has produced several different waterproof membrane families over the years, and this is one area where I would be careful with old specification tables. The original version of this article listed DVstorm, DValpine and DVexpedition at fixed 10,000, 20,000 and 30,000 mm figures, but eVent’s product ranges and naming have evolved since then.
Rather than assuming those old numbers still describe every current eVent fabric, I would check the technical specification of the exact garment or membrane you are buying. Different eVent technologies are designed around different balances of waterproofing, air permeability and breathability.
You can see the current range through eVent Fabrics, and we also have a detailed GORE-TEX vs eVent comparison.
What Is the Hydrostatic Head Rating of Pertex?
Pertex is another brand where the exact waterproof rating depends on the particular fabric rather than simply the word “Pertex.” Products such as Shield, Shield Pro and newer air-permeable constructions have used different membranes and performance targets over time.
The old article listed fixed HH and breathability figures for several Pertex products, but I would not leave those historic numbers on the page as though they describe the entire current range. Manufacturers change laminates, test methods and specifications, while individual clothing brands may also use customised fabric constructions.
If you are looking at something made from Pertex Shield Air or another Pertex membrane, check the specification for the actual garment rather than assuming every version has the same HH rating.
Does DWR Increase Hydrostatic Head?
Not in the simple way people often assume. A durable water-repellent treatment, or DWR, helps water bead and roll off the outside surface instead of saturating the face fabric.
That can dramatically improve how a jacket feels and performs in rain, especially because a saturated face fabric often becomes cold and clammy and reduces effective breathability. But DWR is not the same thing as the waterproof membrane or coating that provides most of the hydrostatic resistance.
A fabric can therefore have an excellent waterproof membrane underneath a worn-out DWR finish. The jacket may feel horrible and “wet out” externally while the membrane itself is still preventing liquid water from passing through.
Can You Increase the Hydrostatic Head of a Tent?
You cannot simply spray a tent and turn a factory-tested 2,000 mm fabric into a certified 5,000 mm fabric. The original HH rating comes from the construction and waterproof coating of the material as tested.
You can, however, restore or improve real-world water resistance. Reproofing an ageing coating, refreshing water repellency and resealing damaged seams can make an old tent perform far better in rain.
That is an important distinction: you are improving the condition of the waterproof system rather than officially changing the fabric’s laboratory HH specification.
Does Hydrostatic Head Reduce With Age?
It can. Waterproof coatings and laminates are exposed to folding, abrasion, UV light, dirt, oils, washing and general wear, all of which can eventually reduce performance.
This is one reason an old tent with a huge original HH rating can leak while a newer tent with a lower headline figure stays perfectly dry. A laboratory number describes the tested fabric in a particular condition, not a guarantee of identical performance forever.
Hydrostatic Head FAQs
Does 10,000 mm Mean a Jacket Can Handle Heavy Rain?
A 10,000 mm fabric has good resistance to water pressure and is commonly used in genuinely waterproof outdoor clothing. But the number alone cannot tell you exactly how the finished jacket will perform during a six-hour storm.
Hood design, zips, seams, ventilation, backpack pressure and the condition of the outer fabric all affect what happens in real use.
Is 3,000 mm Enough for UK Camping?
For a good tent flysheet, I would generally be happy with 3,000 mm for normal UK camping. I would pay just as much attention to seam sealing, groundsheet construction and how well the tent pitches in bad weather.
Is 30,000 mm Better Than 20,000 mm?
In the same test, a 30,000 mm fabric resisted more hydrostatic pressure than a 20,000 mm fabric. Whether that actually makes the finished jacket better for you is a completely different question.
If the 20,000 mm jacket fits better, breathes better, has a much better hood and uses better seam construction, I wouldn’t automatically choose the higher number.
Why Are Tent Floors Often More Waterproof Than Flysheets?
Because floors experience pressure. Knees, elbows, sleeping pads and body weight press the material into wet ground, which can force water through fabric far more aggressively than rain simply hitting a flysheet.
That is why a high HH figure often makes more practical sense underneath you than above you.
How to Use Hydrostatic Head Ratings
Hydrostatic head is useful, but I would treat it as one specification rather than a universal waterproof-quality score. A higher number means greater resistance to water pressure under the test conditions, nothing more and nothing less.
When buying a tent, I would generally look for a flysheet around 2,000-3,000 mm or higher from a reputable manufacturer, then check the groundsheet rating and seam construction separately. If I’m buying a waterproof jacket for serious hiking, I generally like seeing figures around 10,000 mm or higher where the manufacturer publishes them, but I care just as much about the hood, seams, breathability and overall design.
Most importantly, compare like with like. A 20,000 mm laboratory result on a square of fabric doesn’t automatically beat a product that uses different testing and excellent finished-garment construction.
Final Thoughts
Hydrostatic head sounds complicated until you strip it back to the basics: it is simply a measure of how much water pressure a fabric can resist before water penetrates it.
The higher the number, the greater the resistance in that test. But tents and jackets are made from much more than one flat piece of fabric, so seams, zippers, design, coatings and age still determine whether you actually stay dry.
Use the HH figure as a useful comparison tool, not a guarantee. Once you understand that, those 2,000 mm, 10,000 mm and 30,000 mm numbers on outdoor gear specifications become much easier to interpret.
[…] its outdoor gear. Some people might find this annoying and wonder why Gore-Tex doesn’t have HH ratings. The answer is a little pretentious but something I do agree with. Gore-Tex purports that column […]