What is Hydrostatic Head? Waterproof HH Ratings Explained

Last Updated on 27/02/2022

What is Hydrostatic Head?

Hydrostatic head (HH) is the measurement used to calculate how much water pressure a waterproof material can take before it leaks.

The unit is displayed in millimeters and refers to the volume of water that can be held in a column before it breaks the seal. For example, a tent with a HH of 3,000 mm will hold a column of water that is 3,000 mm tall before it is breached.

Material with a tighter weave construction or with a durable water repellent (DWR) coating will often have a higher hydrostatic head. This means that you can expect a blackout tent with a higher hydrostatic head number to be better quality or at the very least more waterproof.

Now in all seriousness, you are never going to have a 3,000 mm column of water on top of your tent but this is still a good way to evaluate how waterproof material will be.

What is a Good Hydrostatic Head?

A good hydrostatic head for tents starts at around 3,000 mm and then goes up from there. A good hydrostatic head for waterproof jackets is 10,000 mm and a great HH rating would be 30,000 mm.

That doesn’t mean that a material with a lower hydrostatic head will not keep you dry or that a high hydrostatic head is guaranteed to keep you dry. It just means that the fabric is designed to withstand that much water pressure. The reason I bring this up is that a tent or waterproof garments’ ability to stay dry inside is often dictated by details such as stitching, design, and craftsmanship.

What Is a Hydrostatic Head Worth if Everything Else Leaks Water?

A tent without sealed seams will eventually leak through the pinholes and soak everything inside. Heat bonded seals are stronger and longer lasting than taped seams but you will need some kind of seam protection to stay completely dry. A zip without a storm flap will eventually leak through. There are other examples too but my point is that waterproofing does not just rely on the HH of a material.

When buying a new waterproof jacket or tent, knowing how waterproof it is going to be is often the deciding factor. Understanding what is the threshold for a good hydrostatic head can be really helpful in the decision-making process.

How are Hydrostatic Heads Calculated?

To explain how HH ratings are calculated, here is a short video that shows the process of how it is measured:

HH Ratings for Tents

• 1,000 mm – This is the minimum hydrostatic head required to keep you dry in a rain shower. This would be ok in dry conditions for things like desert camping but might not be too solid in a rainstorm.
• 2,000 mm – Can withstand fairly heavy rain for a few hours before showing signs of weakness.
• 3,000 mm – Around the industry average and waterproof enough to keep you dry in sustained rainfall.
• 4,000 mm – Can be trusted no matter how hard it is raining, no water is getting through this without force.
• 5,000 mm – Tents of this rating would be considered premium and you will rarely if ever need anything higher.

HH Ratings for Waterproof Jackets

• 5,000 mm – If you are looking for a cheap waterproof jacket then this would be the minimum recommended hydrostatic head.
• 10,000 mm – Many lightweights but high-end jackets will be rated at this level, which is good enough to block heavy rain.
• 15,000 mm – Thicker materials may have a higher HH rating and so this around average for a standard rainjacket.
• 20,000 mm – 100% waterproof jackets but will need additional features on the seams and zips to keep you bone dry.
• 30,000 mm – The highest HH rating possible for outdoor gear, this is seriously waterproof and of the highest quality.

What is the HH Rating of Goretex?

Unfortunately for specification geeks, Goretex does not use HH ratings for any of their fabrics. You might then wonder why Goretex are the market leaders in premium waterproof jacket materials if they don’t play by the same rules as other brands? Well, they have a state-of-the-art testing facility where they run their own waterproof experiments to ensure they have the best performance in the industry.

Their rigorous testing includes three different stages to ensure that their technology performs better than their competitors. These stages include lab testing materials, human testing, and in-the-field testing which are different for garments, footwear, and gloves. They are meant to simulate real-life conditions and not just support a column of water for a sustained time period.

Using sophisticated rain and wind simulations, Goretex products are tested after completion to ensure that the seams and zips are just as waterproof as the fabric. This is why they have built such a solid reputation and Goretex costs so much… because they don’t fail when you need them most.

What is the HH Rating of Event

EVent Fabrics have 3 adaptions of their signature material which have HH ratings of 10,000 mm, 20,000 mm, and 30,000 mm. Their most waterproof variation is the Expedition however this is also their least breathable. As you can see below, the breathability of eVent Fabric directly correlates with the HH ratings and the more waterproof the material is, the less breathable it becomes.

• eVent DV Storm – HH rating 10,000 mm / Breathability 30,000 g/m²
• eVent DV Alpine – HH rating 20,000 mm / Breathability 20,000 g/m²
• eVent DV Expedition – HH rating 30,000 mm / Breathability 10,000 g/m²

We compare eVent Vs Goretex performance here.

What is the HH Rating of Pertex

Established in 1979, Pertex has a fairly large range of fabrics and lots of experience when it comes to waterproof material. They have their original Sheild waterproof fabric and then a growing number of variations offering varying degrees of water resistance. Their newest Pertex Sheild AP fabric is designed with an air permeable nanofibre membrane to provide ultimate breathability, although we couldn’t find a rating in g/m². Pertex is thought to be more breathable than Goretex and is also incredibly lightweight.

• Pertex Shield – HH rating 10,000 mm / Breathability 6,000 – 7,000 g/m²
• Pertex Shield Pro – HH rating 20,000 mm / Breathability 20,000 g/m²
• Pertex Shield AP – HH rating 20,000 mm

Hydrostatic Head: FAQ’s

When you see a number like HH 3,000 mm written on a product description it is easy to feel a little sheepish if you don’t know what it means. Infact, you probably found this article after searching, “what is hydrostatic head?” or “what does HH mean”, or “what is a good HH rating for tents”. That’s ok, we all learn something new every day. Here are some frequently asked questions about hydrostatic head ratings.

Is a 2000 mm Tent Good in The Rain?

A tent with a HH rating of 2000 mm will keep you dry in most rain showers however you may notice that in heavy downpours, small droplets of water begin to get through and drip down. This rarely happens but we prefer a slightly higher rating for peace of mind.

Is 5000 mm Good for A Tent?

A tent with a 5000 mm hydrostatic head rating will withstand huge amounts of water before it breaches through the fabric. With this kind of waterproofing, you can be sure your tent is good enough for the worst weather. This is because the fabric is so tightly weaved that it will take a column of water taller than 5000 mm to breach through.

Can I Improve the Hydrostatic Head of My Tent?

You cannot increase the hydrostatic head of your tent or waterproof clothing as this is determined by the weave construction of the material. BUT, you can improve the water resistance by using waterproof treatment sprays and sealing seams with silicone.

How to Use Hydrostatic Head Ratings

So now that you understand what a hydrostatic head is, you can use this newfound information to buy a new tent or waterproof jacket with confidence. So, what have we learned?

When buying a tent, look for a HH rating of 2,000 mm minimum and aim for a tent with 5,000 mm if at all possible. When buying a waterproof jacket, look for a HH rating of 10,000 mm minimum and if your budget allows, consider Goretex as one of the best. The reason tents have a lower HH rating than garments is because they are made with much thinner and less hard-wearing fabric.

We hope you found this informative article about hydrostatic heads useful and will know what it means next time you see it.