What Does Hydrostatic Head Mean? A Guide to Tactical Waterproof Ratings

When you are choosing a waterproof jacket, overtrousers, shelter or other tactical outdoor equipment, you will often see a waterproof rating measured in millimetres — for example 5,000mm, 10,000mm or 20,000mm HH.

But what does that number actually mean when you are operating outdoors in persistent rain, carrying a pack or spending hours exposed to the elements?

Hydrostatic head (HH) is one of the most useful ways of comparing the waterproof performance of different fabrics. However, the number alone does not tell you everything about how a garment or shelter will perform in the field.

In this guide, we explain hydrostatic head, what different ratings mean and what to consider when choosing waterproof equipment for tactical and demanding outdoor use.

What is hydrostatic head?

Hydrostatic head, normally shortened to HH, measures how much water pressure a fabric can withstand before water begins to penetrate it.

During testing, the material is subjected to increasing water pressure. Its rating represents the height of a column of water the fabric can withstand before moisture begins to pass through it.

A fabric rated to 5,000mm HH, for example, can withstand the pressure produced by a five-metre column of water under test conditions.

The principle is straightforward:

The higher the HH rating, the greater the fabric's resistance to water pressure.

That becomes particularly relevant when equipment is being used for more than a quick walk in a rain shower. Kneeling on wet ground, wearing webbing or a loaded pack over a waterproof jacket, and spending prolonged periods exposed to driving rain can all place additional demands on waterproof equipment.

Why does hydrostatic head matter for tactical clothing?

For tactical users, waterproof clothing may need to perform while the wearer is moving, carrying equipment or remaining static for extended periods.

A jacket could be exposed to hours of rain during a patrol, exercise or observation task. Pack straps and load-carrying equipment can also press wet fabric against the body.

This is why waterproof performance needs to be considered in the context of how the garment will actually be used.

For occasional outdoor use, a moderate HH rating may be sufficient. For prolonged exposure and demanding operational environments, a higher waterproof rating provides a greater margin of protection.

What do different hydrostatic head ratings mean?

There is no single HH figure that is right for every situation, but the following provides a useful guide.

Around 1,000mm–3,000mm HH

At the lower end of the scale, fabrics provide protection against lighter rain and showers.

These ratings are commonly associated with lightweight shelters and equipment intended for relatively moderate conditions.

They can be perfectly suitable where low weight and packability are priorities, but would not normally be the first choice for waterproof tactical clothing expected to deal with sustained severe weather.

Around 5,000mm HH

A 5,000mm HH fabric provides a more substantial level of waterproof protection and can be appropriate for general tactical and outdoor use.

Highlander's Typhoon Waterproof Trousers, for example, use AB-TEX Tri-Max fabric with a waterproof rating of 5,000mm HH and a breathability rating of 5,000g/m².

They also feature taped seams and two-way full-length side zips, allowing them to be put on or removed quickly over other clothing and footwear — particularly useful when conditions change rapidly in the field.

Around 10,000mm HH

Moving into five-figure ratings provides increased resistance to water pressure and is worth considering when equipment will be used regularly in prolonged rain.

At this level, however, HH should not be viewed in isolation. Breathability becomes increasingly important, particularly when moving quickly, carrying a load or working hard for extended periods.

Keeping rain out is only half the job. A good waterproof system also needs to manage the moisture generated inside the garment.

20,000mm HH: protection for demanding conditions

For prolonged exposure to severe weather, a 20,000mm HH rating represents a high level of waterproof performance.

The Highlander Cyclone Pro Waterproof Jacket has been developed with demanding conditions in mind. Its Prot3X-LTE shell has a 20,000mm HH waterproof rating combined with 15,000g/m² breathability, providing high waterproof resistance while helping moisture escape during active use.

The Cyclone Pro also uses taped seams and waterproof zips, because the performance of a waterproof garment depends on much more than the face fabric alone.

At approximately 484g in size L, the jacket is designed to balance durability and weather protection with packability. Its 30D mechanical-stretch ripstop fabric also allows greater freedom of movement than a rigid waterproof shell.

For tactical use, details matter. The Cyclone Pro includes a wire-peaked adjustable and concealable hood, which is also helmet compatible, adjustable cuffs and an elasticated drawcord hem to help create a weather-resistant fit. The HMTC version additionally incorporates a rank slide with hook-and-loop fastening.

Available in Ranger Green, Black and HMTC, it offers a high-performance waterproof option without the bright colourways commonly associated with civilian mountaineering shells.

Waterproofing is about more than the HH number

A high HH figure is useful, but it should never be the only specification you look at.

A waterproof garment is a complete system. Fabric, seams, zips, hood, cuffs, hem and breathability all contribute to its real-world performance.

Taped seams

Every time waterproof material is stitched, a series of tiny holes is created.

Without additional protection, these areas can become potential entry points for water. Taped seams cover the stitching on the inside of the garment or shelter to maintain waterproof protection.

This is particularly important during prolonged exposure, when even a relatively small weakness can eventually allow moisture through.

Waterproof zips and storm protection

Zips are another potential weak point.

On equipment designed for demanding wet-weather use, look at how the main zip and pockets are protected. Waterproof zips or well-designed storm flaps can help prevent wind-driven rain reaching the layers underneath.

The Cyclone Pro, for example, uses a waterproof central zip alongside its taped construction.

Hood design

A good hood can make a significant difference in bad weather.

For tactical use, it needs to provide protection without unnecessarily restricting peripheral vision or interfering with other equipment.

The Cyclone Pro's adjustable, concealable hood has a wire peak and is designed to be compatible with a helmet, helping provide weather protection while maintaining visibility.

Cuffs and hem

Water often gets into clothing at openings rather than directly through the fabric.

Adjustable cuffs allow a jacket to be secured around gloves or the wrist, while an adjustable hem helps reduce wind and rain entering from below.

These seemingly small details can become particularly important after several hours outside.

Waterproofing vs breathability

A waterproof jacket has two competing jobs: keeping rain out and allowing moisture generated by your body to escape.

During a static task, moisture build-up may be relatively limited. Start moving quickly across rough terrain with a pack, however, and the situation changes.

Sweat that cannot escape can condense inside the garment. The wearer can then feel wet even though rain has not penetrated the outer fabric.

That is why breathability should be considered alongside HH.

The Cyclone Pro combines its 20,000mm HH waterproof rating with 15,000g/m² breathability, making it particularly relevant for active use where both weather protection and moisture management are important.

Your layering system matters too. Even a highly breathable waterproof shell will struggle if moisture is trapped by unsuitable layers underneath.

Waterproof trousers matter too

A high-performance waterproof jacket only protects part of the body.

When you are moving through wet vegetation, kneeling, sitting on damp ground or spending extended periods in heavy rain, waterproof overtrousers become an important part of the system.

The Highlander Typhoon Waterproof Trousers provide 5,000mm HH waterproofing and 5,000g/m² breathability, with taped seams and adjustable hems.

Their two-way full-side zips are particularly practical for field use because they make it easier to pull the trousers on over boots when the weather changes without having to remove footwear first.

Combining waterproof upper and lower layers allows the wearer to adapt their protection according to the conditions and task rather than relying on a jacket alone.

What waterproof rating do you need?

The right level of waterproofing depends on what you are doing and how long you expect to remain exposed.

For general outdoor use and intermittent rain, a moderate waterproof rating may be sufficient. If you are spending long periods outdoors, operating in exposed terrain or expecting persistent heavy rain, a higher HH rating gives you additional protection.

Consider:

• Duration: are you outside for an hour or an entire day?

• Activity level: will you be static or moving hard under load?

• Terrain: are you working in open, exposed ground or sheltered woodland?

• Load carriage: will you be wearing a daysack, bergen, plate carrier or webbing over the jacket?

• Weather: are you preparing for showers or prolonged wind-driven rain?

• Layering: does the shell leave enough room for insulation underneath?

• Packability: does your waterproof layer need to remain in your pack until conditions deteriorate?

• Breathability: will you generate significant heat and moisture while moving?

For tactical users, it is often better to think in terms of a waterproof system rather than an individual garment.

The shell needs to work with your base layers, insulation, trousers, footwear and load-carrying equipment.

Looking after tactical waterproof clothing

Even high-performance waterproof clothing needs to be looked after.

Dirt, oils and repeated use can affect the performance of the outer surface over time. Clean waterproof garments according to their care instructions and avoid ordinary detergents or treatments that could damage the waterproof membrane or water-repellent finish.

Allow wet equipment to dry properly before storing it for extended periods.

If water stops beading on the outside of the garment, the durable water-repellent treatment may eventually need refreshing. This does not necessarily mean that the waterproof membrane itself has failed, but a saturated outer fabric can reduce breathability and make the garment feel colder and heavier.

The bottom line

Hydrostatic head tells you how much water pressure a fabric can withstand before moisture begins to penetrate it.

For tactical equipment, however, the highest number on the label should not be the only consideration.

The conditions you operate in, the length of exposure, your activity level, breathability, taped seams, zips, hood design and compatibility with the rest of your equipment all matter.

For users who need a lightweight waterproof shell capable of dealing with demanding conditions, the Highlander Cyclone Pro Waterproof Jacket combines 20,000mm HH waterproofing with 15,000g/m² breathability, taped seams, waterproof zips and a helmet-compatible hood in a package weighing approximately 484g in size L.

Pairing an appropriate waterproof jacket with waterproof overtrousers creates a more complete wet-weather system, helping you stay protected when conditions deteriorate and the task still needs to continue.

Explore Highlander Forces waterproof clothing and build a wet-weather system designed for demanding conditions.