In brief: Hydrostatic head measures resistance to water pressure; MVTR measures moisture vapour transfer under a specified test; denier describes yarn mass; and GSM is fabric weight per square metre. Read these figures alongside the jacket’s construction and the conditions you expect.
Most outdoor gear comes with a wall of numbers on it. Hydrostatic head, denier, GSM, MVTR. Nobody explains what any of it means, so it either gets ignored or trusted blindly. I'd rather explain it, because once you know what you're looking at, you can actually judge whether a jacket is going to hold up or not.
Hydrostatic head
This is the waterproofing number, measured in millimetres. It tells you how much water pressure the fabric can take before it lets water through. A 20,000mm result indicates high resistance to water pressure in that test. It is not a universal pass mark for a waterproof jacket: seam sealing, closures, wear and the complete garment also affect protection.
The Fellguard is rated to 20,000mm, and it's not just a number we put on the page. We had it independently tested by SGS, and it exceeded that figure across every sample they tested. That's the difference between a claim and a result.
Not all waterproofing works the same way
There are a few genuinely different approaches to making a fabric waterproof, and most people have only heard of one or two.
The most familiar is a microporous membrane, tiny holes small enough to block liquid water but large enough to let vapour through. Gore-Tex is the famous version of this. eVent works on a similar principle but skips an inner layer that older Gore-Tex designs used, so vapour escapes more directly rather than needing heat to push it out.
The other main approach is a non-porous, hydrophilic membrane, no holes at all. Instead, moisture moves through the material molecule by molecule, pulled from the warm, humid inside toward the cooler, drier outside. This is what the Fellguard uses, a laminated PU membrane. Sympatex works on the same principle and is built to be more easily recycled. Toray Dermizax, made in Japan, is another one in this family, known for having some stretch built into the membrane itself.
There's also NeoShell, which is more air-permeable than most membranes, it actually lets a small amount of air move through, which some people find breathes better during high-output activity, at a slight cost to wind resistance.
And then there's the oldest approach of all, which uses no membrane whatsoever. Ventile is a tightly woven cotton developed in Britain during the Second World War for pilot survival suits. The weave is dense enough that the cotton fibres swell when wet and seal themselves shut. No coating, no laminate, just the weave itself doing the job. It's heavier and needs more looking after than modern fabrics, but it's a genuinely different way of solving the same problem, and it's still used today for people who want something more repairable and less synthetic.
Breathability (MVTR)
Waterproofing on its own is easy, you can wrap someone in a bin bag. The hard part is doing that while still letting sweat vapour escape, otherwise you're just as wet from the inside. That's what MVTR measures, how much moisture the fabric lets out over a day. The Fellguard's tested figure came in at over 26,000g/m²/24h, well above what we list on the site.
Compare MVTR figures only when the test method and conditions are comparable. Different methods can produce very different numbers, and a laboratory figure cannot promise that you will stay sweat-free on a climb.
Denier
This is about durability, not warmth. It measures yarn mass: one denier is one gram per 9,000 metres of yarn. It can help compare fabric weight, but fibre type, weave and construction also affect toughness. The Fellguard uses a 75-denier face fabric, heavy enough to resist abrasion and tearing on rock and scrub, without going so heavy it stops packing down small.
GSM: fabric weight
GSM means grams per square metre. A 260 GSM fleece fabric weighs 260 grams for each square metre of material. It is not the weight of the finished garment, and it does not measure warmth on its own; fibre, thickness and structure matter too.
PFC-free
A lot of waterproof coatings historically used fluorocarbons, chemicals that don't break down and have been linked to health and environmental concerns. Ours is C0 PFC-free, compliant with current UK and EU PFAS regulations. It does the same job without leaving that behind.
Merino, for the socks
Different problem, similar principle. Merino wool regulates temperature in both directions, warm when it's cold, cool when it's warm, and it resists odour naturally, which synthetic fibres generally don't. That's why it's in the socks rather than a synthetic blend.
None of this is complicated once it's explained. It's just usually left out.
Waterproof fabric rating FAQs
What does hydrostatic head measure?
Hydrostatic head measures how much water pressure a fabric can resist before water passes through it. It is expressed in millimetres, but the finished jacket's seams, zips and construction matter too.
What does MVTR mean?
MVTR stands for moisture vapour transmission rate. It describes how much water vapour can move through a square metre of fabric over a set period; a higher figure generally indicates greater breathability under the test method used.
Does a higher denier make a jacket more waterproof?
No. Denier describes yarn thickness and is more closely associated with weight and durability. Waterproofness comes from the membrane or coating, seam sealing and the complete garment construction.
See how those parts are assembled in our guide to 2-layer and 3-layer waterproof jackets.
Testing reference
Hydrostatic-head testing is standardised through methods including ISO 811 for resistance to water penetration. Results still need to be read alongside garment construction, seams and intended use.