What the numbers on your waterproof jacket actually mean

Hydrostatic head, breathability and layer construction explained, including why breathability figures from different brands can't be compared with each other.

By GearScouting · 7 min read

Waterproof jacket spec sheets are full of numbers. 10,000mm. 20,000mm. 20,000g. RET 6. Most product pages display them without explaining what they mean or what you actually need.

Here's what they mean in plain terms, and, for breathability, why the number on one brand's jacket often can't be compared with the number on another's at all.

Hydrostatic head: how waterproof is waterproof?

Hydrostatic head (HH) is the standard measure of waterproofing, tested under ISO 811: a fabric specimen is clamped and water pressure is raised against it at a controlled rate, commonly 10cm of water column per minute, until moisture pushes through. The result is expressed as the height of water column that pressure equates to, which is why, as Craghoppers put it, a higher number means more water pressure the fabric can endure.

10,000mm means the fabric withstood the pressure a ten-metre column of water would exert.

Why you need so much more than sounds logical

Ten metres of water sounds absurd for a jacket. The reason the number needs to be that high is the gap between a flat swatch in a lab and a body in motion on a hillside.

Pack straps concentrate significant pressure onto small areas of fabric. Wind-driven rain arrives with force rather than falling vertically. Kneeling, sitting on wet ground and leaning against rock all apply sustained local pressure. The rating needs headroom for all of it.

What rating do you actually need?

For context, 1,500mm is the figure the industry treats as the floor for calling a fabric waterproof - low enough to be meaningless for hill use. No UK legislation defines the term; the number comes from standards practice, and retailers repeat it as though it were law.

These tiers are where the industry converges rather than a published standard. No independent testing body issues them.

Why some jackets don't have a figure at all

Gore doesn't publish a hydrostatic head figure for individual Gore-Tex garments. The brand operates on a pass/fail standard backed by its guaranteed-to-keep-you-dry warranty rather than a per-product number, and the 28,000mm figure you'll see quoted around the internet is a laminate-level figure circulated by retailers, not a spec Gore publishes per jacket.

So a blank in the hydrostatic head column doesn't mean a jacket is less waterproof. It means that brand doesn't report it. Across the jackets in our spec table, more than half have no published hydrostatic head figure at all.

Breathability: the number most people misread

Breathability determines whether your sweat escapes or condenses inside the jacket. A shell can have an excellent HH rating and poor breathability, which means it keeps rain out and traps sweat in. Both matter.

The complication is that brands measure it two completely different ways.

MVTR — higher is better

Moisture vapour transmission rate, written as g/m²/24h, measures how much water vapour passes through a square metre of fabric in a day. Most non-Gore brands report this.

• 10,000g/m²/24h — adequate for moderate activity at a steady pace
• 20,000g/m²/24h and above — the target for aerobic hillwalking, scrambling, or anything where you're working hard

RET — lower is better

Resistance to evaporative heat transfer, measured under ISO 11092. It expresses how much the fabric resists moisture passing through, so the scale runs the opposite way. Inov8 give the working bands as:

• 0 to 6 — excellent
• 7 to 13 — good
• 14 to 20 — ok
• Above 20 — poor

Gore reports in RET rather than MVTR, though it rarely publishes a per-garment figure, which is why the breathability column is mostly blank for Gore-Tex jackets in our table.

Why you can't compare the two — or, often, two MVTR figures

RET and MVTR are not convertible. They're different measurements from different test methods. There is no reliable formula to turn RET 6 into a g/m²/24h figure. If you're comparing a Gore-Tex jacket against a Pertex Shield jacket, you have two numbers that cannot be placed on the same scale.

Worse, two MVTR figures often aren't comparable either. There are several accepted test methods — JIS L1099 A1, JIS L1099 B1, ASTM E96 and others — and they produce substantially different results on identical fabric. The B1 method typically returns figures several times higher than A1 on the same material. Brands almost never state which method they used.

The practical consequence: a jacket advertised at 20,000g/m²/24h is not necessarily more breathable than one advertised at 10,000g/m²/24h. It may simply have been tested using a more generous method.

What to do instead. Use breathability figures to compare jackets within a single brand's range, where the test method is likely consistent. Across brands, treat the figures as a rough tier indicator only, and weight construction, venting and independent testing more heavily. A jacket with pit zips and a 10,000g rating will often feel more comfortable on a climb than a 20,000g jacket without them.

Our spec table records which measurement each brand uses, so you can see at a glance where a comparison is valid and where it isn't.

Construction: 2-layer, 2.5-layer, 3-layer

2-layer laminates the membrane to the outer face fabric, with a separate loose-hanging liner inside. More packable and often softer, but the liner can cling when damp.

2.5-layer adds a printed or embossed inner surface to protect the membrane instead of a full liner. Lighter and more packable — common in ultralight and running shells, less durable over years of hard use.

3-layer bonds outer fabric, membrane and inner face fabric into a single laminate. The most durable construction and generally the right choice for regular mountain use.

What the numbers don't tell you

Neither rating says anything about seams, zips or construction. A 20,000mm fabric in a jacket with critically taped seams and an unprotected front zip will still let water in at those points. Fit, hood adjustment and venting affect real-world comfort more than a few thousand millimetres either way.

The specs narrow the field. They don't pick the jacket.

Frequently asked questions

What hydrostatic head rating do I need for UK hillwalking?

10,000mm is the realistic minimum for typical UK conditions. For mountain use in Scotland, Wales or the Lake District in autumn and winter, 20,000mm and above gives proper margin for pack-strap pressure and wind-driven rain.

What does 10,000mm waterproof mean?

The fabric withstood the water pressure a ten-metre column would exert before moisture pushed through.

Why doesn't my Gore-Tex jacket have a hydrostatic head rating?

Gore doesn't publish hydrostatic head figures for individual garments, working instead to a pass/fail standard backed by its guaranteed-to-keep-you-dry warranty. A missing figure reflects the brand's reporting policy, not weaker waterproofing.

Can I compare breathability ratings between brands?

Not reliably. Some brands report MVTR in g/m²/24h where higher is better; others report RET under ISO 11092 where lower is better, and the two can't be converted. Even two MVTR figures may come from different test methods that produce very different numbers on the same fabric. Compare within a brand's range, not across brands.

Is RET 6 good?

RET 6 sits right at the edge of the excellent band, the highest number that still qualifies, and it's the range good technical shells occupy. Remember the scale is inverted compared with g/m²/24h: with RET, lower is better.

What's the difference between 2-layer and 3-layer construction?

2-layer bonds the membrane to the outer fabric with a separate loose liner inside; 3-layer bonds all three layers into one laminate. 3-layer is more durable and generally performs better over years of use. 2-layer and 2.5-layer are lighter and pack smaller.

Is a higher hydrostatic head always better?

Better on paper, with diminishing returns. The step from 5,000mm to 10,000mm is meaningful for UK hillwalking. Beyond 20,000mm the difference matters mainly at the extremes of mountain use, and other factors — seams, venting, fit — will affect your day more.

Sources: Craghoppers: What is Hydrostatic Head? · Inov8: Waterproof Breathability Ratings Explained · Gear Assistant: What is Hydrostatic Head? · Snow + Rock: Waterproof Ratings and Breathability Explained

See these figures on real jackets: our waterproof jacket spec table lists current UK models with ratings, layer construction, weight and today's price — plus which measurement each brand uses.

Related reading: why "waterproof" doesn't always mean waterproof, and how Gore-Tex, Pertex Shield and eVent compare once you know what the numbers mean.

The waterproof jacket spec table · Watch a product · All field notes