Waterproof Ratings Explained: What the mm and Breathability Numbers Mean

Checked on 18 October 2026. Figures come from the published standards and from manufacturer guidance, linked below. Brands test in different ways, so always read what test a rating refers to.

What does a waterproof rating in mm mean?

It is the result of a hydrostatic head test. A sample of fabric is clamped under water. The pressure is raised until water comes through. The result is given as the height of a column of water in millimetres. So a 10,000mm fabric held back the pressure of a water column 10 metres high before it leaked.

The international method is ISO 811:2018, "Textiles: Determination of resistance to water penetration: Hydrostatic pressure test". ISO confirmed it as current in September 2025. You will also see it written as EN ISO 811 or BS EN ISO 811.

Two things the number does not tell you:

• It tests the fabric, not the jacket. Most rain gets in through seams, zips, pockets and the hood.
• It is usually measured when the fabric is new. Wear, rubbing and washing can lower it over time.

Is any jacket 100% waterproof?

A fabric either lets water through or it does not. Gore-Tex makes this point in its guide when is a garment waterproof? (published May 2017). It says a garment cannot be "a bit waterproof".

The same article makes a less obvious point. It cites testing at EMPA, the Swiss materials research institute, in which some fabrics rated below 120mm stayed waterproof in simulated rain. It also says fabrics rated above 20,000mm when new can start to leak within a few uses or after home washing. The lesson is that a big number on the label is not the whole story. Durability and construction count.

In practice, a jacket keeps you dry when all of these hold up:

• a waterproof fabric or membrane
• fully taped or welded seams
• covered or water-resistant zips
• a hood and cuffs that seal around you

Feeling damp inside does not always mean a leak. Sweat that cannot escape condenses on the inside of the jacket. That is a breathability problem, covered below.

The EN 343 classes: an official scale for the whole garment

Outdoor brands mostly quote mm figures from their own testing. The European standard for rainwear is EN 343:2019, "Protective clothing: Protection against rain". It is written for workwear, so few high-street jackets carry it. It is still useful, because it grades seams as well as fabric.

Thresholds as published by Skanwear's EN 343 summary. We converted pascals to millimetres at 1mm of water = 9.80665 Pa. The pre-treatments simulate wear and cleaning, so classes 2 to 4 test the fabric after it has been aged. Class 4 is the top class.

Look at those conversions. The top class needs about 2,000mm after ageing. That is far below the 10,000mm to 20,000mm figures common on outdoor labels. The two numbers measure different things in different ways, so a 20,000mm jacket is not ten times better than an EN 343 class 4 coat.

Breathability: RET and g/m²/24h

Breathability is how well water vapour from your body gets out through the fabric. There are two common ways to measure it, and they use different units.

• RET (resistance to evaporative heat transfer). Measured on a sweating hot plate under ISO 11092. Lower is better.
• g/m²/24h (moisture vapour transmission rate). Grams of water vapour through a square metre of fabric in 24 hours. Measured with cup methods such as JIS L 1099, ASTM E96 or ISO 15496. Higher is better.

The common RET bands, as set out by Decathlon's explanation of the ISO 11092 test:

EN 343 grades breathability too, from class 1 (RET above 40) to class 4 (RET of 15 or less).

Compare like with like. A 2013 SGS technical review of the test methods shows they use different set-ups and report different units. So treat a g/m² figure from one method as not directly comparable with one from another, or with a RET.

Waterproof, water-resistant or water-repellent?

• Waterproof: the fabric stops water under pressure, and the seams are sealed. This is the only one that keeps you dry in sustained rain.
• Water-resistant: the fabric slows water down but does not stop it. Fine for a shower between the café and the museum. Not for a wet day out.
• Water-repellent: a surface finish that makes water bead and roll off. It is usually called DWR (durable water repellent). It sits on top of waterproof and non-waterproof fabrics alike, so it tells you nothing on its own.

Choosing a packable waterproof for a trip

For most trips from the UK, a light waterproof that packs into its own pocket earns its place. It fills the gaps in a case, or sits in an under-seat bag. Our cabin bag guide has the free-bag sizes. Wear it on the journey if space is tight.

What to check on the label and the product page:

• A stated waterproof rating, and how it was tested. "Waterproof" with no figure or test is a weaker claim.
• Taped seams. "Fully taped" beats "critically taped", which seals only the main seams.
• A hood that adjusts. A hood that blows back or blocks your view will not get used.
• Packed size and weight. Compare them against the space you actually have.
• A breathability figure, if you will walk a lot or travel somewhere warm and wet. Note which unit it uses.

We do not test jackets. This is a checklist for reading labels, not a ranking of products.

How to keep a waterproof jacket waterproof

If water stops beading and the outer fabric soaks through, the DWR has worn off. The jacket may still be waterproof underneath, but it will feel cold and clammy. Gore-Tex's care instructions for outerwear give this routine:

• Wash: do up the zips and flaps. Machine wash at 40°C with a small amount of liquid detergent. Rinse twice and use a low spin.
• Avoid: powder detergent, fabric softener, stain removers and bleach.
• Reactivate the DWR: once dry, tumble dry on a warm, gentle setting for another 20 minutes. Or iron on a gentle setting with a towel between the iron and the jacket, without steam.
• Reproof: if water still does not bead after washing and drying, apply a spray-on or wash-in DWR product. Then reactivate it with heat as above.

These steps are Gore's guidance for Gore-Tex garments. Some fabrics must not be tumble dried, so always follow the care label on your jacket first.

Quick answers

• Is 5,000mm waterproof? It is a lab result for the fabric when new. Whether the jacket keeps you dry depends on the seams, zips and hood as well.
• Is a higher mm rating always better? No. Gore cites cases where very high ratings did not last. Construction and durability matter more past a certain point.
• What does hydrostatic head mean on a tent? The same test. It is the water column the flysheet or groundsheet fabric held back.
• Do I need Gore-Tex? Not necessarily. It is one brand of membrane among several. Compare the stated rating, the test used and the seam construction.

Sources, checked 18 October 2026:

• ISO 811:2018, hydrostatic pressure test
• EN 343 classes summary (Skanwear)
• Gore-Tex: when is a garment waterproof?
• Gore-Tex care instructions for outerwear
• Decathlon: how RET is measured (ISO 11092)
• SGS review of water vapour permeability test methods (2013)