Key takeaways
- Backpack straps and hip belts press wet fabric against your body for hours.
- Kneeling or sitting on wet ground concentrates your body weight on a small area, which can push water through lower-rated fabric.
- Wind-driven rain hits harder than vertical rain.
- Age and abrasion lower a fabric’s effective HH over time, so a higher starting number leaves more margin.
A waterproof rating in mm is the height of a column of water, in millimetres, that a fabric can hold back before water is forced through it. A 10,000 mm waterproof jacket fabric resisted a 10-metre column of water in the lab; a 20,000 mm fabric resisted 20 metres. For rain jackets, around 10,000 mm handles moderate to heavy rain, 20,000 mm and up covers long storms and pressure from pack straps or kneeling, and anything under about 5,000 mm should be treated as showerproof rather than truly waterproof.
The number is called hydrostatic head (HH). It is useful, but it only describes a flat piece of fabric under lab conditions. Seams, zips, hood design and the condition of the DWR finish decide whether a whole jacket keeps you dry.
Our top picks
Columbia Men's Watertight II Waterproof Rain JacketOmni-Shield finish is designed to resist liquid absorption and dry fast
Packs into its own hand pocket for compact carrying
Abrasion-resistant chin guard and zippered pockets add practical protection
Single color and size offered in this listing
Listing does not state a specific waterproof figure in millimeters
Columbia Men's Glennaker Lake II Rain JacketStow-away hood tucks into the collar when not needed
Packable design folds into its own hand pocket
Adjustable hem and elastic cuffs help seal out wind and rain
Listing does not provide a specific waterproof figure
Only one color and size combination offered here
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How the test works
In the standard method (ISO 811 and similar tests), a fabric sample is clamped under a tube, and water pressure is increased steadily until three drops appear on the other side. The pressure at that point, expressed as an equivalent water column height, is the rating. Because pressure and water height are interchangeable, you can convert: 1 psi is about 704 mm, so a 10,000 mm fabric withstands about 14 psi, and a 20,000 mm fabric about 28 psi.
There is no legal minimum for calling a garment waterproof. As a rule of thumb, many in the industry treat roughly 1,500 mm as the floor for “water-resistant,” and most outdoor brands do not call a jacket waterproof below 5,000 mm.
Waterproof rating chart
| Rating (mm) | What it handles | Typical garment |
|---|---|---|
| 0–1,500 | Light drizzle, brief exposure | Windbreakers, softshells, fashion raincoats |
| 1,500–5,000 | Light rain, short walks, low pressure | Budget rain jackets, ponchos |
| 5,000–10,000 | Moderate rain, several hours with low activity | Entry-level hiking and city rain jackets |
| 10,000–15,000 | Moderate to heavy rain, active use | Mid-range hiking jackets, stretch shells |
| 15,000–20,000 | Heavy rain, long exposure, backpack pressure | Quality 2.5L and 3L rain shells |
| 20,000+ | Sustained storms, high pressure (kneeling, sitting, straps) | Mountain hardshells, ski and alpine jackets |
Real jackets and their published ratings
| Jacket | Construction | Waterproof rating | Breathability |
|---|---|---|---|
| Rab Kinetic Alpine 2.0 | 3L Proflex stretch | 10,000 mm | 35,000 g/m²/24h |
| Rab Downpour | 2.5L Pertex Shield | 20,000 mm | 20,000 g/m²/24h |
| Rab Downpour Light | 2.5L Proflex, 20D | 20,000 mm | 20,000 g/m²/24h |
| Black Diamond Highline Stretch Shell | 3L BD.dry | 20,000 mm | 20,000 g/m²/24h |
| Arc’teryx Beta AR | 3L Gore-Tex Pro ePE | 28,000 mm | RET under 13 |
The Rab pair shows the trade-off clearly. The Kinetic Alpine 2.0 gives up waterproofing (10,000 mm) for stretch and very high breathability, which suits climbers working hard in showers. The Downpour doubles the HH for long wet days but breathes less. Higher is not automatically better; it depends on how long you will be in the rain and how hard you will be working.
10,000 mm vs 20,000 mm: when the difference matters
Falling rain alone exerts very little pressure, so in theory even a 5,000 mm fabric resists it. The extra headroom matters because of real-world pressure points:
- Backpack straps and hip belts press wet fabric against your body for hours.
- Kneeling or sitting on wet ground concentrates your body weight on a small area, which can push water through lower-rated fabric.
- Wind-driven rain hits harder than vertical rain.
- Age and abrasion lower a fabric’s effective HH over time, so a higher starting number leaves more margin.
A 10,000 mm waterproof jacket is a sensible choice for city use, day hikes and active use in showers. A 20,000 mm waterproof jacket makes more sense for backpacking, all-day exposure and places where storms last for hours.
Why ratings are hard to compare across brands
Test methods differ. Pressure can be ramped up at different speeds, and faster ramps produce higher numbers. One workwear maker found the same fabric scored 11,000 mm under EN ISO 811 at 60 cm per minute and 22,000 mm under the Japanese JIS L 1092 B method at 100 cm per minute. Brands rarely say which method they used. Some brands, including Gore and several mainstream labels, do not publish an HH figure at all and instead guarantee the finished garment. Treat a single number as a rough class, not a precise score.
Work wear: EN 343 classes
Protective rain clothing in Europe uses the EN 343 standard instead of a raw mm figure. Under the 2019 version, fabric must reach at least 8,000 Pa (about 816 mm) for Class 1 and 2, 13,000 Pa for Class 3, and 20,000 Pa (about 2,040 mm) for Class 4, with Classes 2 to 4 measured after abrasion, flexing and washing pre-treatment. Those numbers look low next to outdoor ratings, but the pre-treatment makes the test much harsher, so they are not directly comparable.
What the mm number does not tell you
- Seam sealing. Unsealed seams leak regardless of fabric rating. Look for “fully taped” or “welded” seams.
- Zips and closures. Water-resistant zips or a storm flap are needed; the HH test ignores them.
- DWR condition. When the face fabric wets out, the jacket feels clammy even if the membrane holds.
- Breathability. Measured separately, as MVTR in g/m²/24h (higher is better) or RET (lower is better).
What does waterproof rating mean for your purchase?
Use the rating as a first filter: skip anything below 5,000 mm for real rain, choose 10,000 to 15,000 mm for general hiking and commuting, and aim for 20,000 mm or more if you carry a pack in long storms. Then check seams, hood and closures, because those decide whether the rating translates into a dry torso.
