Analysis
Ten Thousand Millimetres of What?
Jackets, pants and dog coats quote waterproof ratings like 10K or 20K without naming the test, and boots usually publish nothing comparable at all. Here is what the number measures and what to read instead.
Read enough winter gear pages and the numbers start to look like a currency. A ski shell is 20K. A pant is 15K. A dog raincoat is “above 10,000 mm”. The implied deal is simple: bigger number, drier you.
Across six comparisons we ran this season, ski jackets, ski pants, rain jackets, snow boots, rain boots and dog raincoats, here is what the makers actually published:
| Product | Published rating | Seams |
|---|---|---|
| Strafe Pyramid Jacket | 20K/20K | Fully sealed |
| Strafe Capitol Pant | 20K/20K | Fully sealed |
| Flylow Chemical Pant | 20k/20k | Fully taped |
| Obermeyer Raze Jacket | 15k/15k, some colours only | Critical seams |
| Outdoor Research Snowcrew Jacket and Pant | None, only OR’s own scale | Fully taped |
| Hurtta Monsoon Coat II ECO | Water column above 10,000 mm | All seams taped |
| Ruffwear Sun Shower | 3,000 mm | Not stated |
| Chilly Dogs Harbour Slicker | None | Not stated |
| Gold Paw Rainpaw | None | Not stated |
None of the ratings we recorded comes with the name of the test behind it. And the boots, all eight of them, publish no comparable figure at all.
What the number measures
The millimetre figure is a hydrostatic head. The method is old and simple: clamp a piece of fabric, put water against one face, raise the pressure steadily and note the point at which water forces its way through. The result is expressed as the height of a water column that would exert the same pressure. A 10,000 mm fabric resisted the pressure of a column ten metres tall. “20K” is the same idea with the zeros removed.
It is a published, standardised test. The British Standards Institution lists it as BS EN ISO 811:2018, a document that “specifies a hydrostatic pressure method for determining the resistance of fabrics to penetration by water”. Other methods exist alongside it, and each one fixes its own details: how fast the pressure climbs, how large the test area is, how the sample is held and when a leak counts.
That is where the comparability goes. Two shells can both print 20K and have been measured under different methods, on new fabric or on washed fabric, on the flat panel or nowhere near a seam. A rating with no method named is a result with the conditions removed. It still tells you something, mostly that the maker bothered to test, but it cannot settle a choice between two brands that each printed the same number.
A fabric number on a garment made of seams
The more useful point is what the number does not cover. A hydrostatic head describes a swatch. A jacket is a swatch cut into pieces and sewn back together, and every stitch is a row of holes.
That is why the seam column in the table matters more than the rating column. Outdoor Research publishes no waterproof figure for the Snowcrew jacket or pant, only its own internal scale, and both are fully seam taped. Obermeyer publishes 15k/15k for the Raze, but only for the colours made in its HydroBlock Pro stretch fabric; several other colours use HydroBlock Sport with no rating given, and the jacket’s seam sealing is listed as critical seams, meaning the key seams are taped rather than every one. On paper the Raze has the number. In the comparison, the Snowcrew had the construction, and it was the pick.
The rain jacket comparison never needed a hydrostatic figure to make its calls. What separated the four was layer construction, a 3-layer shell against a 2.5-layer build with a printed interior, whether the water-repellent finish can be restored, and whether the seams hold under stress. The skip was a nonwoven suit whose fabric is the membrane: waterproof out of the bag, with no repellent finish to restore and seams that can leak once the fabric is stressed. No rating would have shown you that.
Why boots skip the number
Boots publish temperatures, liners and sole compounds, and almost never a millimetre figure. The reason is that most of a boot is not a fabric in the sense the test describes. A rubber or neoprene shell does not let water through the material itself in normal use. What fails is a seam, the bond between upper and sole, or the top of the shaft.
So boot pages describe those parts instead. Muck calls its Arctic Sport II 100% waterproof, built on a 5mm neoprene bootie with extended rubber coverage. Kamik describes the Snovalley as seam-sealed waterproof suede and leather over a rubber base. KEEN names its KEEN.DRY membrane. Baffin builds the Tessa on an Arctic Rubber shell. In rain boots the material is the whole argument, natural rubber on the Hunter, triple-dipped latex neoprene on the XTRATUF, and the number that actually decides whether your feet stay dry is shaft height, which runs from 12.5 inches on the Kamik Miranda to 15 inches full height on the XTRATUF.
The absence is not a gap in disclosure. It is the right spec being published for the product.
The dog coat case
Dog raincoats are where the number is most informative, because so few brands print one. Hurtta states a water column above 10,000 mm with all seams taped. Ruffwear publishes the full shell for the Sun Shower: 70D/210D nylon ripstop, a 3,000 mm rating, a nonfluorinated water-repellent finish and a PU coating. That openness is worth something, and it is also why we could say 3,000 mm is less than a third of Hurtta’s figure.
Chilly Dogs and Gold Paw both describe waterproof fabric and publish no rating and no seam taping. The Gold Paw also calls itself wind resistant rather than windproof and is positioned for layering over fleece. In our comparison, the coat with no number was the skip, and the reason was not the missing number alone. It was the missing number plus the missing seam statement, which together leave nothing to check.
What to read instead
When a product page offers a rating, use it as a filter within a brand, or as a signal that the maker tests at all. Then read the parts that decide whether water gets in:
- Waterproof or water-resistant. Arctix describes its Tundra jacket and its insulated snow pants as water-resistant and wind-resistant. That one word separates a winter coat from a ski shell more reliably than any figure.
- Seam treatment. Fully taped or fully sealed beats critical seams, every time, regardless of the fabric’s rating.
- Which fabric, on which colour. If a rating applies to some colourways only, find out which fabric your colour uses before you buy.
- Maintenance of the face fabric. Look for a water-repellent finish you can restore. Even the Arc’teryx Beta, the most expensive shell in our rain jacket comparison, needs its finish re-proofed like any other jacket.
- For boots, height and joins. Shaft height, seam sealing and liner details answer the question a hydrostatic head never asks.
A big number on a jacket is not a lie. It is a lab result for a square of cloth, printed without the lab’s notes. Read it the way you would read a single test score with the exam paper missing, and then go and read the seams.
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