| What it measures | Water pressure resisted (hydrostatic head) |
| Unit | Millimeters (mm) |
| Higher | = more waterproof |
| Rough guide | 1,500 min · 10,000 rain · 20,000+ hard rain |
A waterproof rating in millimeters (the hydrostatic head) measures how tall a column of water a fabric can withstand before water leaks through — the higher the number, the more waterproof. Roughly, 1,500 mm is minimally waterproof, 10,000 mm handles steady rain, and 20,000 mm+ resists hard rain and pressure. It’s a key spec for comparing rain gear and tents.
Read it with breathability
Balance it against breathability and weight; real dryness also needs a working DWR and sealed seams, as in 3-layer construction.
Frequently asked questions
What does a waterproof rating in mm mean?
It's the hydrostatic head: the height in millimeters of a column of water a fabric can hold back before water seeps through in a standardized test. A 10,000 mm rating means a 10-meter column of water. The bigger the number, the greater the water pressure the fabric resists.
What waterproof rating do I need?
As a rough guide: around 1,500 mm is the minimum to be called waterproof and handles light rain; 5,000–10,000 mm copes with moderate to steady rain and light pressure; 20,000 mm and above resists heavy, prolonged rain and pressure from pack straps and sitting. Higher is more weatherproof but often less breathable and pricier.
Is a higher waterproof rating always better?
Not necessarily — beyond what you need, a very high rating can mean a heavier, stiffer, or less breathable fabric, and real-world dryness also depends on the DWR, seam sealing, and zippers. Balance the waterproof rating against breathability and weight for your intended use.
Sources
- Hydrostatic head testing — The Mountaineers
- Waterproof fabrics — American Hiking Society