Waterproof ratings explained: what 10,000 mm really means

A waterproof rating in millimeters (mm) is the result of a lab test called a hydrostatic head test. A sample of fabric is clamped under a column of water, or an equivalent rising water pressure, and the pressure is increased until water is forced through the fabric. The height of the water column at that point, in millimeters, is the rating. A 10,000 mm fabric held back the pressure of a 10-meter column of water before it leaked.

The method is defined in standards such as ISO 811, where the specimen is put under steadily increasing pressure until water comes through in three places. The current edition is ISO 811:2018. Other test standards exist, so two brands’ numbers are only strictly comparable when they used the same method.

Rough tiers brands use

There’s no official scale, but most outdoor brands and retailers group ratings roughly like this:

Treat these as guidelines rather than promises. Brands write their own descriptions, and some membrane makers don’t publish a millimeter figure at all and rely on their own test protocols instead.

Why pressure matters on the trail

Rain falling on a jacket exerts very little pressure. The higher ratings matter because real use concentrates pressure on small areas: shoulder straps and hip belts press wet fabric against you, kneeling on wet ground pushes water into the knees of rain pants, and sitting on a wet rock does the same to the seat. A 10,000 mm jacket may stay dry walking in steady rain and still wet through under a heavily loaded backpack strap after a few hours.

Tents use much lower numbers, and that’s normal

Tent flies and floors are often rated between about 1,200 mm and 3,000 mm. That isn’t a sign of a poor tent. A taut flysheet sheds water rather than resisting it under pressure, and very high coatings add weight. Floors get the higher figures because you kneel and lie on them.

Two lightweight backpacking tents show the typical pattern. NEMO lists the Dragonfly OSMO’s rainfly at 1,500 mm and its floor at 2,000 mm. MSR lists the Hubba Hubba NX with a 1,200 mm rainfly coating and a 3,000 mm floor coating.

A footprint or ground sheet protects a thinner floor from abrasion, which is usually what ends a floor’s waterproofing first.

What the number doesn’t tell you

The rating describes the fabric, not the whole garment or tent. Water usually gets in somewhere else first:

• Seams. Stitch holes leak unless the seams are taped or sealed. Look for “fully seam-taped” or “fully seam-sealed”.
• Zips. Coated water-resistant zips and storm flaps matter, especially on the main front zip in driving rain.
• Hood and cuffs. A hood that doesn’t move with your head, or loose cuffs, let rain run inside.
• Wet-out. When the outer fabric’s durable water repellent (DWR) finish wears off, the face fabric soaks up water. The membrane underneath may still be waterproof, but breathability drops and the jacket feels clammy, which many people mistake for leaking. Washing and reproofing restores it.
• Age and abrasion. Coatings and membranes wear, especially where packs rub.

How to use the rating when buying

1. Match it to your weather. For occasional showers on day hikes, 5,000–10,000 mm is usually plenty. For multi-day trips where you can’t dry out, or for mountains, look at 15,000–20,000 mm and above.
2. Check construction before chasing a higher number. A well-built 10,000 mm jacket with taped seams and a good hood beats a poorly designed 20,000 mm one.
3. Read the breathability figure too. Waterproofness keeps rain out; breathability lets sweat out. Our breathability guide explains those numbers.
4. For tents, look at the floor rating and the pitch. A tight, well-guyed fly with a 1,200 mm coating can be drier in practice than a sagging fly with a higher number.

Rain shells to compare

Most brands don’t publish a millimetre rating for their jackets, so the table below sticks to what they do list: weight, construction and the waterproof fabric or membrane. Open any jacket for its full spec sheet and sources.

Sources

1. ISO 811:1981 — Textile fabrics, resistance to water penetration, hydrostatic pressure test (record, ANSI Webstore; superseded by ISO 811:2018) (accessed Oct 6, 2026)
2. NEMO Equipment — Dragonfly OSMO product specifications (accessed Oct 6, 2026)
3. Cascade Designs (MSR) — Hubba Hubba NX 2-Person specifications (accessed Oct 6, 2026)