Waterproof Ratings Explained: What the mm Numbers Mean

Here are waterproof ratings explained in one line: the mm number tells you how tall a column of water a fabric can hold back in a lab before it leaks, so a 10,000mm fabric resists the pressure of a 10-metre water column. For hiking, 5,000mm copes with showers, 10,000mm handles a full day of steady rain, and 20,000mm or more is what you want for storms, heavy packs and days when you'll kneel or sit in the wet. The number only tells part of the story, though. Seams, the outer coating and the membrane decide whether you actually stay dry.

This guide is the hub for everything on HikeHeaven about rain protection. If you already know you want a specific class of jacket, jump straight to the best 10,000mm waterproof jackets or the best 20,000mm waterproof jackets. If you want to understand what you're paying for first, read on.

Waterproof Ratings Explained: Which Rating Do You Need?

Here's the short version. Match the rating to the wettest conditions you regularly hike in, not the average day.

If you're stuck between two rows, go up one.

How the Hydrostatic Head Test Works

The mm figure comes from a hydrostatic head (HH) test. A sample of fabric is clamped under a column of water, or a pressurised water chamber, and the pressure rises steadily until water droplets push through to the other side.

The main standards are:

• ISO 811 (also published as EN 20811): a 100 cm² sample, water pressure rising at a fixed rate. The result is the pressure at which the third drop appears.
• JIS L 1092: the Japanese standard, used by many Asian fabric mills. It has a low-pressure method (a rising water column) and a high-pressure method for fabrics that go well beyond a few metres of head.
• AATCC 127: the American equivalent, which works on the same principle.

All three report the result as a height of water, usually in millimetres. That's just a convenient way of expressing pressure. Each millimetre of water column equals about 9.8 pascals, so 10,000mm is roughly 98 kPa, or close to one atmosphere. A 20,000mm fabric holds back about twice that.

Two caveats matter. The test is usually done on new, flat fabric, not a jacket that has been worn, washed and rubbed by pack straps for a season. And brands rarely say which standard they used, so one brand's "10,000mm" isn't always comparable with another's.

Why Pack Straps, Kneeling and Sitting Matter

Here's the part most retailer guides skip. Falling rain barely pressurises your jacket at all. Wind blowing at 100 km/h only pushes on fabric with roughly 50mm of water pressure. Raindrop impacts are brief and tiny. So why do jackets leak?

The answer is pressure points. Pressure is force divided by area, and your body is good at concentrating force.

• Kneeling: a 70kg hiker kneeling on one knee puts around 350 newtons through a contact patch of about 25 cm². That works out at roughly 140 kPa, or about 14,000mm of water pressure. Kneel on wet ground in a 10k waterproof and the fabric under your knee can leak, even though the rating is "good".
• Sitting: on a flat surface your weight spreads out and the pressure stays low. Perch on a sharp rock edge or a wet log and the local pressure jumps, and you sit there for twenty minutes while it soaks through.
• Pack straps and hip belts: the average pressure under a shoulder strap is modest, but straps rub, flex and pump water into the face fabric, wear away the water-repellent coating, and block breathability. The result is a damp patch that's part leak, part trapped sweat.

That's why the best 20,000mm waterproof jackets make more sense for backpackers than for casual walkers. The extra headroom covers the moments when you press the fabric hard against something wet. It's also why waterproof trousers need a decent rating: you sit and kneel in them far more than in a jacket. Our guide to cheap rain pants covers affordable pairs at 10,000mm.

Water Resistant vs Waterproof

There's no single legal line between the two for outdoor clothing, but the industry convention is fairly consistent:

• Water resistant means the fabric sheds light rain thanks to a tight weave or a coating, but it isn't built to stop water under pressure. Seams usually aren't taped. Expect ratings under about 1,500mm, or no rating at all.
• Waterproof means a coated or laminated fabric with a measured HH rating, usually 3,000mm or more, plus taped seams. Without taped seams, a high rating is close to meaningless, because water simply enters through the stitching holes.

For reference, the European standard for protective rainwear, EN 343, sets its lowest water penetration class at 8,000 pascals, which is only around 800mm. That tells you how low the bar can be for something to be called waterproof in a technical sense. For hiking, treat anything under 5,000mm as a showers-only layer.

What Matters More Than the Number

Once you're above about 10,000mm, the HH figure stops being the main reason jackets fail. Three other things decide whether you stay dry.

Seams

Every stitch is a hole. Fully taped seams have waterproof tape bonded over every seam inside the jacket. Critically taped seams only cover the most exposed ones, usually shoulders and hood. For hiking in real rain, insist on fully taped. Zips matter too: look for water-resistant zips or storm flaps over the main zip.

DWR (Durable Water Repellent)

DWR is the coating on the outer fabric that makes water bead and roll off. When it wears out, the face fabric soaks up water and "wets out". The membrane underneath may still be waterproof, but a soaked outer layer feels cold, gets heavy and stops the jacket breathing, so sweat condenses inside and you feel wet anyway.

Most brands have now moved to PFAS-free DWR treatments, which are better for the environment but tend to need re-proofing more often. Wash your jacket with a technical cleaner and reapply DWR when water stops beading.

The membrane or coating

The waterproof layer itself comes in two broad forms:

• PU coatings: a polyurethane layer painted onto the inside of the fabric. Cheap and effective, but usually less breathable and less durable.
• Membranes: a thin film laminated to the fabric, such as Gore-Tex, eVent, or brand-specific options like Marmot's NanoPro and Outdoor Research's AscentShell. Membranes generally offer higher ratings and better breathability for the weight.

How the layers are built also matters. A 2-layer jacket has a loose inner lining, a 2.5-layer jacket has a printed protective layer on the inside, and a 3-layer jacket bonds face fabric, membrane and inner fabric together. 3-layer shells are the most durable under pack straps.

Breathability Ratings Explained (MVTR and RET)

A jacket that keeps rain out but traps every drop of sweat will still leave you soaked on a steep climb. That's where breathability ratings come in. There are two systems, and they work in opposite directions.

MVTR (moisture vapour transmission rate) is given in g/m²/24h: how many grams of water vapour pass through a square metre of fabric in a day. Higher is better. It's usually measured with the JIS L 1099 cup methods, and the two common variants (A-1 and B-1) give very different numbers for the same fabric, so compare MVTR figures within one brand rather than across brands.

RET (resistance to evaporative heat transfer) comes from the ISO 11092 "sweating hotplate" test. It measures how much the fabric resists moisture escaping. Lower is better.

Labels often pair the two ratings, so "10K/10K" means 10,000mm waterproof and 10,000 g/m²/24h breathable. "20K/20K" is the common high-end equivalent.

Real Jackets, Real Ratings

Putting the numbers into context with jackets reviewed elsewhere on HikeHeaven:

• 10k class: the Marmot PreCip Eco (NanoPro membrane, fully taped seams, pit zips) and the Columbia Watertight II (Omni-Tech, sealed seams, very packable) both sit in the 10,000mm bracket. They're the right call for day hikes and three-season trails. Full comparison in our 10,000mm waterproof jacket guide.
• 28,000mm Gore-Tex class: the Marmot Minimalist GTX uses a Gore-Tex membrane, the family widely quoted at 28,000mm, with 100% taped seams and a PFC-free DWR. It's a sensible step up when your hikes regularly run into full days of rain.
• Breathability first: the Outdoor Research Foray 3L uses an air-permeable AscentShell membrane in a 3-layer build. It's the one to look at if you sweat more than you stand in rain.

More shells above 20,000mm are compared side by side in our 20,000mm waterproof jackets guide. For a broader buyer's guide covering budgets and features, see waterproof hiking jackets.

Gore itself doesn't print a mm figure on its swing tags. It tests finished garments in rain rooms instead, so treat the quoted 28,000mm as a benchmark rather than a label claim. The same membrane shows up in footwear, as our look at whether Adidas Terrex shoes are waterproof explains.

Tents, Dry Bags and Other Gear

Jackets aren't the only gear with an HH rating, and the right number changes with how the fabric is used.

Tent flysheets sit taut and steep, so rain runs off without building pressure. A fly of 1,200mm to 3,000mm is plenty for most three-season tents, and very high ratings often mean heavier PU coatings that sag when wet. Tent floors are a different story. You kneel, sit and sleep on them over wet ground, so 3,000mm to 10,000mm is a sensible range, with a footprint adding extra margin.

Dry bags usually skip mm ratings and use IP codes instead, such as IPX7 (short immersion) or IPX8 (longer submersion). Our guide to the best dry bags for hiking covers which ones are rain-proof and which survive a dunking.

How to Read a Waterproof Label

When you pick up a jacket in a shop or scroll a product page, check these in order:

1. HH rating: is it listed? "Waterproof" with no number is a warning sign. Aim for 10,000mm or more for regular hiking.
2. Seams: "fully taped" or "fully sealed" is what you want. "Critically taped" is a compromise.
3. Breathability: an MVTR in g/m²/24h, or a RET value. Remember they run in opposite directions.
4. Construction: 2-layer, 2.5-layer or 3-layer. Heavier pack, more layers.
5. DWR: PFAS-free is now standard. Plan to re-proof it.
6. Vents: pit zips or side zips if you hike hard or in warm, wet weather.

A high HH rating with nothing on seams or breathability deserves suspicion.

Frequently Asked Questions

The Bottom Line

The mm rating is a useful starting point. Use 10,000mm as your baseline for regular hiking, step up to 20,000mm or more for wet climates, heavy packs and mountain weather, then check the seams, DWR and vents before you buy. Ready to choose? Start with the 10,000mm jacket guide for day hiking, or the 20,000mm jacket guide if you hike where the rain doesn't let up.