Waterproof Ratings Explained: What the mm Actually Means

Two jackets can both say waterproof on the label and behave nothing alike in a Wicklow downpour. The difference is usually one number you might have scrolled straight past: the waterproof rating, written in millimetres. Ten thousand mm, twenty thousand mm, sometimes no number at all. This guide explains what that mm figure actually measures, what each level buys you in real Irish weather, and why the rating on its own never tells the full story.

What the mm actually measures: hydrostatic head

The mm figure comes from a lab test called hydrostatic head. A piece of the jacket fabric is clamped under a sealed tube, and water is added to the tube a little at a time. The height of the water column when the first drops push through is the rating. Fabric that holds back a ten metre column before it leaks is rated 10,000mm; one that holds twenty metres is 20,000mm. The higher the number, the more water pressure the fabric can take before it gives.

Pressure is the word that matters. Rain is rarely just water landing on you. It is a pack strap pressing wet fabric into your shoulder, the seat of your trousers on a damp rock, a knee on the ground, wind driving rain hard against your chest. The hydrostatic head test stands in for all of that, which is why the same jacket can shrug off a shower and still wet through under a heavy pack in sustained rain.

The waterproof ratings scale, from showerproof to mountain-ready

Ratings run across a wide range, and the label rarely explains where a given number sits. Here is what each band means once you are actually outside. Anything below about 1,500mm is not classed as waterproof at all under the standard most brands test to; it is water-resistant, and honest labels will say so.

A jacket rated around 10,000mm is the sensible floor for anyone spending a full day out in the kind of rain Ireland does best. The Trailpeak jackets we stock, available exclusively at 53 Degrees North, sit exactly here at 10,000mm Ventron, which keeps a hillwalker dry through hours of steady rain without paying for mountaineering-grade fabric they will not use. If your days out involve exposed ridges, winter conditions or a heavy pack for hours at a time, that is when the jump to 20,000mm and beyond earns its price.

If most of your wet-weather wear is the walk to the car or the school run, a showerproof jacket is often the more comfortable, more breathable choice, and we keep those separate for exactly that reason.

One caveat worth knowing: brands do not all arrive at their numbers the same way, because the test can be run to different standards. Read the rating as a band, not a decimal-precise promise. A 10,000mm jacket from one brand and a 12,000mm jacket from another are, for practical purposes, in the same class.

Waterproof, water resistant and water repellent: what the labels mean

Three words get used loosely on labels, and they do not mean the same thing. Getting them straight is half the battle.

Waterproof means the fabric stops water under pressure and over time, backed by a hydrostatic head rating and, on a good garment, taped seams. It is the only one of the three you can rely on in sustained rain.

Water resistant is a step down. It sheds light rain and drizzle for a while, then wets through. Showerproof is the same idea by another name, and most showerproof jackets sit under about 5,000mm. They are lighter and more breathable, which is why they suit commutes and everyday wear rather than a day on the hills.

Water repellent describes the DWR finish on the outer face, the treatment that makes rain bead and roll off. It is a surface effect, not a barrier: a water-repellent finish on its own is not waterproof, and it wears away with use, which is why reproofing matters. On a proper waterproof jacket the finish and the membrane work together, the DWR shedding most of the water so the membrane underneath is never overwhelmed.

In short: water repellent sheds, water resistant slows, waterproof stops. If a label only claims water resistant or shower resistant, treat it as a light-rain layer, not a wet-day one.

Why the waterproof rating is only half the story

Two jackets with the same mm rating can still perform very differently, because the number only describes the flat fabric. Everything else is down to how the jacket is built.

Seams come first. Every seam is a line of needle holes, and water walks straight through them unless they are sealed with tape on the inside. Fully taped seams seal every one; critically taped seals only the most exposed; untaped leaves them open no matter how high the fabric rating. It is the single most common reason a well-rated jacket still lets water in.

Then there is the outer face and its DWR, the durable water repellent finish that makes rain bead up and roll off. When the DWR is working, water beads; when it wears off, the outer fabric soaks up water and goes dark and heavy. That is wetting out, and it is not the same as leaking. The membrane underneath can still be keeping you dry while the outer looks soaked. It does kill breathability and make the jacket feel clammy, which is why reproofing matters, more on that below.

Breathability is the other number, usually written in grams (g/m2/24hrs) or as an MVTR figure. A very high waterproof rating with poor breathability traps your own sweat, and you end up wet from the inside instead. The best jackets balance the two rather than maxing out one.

Finally, design decides where water actually gets in: the hood and its adjusters, storm flaps over the zip, the cut of the cuffs and hem, and whether there are pit zips to dump heat on the climb. A hood is only as good as its adjusters. None of that shows up in the mm figure.

A few jackets carry no mm rating at all, because they use a different system that moves moisture out rather than blocking it with a rated membrane. Those are not worse, they just cannot be compared to a 10,000mm jacket like for like.

What waterproof rating you actually need in Ireland

Match the rating to how you are actually going to use the jacket, not to the biggest number on the rail. For everyday wear, commuting and the school run, showerproof to around 5,000mm is usually plenty, and it will breathe better and pack down smaller than heavier fabric. For regular walking and a full day in the hills, around 10,000mm with fully taped seams is the reliable choice. For the mountains, winter, or long hours under a pack in driving rain, look for 20,000mm and up, and pair it with waterproof overtrousers so your legs get the same protection your top half does.

The Trailpeak Waterproof Jacket is a straightforward example of the 10,000mm class done well: 10,000mm Ventron fabric, fully sealed seams and a sealed front zip, underarm vents to let heat out on the climb, and an adjustable hem, all in GRS certified recycled polyester. It comes in men's and women's fits, with matching overtrousers if you want head-to-toe cover.

If cold is as much the problem as rain, an insulated waterproof combines the membrane with warmth in one layer. For the highest-performing membranes, GORE-TEX is a class of its own and we group it together so you can find it easily. And children need the same protection at their own scale, not a scaled-down showerproof, so we keep kids' waterproofs separate too.

How to keep the rating you paid for

A waterproof rating is not fixed for life. As the DWR wears and the fabric picks up dirt, sweat and body oils, water stops beading and the jacket wets out faster and breathes worse. The good news is that most of that is reversible, and it comes down to two steps.

First, clean it. Washing a waterproof in ordinary detergent leaves residues that hold water against the fabric and clog its breathability, which is why a technical cleaner made for the job is worth it. Nikwax Tech Wash lifts out the dirt and body oils a normal wash leaves behind, without stripping the water-repellent finish, and restores the repellency and breathability the fabric had when it was new. It goes straight in the machine or can be done by hand.

Then reproof it. Cleaning gets the jacket back to a clean baseline, but the durable water repellent that makes rain bead and roll off does eventually wear away, and that is what TX.Direct Wash-In renews. It adds a fresh water-repellent layer across the whole garment, cuts how much water the face fabric soaks up, and revives breathability, all in a normal wash cycle with no tumble drying needed. The two come together as a Tech Wash and TX.Direct twin pack, which is the simplest way to do both steps in one go.

Done a couple of times a season, this is the cheapest performance upgrade you can make, and it is why a jacket that is looked after will out-perform a far pricier one that has never been washed.

Once you know what the mm actually means, the rail gets a lot easier to read. Whether you need a showerproof layer for the commute or a 20,000mm shell for the tops, you can browse the full range by rating and fit and match the jacket to the weather you actually walk in.