Every waterproof jacket sold in Britain claims to be waterproof. Some of them are, in the sense that a bin bag is waterproof and about as pleasant to wear. Others keep water out for twenty minutes and then let it through the shoulders. The numbers on the swing tag are supposed to tell you which is which, and most people have no idea what they mean.
Here is what the figures actually measure, what they do not measure, and why a jacket that passed a lab test can still leave you damp on a walk to the station.
Hydrostatic head: The water column test
Hydrostatic head is the number quoted in millimetres, usually as something like 5,000mm or 20,000mm. It comes from a lab test in which a column of water is held against the fabric and increased until water starts to come through. The height of that column at the point of failure, in millimetres, is the rating.
So a 10,000mm fabric resisted a ten metre column of water before leaking. That sounds absurd until you realise that the pressure created by a rucksack strap on a wet shoulder, or by kneeling on wet ground, is a great deal higher than rain falling on flat fabric.
| Rating | Realistically handles |
|---|---|
| Under 1,500mm | Light drizzle, briefly. Usually described as water resistant rather than waterproof. |
| 1,500mm to 5,000mm | Light to moderate rain, short exposure. Fine for a dash to the car, not for standing on a touchline. |
| 5,000mm to 10,000mm | Sustained rain, everyday use, commuting. The sensible range for most people in Britain. |
| 10,000mm to 20,000mm | Heavy rain for hours, rucksack pressure, hill walking. |
| 20,000mm and above | Mountain and expedition use. Genuinely useful in the hills, and overkill for the school run. |
One important caveat: the figure 1,500mm gets quoted everywhere as the point at which something counts as waterproof. Treat that as a widely used industry rule of thumb rather than a legal definition, because there is no UK law policing the word "waterproof" on a coat. The number matters more than the word on the label.
Breathability: The number that matters just as much
Keeping rain out is only half of staying dry. The other half is letting your own sweat vapour escape, because a jacket that traps it leaves you wet from the inside and just as cold.
Breathability is quoted in two ways, and they run in opposite directions, which is exactly the sort of thing that makes comparison shopping infuriating.
- MVTR, moisture vapour transmission rate, in grams per square metre per 24 hours. Higher is better. Around 5,000 g/m²/24h is reasonable for everyday use, 10,000 and above is good, 20,000 and above is aimed at high-output activity.
- RET, evaporative resistance. Lower is better. Under about 6 is very breathable, 6 to 13 is moderately breathable, and above 20 is not breathable in any meaningful sense.
Manufacturers pick whichever measure flatters their fabric, and different test methods produce different numbers for the same material, so use them for rough comparison rather than treating small differences as meaningful.
The trade-off is real. Very high waterproof ratings usually come with lower breathability, because both depend on the same membrane. For most British use, a 10,000mm jacket that breathes well beats a 20,000mm jacket that does not.
EN 343: The standard behind the symbols
If a jacket is sold as protective clothing against rain, you may see a reference to EN 343, the European standard for exactly that. It tests two things separately: resistance to water penetration and resistance to water vapour, and awards a class for each.
The useful thing to understand is the structure. Two numbers, one for keeping rain out and one for letting vapour through, each rising with performance. A garment can be top class for waterproofing and low class for breathability, and the standard makes that visible where a marketing claim would not.
Most high street rainwear is not tested to EN 343 at all, because the standard is aimed at protective workwear rather than fashion. Its absence does not mean a jacket is bad, but its presence tells you somebody has actually measured something.
Why your jacket wets out
Here is the scenario everybody recognises. You have a good jacket. It kept you dry for two years. Now, in heavy rain, the outer fabric goes dark and soggy, and you feel cold and clammy even though nothing has actually leaked through.
That is wetting out, and it is not the membrane failing. The outer face fabric of a waterproof jacket is treated with a durable water repellent finish, DWR, which makes rain bead up and roll off. When that finish wears away, the outer fabric soaks up water instead. The membrane underneath is still working, but a saturated outer layer cannot let vapour through, so your sweat has nowhere to go and condenses inside the jacket.
The fix is straightforward, and it is the opposite of what most people do.
- Wash it. Dirt, sweat and detergent residue all kill DWR. Use a technical wash designed for waterproofs, not ordinary detergent and definitely not fabric conditioner.
- Tumble dry it on low, or warm it with an iron on a low setting through a cloth, if the care label allows. Heat reactivates DWR that is still present.
- Reproof it with a spray-on or wash-in product if heat alone does not restore the beading.
A jacket that is washed and reproofed once or twice a year will outperform a more expensive one that has never been cleaned. Most people are afraid to wash a waterproof, which is precisely why so many of them stop working.
The things the numbers do not cover
Two jackets with identical ratings can perform very differently, because a fabric test says nothing about how the garment is put together.
- Taped seams. Every stitch is a hole. Fully taped seams are sealed; critically taped means only the most exposed ones are. Untaped seams on a high-rating fabric are a waste of a good membrane.
- The hood. A hood that does not adjust, or has no wired peak, funnels water down your face and neck. This is the single most common failure in everyday rainwear.
- Cuffs and hem. Adjustable cuffs stop water running up your arms when you hold an umbrella or a handrail. A drawcord hem stops the jacket ballooning in wind.
- Zips. Water-resistant zips help; a storm flap over the main zip helps more.
- Pit zips. Underarm vents are the fastest way to dump heat, and they do more for real-world comfort than a higher breathability rating.
- Length. A jacket that ends at your waist leaves your thighs soaked, which is fine on a hill and miserable on a commute.
What to buy for British weather
For most people doing most things in the UK, the target is a jacket around 10,000mm hydrostatic head with decent breathability, fully taped seams, an adjustable hood with a peak, and pit zips. That handles a genuinely wet British day without turning into a sauna on the walk from the station.
Go higher only if you are regularly out for hours in the hills or carrying a pack. Go lower only if the jacket lives in a bag for emergencies, where weight and packed size matter more than sustained performance.
And whatever you buy, wash it. The best waterproof you own is the one whose DWR still works.
For how the outer shell fits with everything underneath it, see the layering guide, and the outfit tool will tell you when today is actually a waterproof day.