A 20,000mm waterproof rating tells you something useful about a fabric, but it cannot tell you whether a jacket will keep you comfortable on a muddy climb. Waterproofness, breathability and ventilation describe different things. Understanding the difference makes it easier to choose MTB clothing without paying for an impressive number that solves the wrong problem.
For UK riding, start with the whole garment: sealed seams, useful coverage in the riding position, a hood that works with your helmet and vents you can operate. Then use the fabric figures to refine your shortlist. Our waterproof MTB jacket guide and waterproof MTB trouser guide put those decisions into a buying context.
What does a 10K or 20K waterproof rating mean?
The number normally refers to hydrostatic head: the water pressure a fabric resists in a laboratory test, expressed as the equivalent height of a column of water. A 10K rating means 10,000mm, or ten metres; 20K means 20,000mm. It is a pressure measurement, not a forecast of how many hours you can ride in rain.
A higher result indicates greater resistance under the relevant test conditions. It does not mean that a 20K jacket keeps you dry for twice as long as a 10K jacket, or that every 20K garment performs identically. The fabric sample is only one part of a jacket that also has seams, openings and moving panels.
Durability also matters. Patagonia’s H₂No testing programme, for example, assesses waterproofness and other properties after simulated wear as well as checking garment construction. A headline figure without information about the test or the fabric’s condition tells only part of that story.
Why 20K/20K is not the same number twice
When a product is described as 20K/20K, the first figure commonly means 20,000mm waterproofness and the second 20,000g/m²/24h moisture-vapour transfer. Always check the specification rather than assuming the order. The second number describes water vapour passing through a square metre of material over a day under laboratory conditions.
MVTR stands for moisture vapour transmission rate. Higher values indicate more vapour transfer when measured using the same method. RET measures resistance to evaporative heat transfer, so its direction is reversed: lower RET indicates less resistance. A RET value cannot be compared numerically with an MVTR figure.
Rab explains both tests and their limitations. Test methods and conditions matter, and a fabric result is not a promise that all the sweat you produce will escape. Treat cross-brand league tables cautiously when the manufacturers do not identify comparable methods.
A real example from cycling clothing
Endura lists ExoShell40 with 20,000mm waterproofness and 40,000g/m²/24h breathability. Its ExoShell20 specification lists 10,000mm and 20,000g/m²/24h respectively. Those pairs illustrate why the two properties should be read separately: the fabric name alone is not a waterproof rating.
Check the exact jacket’s model year and product specification, too. A familiar name can span more than one version, and manufacturers can change fabrics. A heavily discounted older jacket may still be useful, but comparing it with the latest version’s figures can lead to a misleading buying decision.
Breathability and ventilation do different jobs
A breathable membrane transfers moisture vapour through the fabric. Ventilation lets air move through an opening, such as an underarm zip or an unzipped front. On a hard climb, opening a vent early can be more useful than waiting until your base layer is saturated and hoping the membrane catches up.
Choose openings that remain usable with your normal riding kit. A backpack can cover a vent or pull fabric tightly against your back; a chest strap can obstruct the zip you intended to open. Try the jacket with the pack, gloves and layers you actually wear, then reach forward into your riding position.
Motor assistance does not remove this problem on an eMTB. Technical climbing still takes effort, while a fast descent can make damp clothing feel cold. Dress for your workload and the likely pauses, and use ventilation before you become uncomfortably hot.
DWR, wetting out and a genuinely leaking jacket
DWR means durable water repellent: the treatment that encourages droplets to bead on the outer face fabric. It is separate from the waterproof membrane. When that treatment becomes less effective, the face fabric can soak up water, feel heavy and cling to layers underneath; this is often called wetting out.
GORE-TEX’s care guidance distinguishes this surface effect from membrane waterproofness. Loss of beading alone does not prove that the waterproof barrier has failed. Follow the garment’s washing and drying instructions, and use a compatible treatment when the manufacturer recommends it.
A persistent wet patch around a damaged seam or a tear deserves closer inspection. Equally, widespread dampness after a hard effort can come from perspiration. Our guide to washing waterproof MTB clothing and restoring water repellency covers the care process; reproofing cannot repair a hole or detached seam tape.
The features to check before buying
- Seams: look for the stated seam-sealing coverage rather than assuming a waterproof fabric makes the entire garment waterproof.
- Fit: sleeves should cover your wrists when reaching the grips, with enough room for a base layer and movement.
- Hood and collar: check helmet compatibility, adjustment and whether turning your head restricts your view.
- Ventilation: make sure openings are accessible with your pack and riding gloves.
- Trousers: check knee-pad clearance, seated coverage and a secure ankle that stays away from the drivetrain.
- Care and repair: read the washing instructions and consider whether damaged panels or zips can be repaired.
What rating should you buy for UK mountain biking?
There is no single number that guarantees the best jacket for every ride. For regular wet-weather riding, we would shortlist properly seam-sealed cycling shells with published waterproof specifications, then prioritise fit, ventilation and durability for the way you ride. A packable emergency layer and an all-day winter shell can sensibly make different compromises.
If two jackets fit equally well and have comparable test information, greater water-pressure resistance can be a useful advantage for sustained wet riding. It should not override a hood that blocks your view or a cut that pulls tight across your shoulders. Build the rest of your base-layer system around managing heat and moisture, rather than expecting the outer shell to do everything.
Quick answers
Is 20K automatically better than 10K? It indicates a higher fabric water-pressure result when the tests are comparable. It does not establish better fit, ventilation or overall comfort.
Does waterproof mean sweatproof? No. Sweat production can exceed the rate at which moisture escapes, especially during hard efforts. Adjust layers and vents as well as considering the fabric.
Does a three-layer jacket mean three jackets’ worth of warmth? No. The term describes the fabric construction. Insulation depends on what the garment and your clothing system actually contain.