What a waterproof rating measures
The millimeter number shows how much water pressure the fabric resisted in a laboratory test. A higher number means the fabric withstood more pressure, but seams, zippers, wear and care still affect how dry the whole garment keeps you.
Compare the whole garment
10,000 mm can be enough for short, dry days. For regular Alpine skiing, consider 15,000 mm as a starting point and 20,000 mm for wetter conditions. Also look for taped seams, secure closures and useful vents. A higher number does not automatically make one jacket better than another.
Our clothing guide helps you match those features to typical resort conditions.
Other details that matter
Seams and closures: check whether seams are fully or partly taped and how the hood, cuffs and zippers close. Try them with your helmet, gloves and layers.
Breathability: check how the brand measured it before comparing numbers. A 20K label and a RET result are not two scores on the same scale. Vents offer a separate way to release heat while skiing.
Warmth: insulation weight can help you compare similar jackets, but it does not tell you exactly how warm you will feel.
How to compare breathability numbers
RET measures resistance to moisture escaping. A lower result means less resistance when the same test method is used. The ISO 11092 test measures water-vapor resistance under controlled conditions; it is not a temperature rating for wearing the jacket.
Grams per square meter over 24 hours measure moisture transfer. You may see this written as g/m²/24h. Rab explains the moisture-transfer and RET tests. Higher figures mean more transfer within the same test, but test conditions differ. A large number from one method is not proof that a garment outperforms one measured another way.
Brand labels need context. Keep a label such as 20K or a brand’s own score in its original format unless the manufacturer explains the units and method. Our jacket comparison and pants and bib comparison show the different formats separately.
Look after the garment
Follow the brand's washing, drying and reproofing instructions. If the outer fabric starts soaking up water, cleaning and reproofing may help. Damage or leaking seams may need a repair.
Quick answers
Is 10,000 mm waterproof enough for skiing? It can be enough for short, dry days. For regular Alpine skiing, consider 15,000 mm as a starting point and 20,000 mm in wetter conditions.
What does a 20,000 mm waterproof rating mean? The fabric resisted the pressure of a 20-meter column of water in a laboratory test. Seams, zippers, condition and care still affect how the complete garment performs.
Why can you feel wet inside waterproof clothing? Sweat and condensation can make the inside feel damp, while worn fabric, damaged seams or open closures can let water in.