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A waterproof jacket’s hydrostatic-head rating describes resistance to water pressure under a test. It is useful information, but it does not tell you how many millimetres of rain you can walk through or how many hours the jacket will keep you comfortable.
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Read the number correctly
Hydrostatic head is expressed as the equivalent height of a water column, in millimetres. A 20,000mm figure may be written as 20K. Under comparable test methods, a higher result indicates greater resistance to water penetration through the tested material.
Rab’s hydrostatic-head explanation provides context for these pressure-based ratings. Check whether a specification describes the fabric and whether the manufacturer identifies the test method.
Do not confuse it with breathability
A jacket may show two similar-looking numbers. Hydrostatic head concerns water resistance; a moisture-vapour figure concerns movement of moisture through the fabric. They answer different questions and cannot be substituted for each other.
The Rab Downpour QWI-06, for example, lists 20,000mm hydrostatic head and a separate 20,000g/m²/24h MVTR figure. The shared number does not make the measurements equivalent.
Inspect the complete jacket
Look at sealed seams, the main zip’s protection, hood adjustment and cuff closure. A strong fabric rating cannot compensate for rain entering an open collar or running up an exposed sleeve. Check how the hem overlaps your trousers when you bend and reach.
Try the jacket over your intended layers and with your rucksack. Turn your head with the hood up, reach for a high shelf and crouch as if adjusting a tent peg. The garment should move without exposing large gaps or pulling uncomfortably.
Match construction to use
A short campsite walk and a long exposed hill day place different demands on a jacket. Consider the expected weather, time away from shelter and abrasion from straps or vegetation. Weight and packed size matter more when you carry the shell for long distances.
Compare the Rab Downpour waterproof jacket as one specification example, checking size and exact generation. Downpour Mountain and older Downpour Eco versions are separate products.
Keep the rating in perspective
A jacket’s condition changes with wear and care. Inspect seams, fabric and fastenings, and follow its washing instructions. Surface water repellency, fabric damage and an opening left exposed are different issues that need different responses.
Use hydrostatic head to compare an identified material characteristic, then choose the jacket whose fit, coverage and practical features suit the trip. The biggest number on its own is not a complete buying decision.
Read our jacket breathability guide for the other number on the specification. If an existing shell looks saturated, use the wetting-out diagnosis guide before assuming its waterproof barrier has failed.
Continue with our waterproof jacket comparison.
Reviewed 10 September 2026. This guide is based on published specifications and guidance.