A GUIDE TO HYDROSTATIC HEAD RATINGS
What is Hydrostatic head rating?
Hydrostatic Head Rating is a measure of how waterproof a certain fabric is, this indicates the amount of pressure the material can withstand before it starts to leak through. This test is measured in millimetres (MM) which refers to the height of a column of water that the fabric can resist. Fabrics which have higher rating are suited for extreme weather conditions, the lower ratings are more suited for calmer conditions.

How is this test conducted?
Test Preparation
- A section of the jacket’s fabric is taken and place onto the Hydrostatic Pressure machine
- The sample fabric needs to be properly secured to prevent any leaks.
Application of Water Pressure
- Water is applied to one side of the fabric at a controlled rate with either a column of water or using a pressurised chamber that applies force in Millimetres of water column.
- The pressure of water increases gradually until water seeps through the fabric.
Measuring water penetration
- This test continues until three drops of water appear on underside of the fabric.
- The pressure at this moment is recorded as the hydrostatic head rating.
An example of this is, if water starts seeping through at 10,000mm, the fabric will then be rated at 10,00mm waterproof.
Industry Standards
In the UK, waterproof workwear must meet BS EN 343 Standards, this is ensuring the safety of workers exposed to rain.
The BS EN 343 Standard is the UK specific standard for waterproof Garments, covering both water penetration resistance and breathability.
Class | Waterproof Rating (mmH₂O) | Performance |
Class 1 | ≥ 800mm | Basic water resistance (light rain, limited protection). |
Class 2 | ≥ 8,000mm | Medium waterproofing (suitable for heavy rain and outdoor work). |
Class 3 | ≥ 13,000mm | Highly waterproof (extreme conditions, professional use). |
What are the benefits of Hydrostatic Head ratings?
Weather Protection: Higher Hydrostatic Head ratings offer better resistance against challenging weather, to ensure you are kept dry and warm whilst working.
Enhanced Durability: The higher hydrostatic head rated jackets are made with stronger materials which makes it less likely to get worn and start tearing.
Comfort and performance: When working in rainy conditions it is important to be in clothing which is providing comfort and keeping you dry, Jackets with higher head rating provides this for you, they prevent discomfort, stops you from feeling the colder weather which ultimately prevents hypothermia.
Breathability: Many high hydrostatic head rated jackets contain breathable fabrics, preventing sweat from building up whilst still staying waterproof.
Increased Longevity: High quality waterproof materials last longer, which reduces the need for frequently replacing jackets.
The drawbacks
Not the only factor in Waterproofing: it is all well and good having a high hydrostatic head rated jacket, but if the seams, zippers and waterproof coatings aren’t very well designed then water can still seep through.
Heavier Material: Jackets with very high ratings often use materials which make them heavier and less flexible.
Higher Cost: Waterproof with higher ratings tend to be on the expensive side due to the advanced materials and coatings used in the making of the jacket.
What is the recommended head rating?
When it comes to choosing the right rain jacket or waterproof trousers there is many different factors to consider.
If you are looking for a heavy duty HI VIS garment which has minimal chance of rain seeping through and suitable for extreme weather conditions, you are looking between 10,000mm – 15,000mm.
If you are wanting a more lightweight, breathable garment but keeps you dry in rainy conditions, you would be looking at a rating of 5,000mm – 10,00mm. This is where ROKWEAR comes in. ROKWEAR offers a range HI IVS Jackets & Trousers that have Hydrostatic ratings from 5500mm to 6000mm as well as strong EN388 ratings. The Milford, Selborne, Witley and Blackmoor are a excellent companion when dealing with heavy rain and general outdoor work.

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