Tent hydrostatic head: what the millimetre rating actually measures

By the Canvasdale team · Last checked 24 October 2026

A guide, not advice. Check your kit maker's figures before you buy or pack.

A tent specification may list a hydrostatic head figure in millimetres. That number describes resistance to water pressure. It is not the depth of rain the campsite can receive, the length of a storm the tent survives, or a complete weather rating.

Understand the measurement

Sea to Summit: hydrostatic head explained describes the test using a fabric sample beneath a column of water. The height of that column when water penetrates is expressed in millimetres. It is a fabric/coating measurement, so first establish which part of the tent the advertised number describes.

The practical comparison is therefore between equivalent claims. A figure for a flysheet should not silently become a figure for the groundsheet. Nor does a larger number, by itself, establish stronger poles or more useful door protection.

As an example of how manufacturers present the distinction, the Outwell: flysheet materials guide describes different flysheet fabrics and their coatings. Treat such figures as manufacturer-stated specifications for the named material, not independent whole-tent test results or a promise that any pitch will remain dry.

Put the number into a buying checklist

For each shortlisted tent, save the exact model and season, the component associated with the rating and the manufacturer's care instructions. Then look for answers to separate construction questions:

• What does the maker say about seam treatment and any preparation before first use?
• How are doors, vents and openings protected when it rains?
• What pitching, tensioning and guying instructions apply?
• What are the stated limitations on location, exposure and length of use?

If the listing gives only a large waterproof number, ask for the missing details. Avoid filling gaps with a retailer's photograph of a similar tent.

Compare the whole arrangement

Imagine two tents with different fabric ratings. One fits the available pitch and has an entrance you can use without carrying wet footwear across the sleeping area. The other has a higher number but an awkward layout. The rating alone does not resolve which arrangement suits the family.

That is a planning example, not a performance comparison. Use the maker's dimensions and instructions for the actual products before you buy.

Keep damp observations separate

Water inside a tent does not, on its own, prove that water passed through the flysheet fabric. Read why is my tent wet inside to think about moisture observations, rather than immediately deciding that the advertised rating has failed.

Before the trip, practise pitching in a suitable place and check the actual components against the manual. If a new tent has damage or an apparent defect, discuss it with the supplier. Do not apply an unfamiliar coating or seam treatment merely because someone online used it on another fabric.

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