Tent Hydrostatic Head Ratings Explained

Every tent listing on Amazon UK quotes a hydrostatic head figure, usually as a bare number with "mm" after it and no explanation. Sellers know it reads as a quality score, which is why the number keeps climbing: 2,000 mm, 3,000 mm, 5,000 mm, 10,000 mm. Almost none of them say what was measured, or on which part of the tent.

It is a genuinely useful specification once you know what it describes — and, more importantly, what it does not. This page is the explanation, not a recommendation: it should let you read a spec sheet and work out whether a tent will keep you dry in the conditions you actually camp in.

How the hydrostatic head test works

Hydrostatic head is measured with a water column test, standardised as ISO 811 (published in the UK as BS EN 20811). A sample of the fabric is clamped under a sealed cylinder, water pressure is applied to one face and increased at a constant rate, and the tester watches the other face. The moment the third droplet of water appears through the fabric, the test stops and the pressure is recorded as a height of water in millimetres.

So a 3,000 mm rating means the fabric held back a three-metre column of water before the third drop came through. That is the whole claim. It is a lab measurement of one flat, new, undamaged piece of material under steadily rising pressure — not a description of a pitched tent in a storm.

What the figures mean in UK conditions

The figure usually quoted as the threshold for calling a fabric waterproof at all is 1,500 mm. Above that, the practical bands look roughly like this:

• 1,500 mm — holds up to ordinary summer rain. Fine for a festival weekend or settled weather, marginal for anything sustained or exposed.
• 2,000–3,000 mm — the sensible floor for UK three-season camping, where a wet night is the expectation rather than the exception.
• 4,000–5,000 mm — worth having for exposed pitches, spring and autumn trips, and long stretches of driving rain.
• 10,000 mm and above — real, but rarely the constraint. At this level the seams, the pitch and the ventilation decide whether you stay dry, not the fabric.

Diminishing returns set in earlier than the marketing suggests. Going from 1,500 mm to 3,000 mm is a meaningful change in what the tent can cope with; going from 5,000 mm to 10,000 mm mostly buys a bigger number, a heavier fabric and a higher price.

Groundsheets are rated far higher, for a reason

Groundsheets are typically quoted between 3,000 mm and 10,000 mm, and that is not manufacturers gold-plating the floor. Rain applies very little pressure to a flysheet. A kneeling adult applies a great deal of pressure to a small patch of groundsheet, and that concentrated load will push water up through a fabric that would shrug off a downpour all night. If a tent quotes a single figure and it is a low one, the floor is the part to be sceptical about.

Where hydrostatic head stops being useful

Tents leak at their seams long before they leak through their fabric. A needle makes a hole; a stitched seam is thousands of them in a line. Factory-taped seams are what close them, and a tent with a high fabric rating and untaped seams is worse in the rain than a modest fabric with tape on every seam. "Fully taped seams" in a spec list is worth more than another 2,000 mm.

The rating also assumes still water. Wind-driven rain arrives with momentum, and anything pressing against the inside of a wet flysheet — a sleeping bag, a rucksack, the back of your head — locally exceeds the static pressure the fabric was tested at and can wick water straight through. This is why the gap between the inner and the fly matters, and why a double-skin tent behaves so differently from a single-skin one with the same rating.

The rating changes as the coating ages

The waterproofing is a coating on the fabric, not the fabric itself, and most affordable tents use polyurethane. PU degrades by hydrolysis — the reaction with water that eventually leaves an old tent's inner surface sticky and flaking — and packing a tent away damp accelerates it considerably. A five-year-old 3,000 mm flysheet that has been stored wet is not a 3,000 mm flysheet any more.

Silicone-coated fabrics (silnylon and its polyester equivalents) do not hydrolyse the same way and hold their performance for far longer, which is part of what you pay for at the lightweight end. The trade-off is that seam tape will not stick to silicone, so those tents need liquid seam sealant applied by hand rather than tape applied at the factory.

One more material difference worth knowing: nylon absorbs water and stretches when it does, which is why a nylon fly needs re-tensioning in the night, while polyester barely moves. Neither is about waterproofing, but a sagging fly touching the inner is a common route for water to get in regardless of the number on the label.

Polycotton and canvas do not play this game

Canvas and polycotton tents often quote no hydrostatic head at all, and that is not an omission. Cotton fibres swell when wet and close the weave, so the fabric seals itself rather than relying on a coating — which is also why a new canvas tent is "weathered in" by soaking it before its first trip. Applying the polyester rating scale to canvas produces a meaningless answer in either direction.

Reading it back on a real listing

Put together, the questions worth asking of a tent listing are narrower than the spec sheet suggests: which fabric does the quoted figure describe, is the groundsheet rated separately and higher, are the seams taped, and is there enough ventilation to deal with the condensation the fabric will not. A tent that answers all four honestly at 3,000 mm will keep you drier than one that answers none of them at 8,000 mm.