A hydrostatic head is the height of a column of water, in millimetres, that a coated fabric resists before water passes through it. It is a laboratory figure published by the maker for a coating, and on a tent it is only meaningful when you have both of them — the flysheet’s and the floor’s.
What the measurement describes
Picture a tube of water standing on a piece of fabric. The taller the column, the greater the pressure at the bottom. The point at which water begins to come through is the fabric’s hydrostatic head, and it is quoted in millimetres of water column.
It belongs to the coating, not to the yarn. Denier describes the yarn the fabric is woven from and predicts weight and abrasion resistance; hydrostatic head describes the polyurethane or silicone layer applied to it. The two frequently rise together, because a heavier fabric can carry a thicker coating, but they measure entirely different things.
One figure is half a specification
This is the point on which this site does not bend. A tent has two rated fabrics, and a maker who publishes only one number has told you half of what you need.
The flysheet figure describes resistance to rain arriving from above. The groundsheet figure describes resistance to water pushed up from below — by a knee, an elbow or a hip on saturated ground — and it is normally the higher of the two, for the straightforward reason that a knee on a wet pitch presses far harder than rain falls.
Where a listing publishes one figure and does not say which fabric it belongs to, treat that as a gap in the specification. Where it publishes both, you can finally compare two tents.
The same number turns up elsewhere
Tent footprints publish a hydrostatic head, and it runs across an enormous range in that family, from ultralight film to heavy coated polyester. Tarps publish one for most coated fabrics, and it is the comparable waterproofing figure between silicone-coated nylon and silicone-coated polyester. Bivy bags publish one too, and there it is the least likely thing to be your problem — essentially everything on sale keeps rain out, and what makes people damp is their own moisture, which is a breathability question instead.
What the figure does not cover
Three things sit outside it, and all three end more nights than a fabric rating does.
Seams. A needle hole is a hole. Taped or welded seams are what turn a rated fabric into a waterproof shelter, and they are a separate line on the specification.
Zips and closures. A waterproof zip is its own claim; an ordinary one under a storm flap is a different arrangement.
Wetting out. A surface that has stopped repelling holds a film of water and feels like a leak without being one. That is a repellency question, governed by the finish on the outer face rather than by the coating figure.
Reading a tent honestly
Write down four numbers before comparing anything: hydrostatic head of the fly, hydrostatic head of the floor, denier of each, and whether the seams are taped. Then read the weight the maker publishes and check which weight it is — a trail weight counts only what stands the tent up, a packed weight counts the pegs, guylines and stuff sack too, and the gap between them runs to several hundred grams.
Four figures and a weight label take a minute to collect and settle most of the argument between two shelters that look the same on a listing page.
What people ask before buying
Is a higher hydrostatic head always better?
It usually comes with a heavier fabric and a thicker coating, so it is a trade rather than an upgrade. The figure worth checking is the one for the part that takes pressure — the groundsheet — because that is where body weight, not rainfall, does the pushing.
Why do some fabrics carry no hydrostatic head figure at all?
Because they are not coated weaves. Dyneema composite is a laminate and is usually described by its own areal weight instead. Where a maker publishes no figure for a coated fabric, that is a gap in the specification and worth saying so rather than assuming a number.
Last reviewed 6 September 2026