Hydrostatic Head Testing: The Standard Behind Waterproof Bag Claims

A bag that claims to be waterproof has to spend its claim somewhere real, and for a fabric the measure is the hydrostatic head. Hydrostatic head testing measures how much water pressure a fabric resists before it leaks, and it is the number behind a “water resistant” or a “waterproof” claim. This guide explains what the test measures, what a result means in practical terms for a bag, and the question to ask a factory before you trust the label.

Verify the waterproof claim on the products range, with the fabric head and the seam construction checked.

What the test measures

Hydrostatic head testing measures the water column height a fabric withstands before water passes through, expressed in millimetres. A fabric is clamped and a column of water is applied from one side, and the height at which the first droplet appears on the other side is the hydrostatic head. A higher number means the fabric resists a taller column of water, so it better resists rain and standing water. The result is a measure of the fabric’s resistance, not of the bag’s seams, because a seam is the point where water gets in even on a high-head fabric.

What a result means for a bag

A low hydrostatic head, in the region of a few hundred millimetres, is enough for a light shower and a splash, while a higher figure is what a serious waterproof bag uses for a longer exposure or heavier rain. The number alone does not make a bag waterproof; the seams have to be welded or taped, and the closure has to be sealed, because water will enter at a seam or a zip long before the fabric leaks. So a bag manufacturer’s claim is the combination of the fabric head and the construction, and you have to ask for both rather than the fabric number on its own.

The question to ask a factory

Ask for the hydrostatic head of the fabric used, the test method, and whether the seams are welded or taped. Ask for a lab report rather than a verbal figure, and confirm the head is measured on the production fabric, not on a hand-picked sample. If the bag is sold to a user who expects it to keep a load dry in the rain, the seams and the closure carry more of the real-world result than the fabric number does. The standards that set the hydrostatic head test and the water-resistance measurement are published by the ASTM standards body, which is where the test method and the reporting convention come from.

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FAQ

What is hydrostatic head testing?

It measures the water column height a fabric resists before water passes through, in millimetres. A higher number means the fabric better resists rain and standing water.

Does a high hydrostatic head make a bag waterproof?

Not on its own. The seams have to be welded or taped and the closure sealed, because water enters at a seam or a zip long before the fabric leaks, so the fabric head is only half the claim.

What should I ask a factory about a waterproof claim?

The hydrostatic head of the fabric, the test method, whether the seams are welded or taped, and a lab report on the production fabric rather than a hand-picked sample.