Hydrostatic Head Rating Chart: Which mm to Specify

The Chart: mm Bands and What They Survive

A hydrostatic head figure becomes useful only when it is tied to a load case. The number itself is the height of a water column, in millimetres, that a flat fabric specimen withstands before water penetrates, measured under ISO 811 or, in North American programmes, AATCC 127. Rain alone imposes very little pressure; what generates pressure in service is contact, strapping and body weight.

The chart below is the frame we use when a buyer asks what figure to put in a specification. It is deliberately expressed as what the band survives rather than as a quality ranking, because the right answer depends entirely on how the product is used. A 3 000 mm fabric on a fashion tote is a good decision. The same fabric on a rafting dry bag is a warranty claim waiting to happen.

Two cautions before you use any chart like this. First, the figure describes new, clean, flat fabric, and it falls after abrasion, laundering and flexing, which is why the lower sections of this page deal with retention. Second, the figure says nothing about the assembled article. A 20 000 mm laminate sewn with an untaped seam leaks at a fraction of that pressure, because the needle holes are the path of least resistance.

There is also a unit trap. Certificates are issued in millimetres, centimetres, kilopascals and millibars, and a figure of 1 000 cm equals 10 000 mm. Transcription errors of a factor of ten are common when a mill certificate is retyped into a sourcing spreadsheet, so state millimetres in your specification and require the same on the report. Our incoming material test protocol issues one unit and one method for this reason.

• Read the band as a load case, not as a quality ranking on a single scale.
• Match the band to how the product is strapped, loaded and pressed, not to how hard it rains.
• Deduct for abrasion and laundering, because the certified figure describes new fabric.
• Never let a high head figure substitute for a seam and closure qualification.
• State the unit, the method and the rate of rise so the number can be reproduced.

What follows is a section by section walk through the bands, with the commercial judgement attached to each. Where a band is commonly over-specified or under-specified, we say so, because those two mistakes are what erode margin and generate returns in equal measure.

1 000-3 000 mm: Shelter and Light Rain

This band covers products whose realistic exposure is light rain, mist and occasional contact with a damp surface. A fashion tote carried from a car to a shop, a promotional bag, a packing cube, or the outer of a shelter that is tensioned rather than compressed all sit comfortably here. The fabric is usually a polyester or nylon with a light coating or a durable water-repellent finish, and the construction is sewn without sealing.

The engineering reason this band works for those products is that nothing presses the fabric against a wet surface. Rain drops land and are shed or evaporate. There is no standing pressure, so the limiting factor is not penetration resistance but wetting and the speed at which the face fabric absorbs water and feels damp to the touch. That is a finish problem, and a good finish is cheaper than a heavy coating.

Where this band is misused is in products that look technical but are not. A bag styled like a dry bag, sold at a fashion price, and claimed as waterproof on the strength of a 3 000 mm fabric will fail the first time a customer straps it to a kayak deck or sets it down in a puddle. If your marketing will use the word waterproof, this band is not defensible, whatever the styling suggests.

For buyers, the decision rule is simple. Use 1 000 to 3 000 mm when the honest claim is water-repellent or water-resistant, when the price band cannot absorb coating cost, and when the product will never be compressed against water. Move up a band the moment the product is strapped down, kneeled on, sat on, or sold with an immersion story. Our waterproof backpack range starts above this band precisely because backpacks get pressed against wet surfaces.

• Use for: fashion totes, promotional bags, linings, shelter fabric, packing organisers.
• Honest claim: water-repellent or water-resistant, never waterproof.
• Construction: sewn, with a durable water-repellent finish for beading.
• Avoid for: anything strapped down, sat on, or sold with an immersion claim.

5 000-10 000 mm: Sustained Rain and Load

This is the commercial workhorse band. It covers sustained rain and, more importantly, the contact loads that a carried bag actually sees: a pack pressed against a wet back, a shoulder strap pulled tight over a soaked panel, a bag set down in a puddle while the user loads a car. Those loads generate far more local pressure than rainfall does, which is why the practical floor sits here rather than at 3 000 mm.

Products in this band are commuter backpacks, school bags, hiking daypacks, rain covers and messenger bags. The fabric is typically a 420D to 600D polyester or nylon with a PU or TPU coating, and the seams are either sewn and taped or, where the design allows, welded. A sewn-and-taped seam is acceptable at this level because the product is not rated for immersion, and the failure mode of a lifted tape edge is slow weeping rather than a flood.

The judgement call inside this band is whether to buy 5 000 mm or 10 000 mm. Buy 5 000 mm when the product is a school or commuter bag at a competitive price and the claim is water-resistant or rainproof. Buy 10 000 mm when the product is sold into a wet climate, when the customer is likely to test it in a shower, or when the same fabric is shared with a higher-rated product in your range and consolidation saves money.

One practical warning about taped seams in this band. Tape adhesion is the failure point, and it degrades with flexing, heat and laundering. If your product will be washed by the end user, or stored hot in a warehouse container, ask for a tape adhesion result after aging rather than on the day of manufacture, and require the seam sealing method to be named in the specification.

10 000-20 000 mm: Technical and Commercial

This is the band where waterproof becomes a defensible word. Products here include dry bags for paddling and marine use, expedition kit bags, camera and electronics protection, dive support bags and anything sold with an immersion claim. The fabric is a coated or laminated construction, commonly 500D to 840D with a TPU film or a heavy PU coating, and the seams are welded rather than sewn.

The technical reason is that immersion and strapped contact impose sustained pressure across a wide area. Ten thousand millimetres is roughly one bar, which is far more than the depth pressure in a typical immersion test, so the fabric is not the constraint at all. What is the constraint is the closure and the seam, which is why this band must always be specified together with a construction requirement rather than as a standalone number.

Within the band, the choice is mostly about cost and feel rather than about performance. A 500D TPU laminate at 10 000 mm is lighter and cheaper than an 840D at 20 000 mm and will pass the same immersion test. Buy the heavier fabric when the product will be dragged over rock, loaded with sharp gear, or sold with a durability story, and buy the lighter one when weight and price matter more.

This is the band our 50 L dry bag OEM programme is built around, with welded seams, a four-roll closure and a hydrostatic head certificate issued per fabric lot. Programmes here run at MOQ 500 pcs per style and colourway, sampling in 6 to 10 working days, bulk in 35 to 50 days, inspected to AQL 2.5 under ISO 2859-1, quoted FOB Xiamen on T/T 30/70.

• Specify the head figure and the construction together; neither is meaningful alone.
• Weld seams rather than sew them at this level; needle holes are the leak path.
• Qualify the closure separately, with an immersion test on the assembled article.
• Choose denier and coating for abrasion and feel, not for extra head.
• Issue a per-lot certificate tied to the roll number for traceability.

20 000 mm+: Where Extra Head Stops Paying

Above about 20 000 mm the fabric has long since stopped being the weakest link. At that level the material resists roughly two bar of standing pressure, while a roll-top closure, a zipper tape or a weld bead will fail at a small fraction of it. Paying for 30 000 mm or 40 000 mm laminate on a bag whose closure leaks is buying a property that will never be exercised, and it takes budget away from the component that would actually improve the product.

There are legitimate uses for very high figures. Specialist military and rescue programmes, products that are stored submerged for long periods, and designs where a heavy coating is needed anyway for abrasion or puncture resistance all justify it. In those cases the high head is a by-product of a specification driven by another property, not the target itself, and that is the healthy way to arrive at the number.

There is also a durability argument for heavier coatings in this region. Coating thickness contributes to abrasion life, so a laminate certified at 30 000 mm may survive being dragged across a gravel bank far better than one at 12 000 mm. If that is the reason you are buying it, specify abrasion resistance as the acceptance criterion and treat the head figure as supporting data, because abrasion is the property you actually care about.

The buyer's test for this band is a simple question: does anything in the product's service life generate the pressure that this fabric resists? For soft goods the answer is almost always no. If it is no, take the money out of the laminate and put it into the closure, the weld quality, the harness comfort or the packaging. That trade is where experienced sourcing teams find margin without losing performance. Our custom dry bag development work starts with exactly this question.

Coating Weight vs Measured Head

Coating weight, usually expressed in grams per square metre, is the manufacturing lever behind the head figure, but the relationship is not linear and not unlimited. A light PU coating of 20 to 40 grams per square metre will produce a low but usable head. Moving to 80 to 120 grams takes you into the technical band. Beyond that, additional coating adds weight, cost and stiffness while the measured head flattens out.

The reason is that penetration is governed by the weakest point in the coating film, not by its average thickness. Once the film is continuous and free of pinholes, extra thickness mostly protects against abrasion and puncture rather than against water entry. That is why a coated fabric can show a big gain in abrasion life from a heavier coating while showing a marginal gain in head.

TPU laminates behave differently from PU coatings in one important way: TPU is weldable by radio frequency, while PU coatings generally are not welded reliably. So choosing TPU is often a decision about seam construction rather than about head. If you want welded seams and an immersion rating, you are choosing TPU, and the head figure comes with it. See our TPU versus PVC comparison for how the material choice moves cost, feel and weldability.

There is also a weight consequence worth costing. Coating weight and fabric denier together determine the grams per square metre of the finished material, which drives both shipping weight and the perceived quality of the product. A bag that is 200 grams heavier than its competitor because of over-specified coating loses on freight, on shelf feel and on price, and gains nothing a customer can perceive.

Head After Abrasion and After Wash

The certified head figure describes new fabric on the day of manufacture. In service, two attacks degrade it: abrasion removes coating thickness locally, and laundering or exposure degrades finishes and can hydrolyse some coatings. A specification that only quotes the as-new figure is specifying a product that exists for one day, which is why retention clauses matter more than the headline number.

A workable requirement is to retest after a defined abrasion cycle, using a method appropriate to coated fabrics, and to accept a stated percentage of the original minimum. Seventy percent is a common commercial threshold. The same approach applies after laundering: run a defined number of cycles, dry, condition and retest. Products that cannot hold the threshold after twenty cycles should not carry a waterproof claim on the label.

Abrasion damage is also highly localised. A dry bag dragged across a gravel bank does not lose coating evenly; it loses it at the contact patches, and those patches then become the leak path. This is why we run abrasion testing on the finished construction and not only on flat fabric, and why our abrasion resistance programme reports the post-abrasion head rather than cycles alone.

1. Set the as-new acceptance minimum in millimetres under ISO 811 at a stated rate of rise.
2. Add an after-abrasion retest with an acceptance threshold, commonly 70 percent of the original minimum.
3. Add an after-laundering retest with the same logic and a defined number of cycles.
4. Test the finished construction, not only flat fabric, so seam and tape damage is included.
5. Store retained specimens from each lot so a disputed retention result can be rechecked later.

Creasing deserves a separate mention because it is not covered by abrasion or wash. A bag folded the same way every day develops a crease line where the coating cracks, and a cracked coating has almost no head along that line. If your product folds in service, specify a crease-flex cycle and a post-flex head figure, or choose a laminate with demonstrated flex life at the temperatures your market actually sees.

Using the Chart in a Sourcing Decision

Turn the chart into a decision by working backwards from the claim you intend to print. If the claim is water-repellent, the 1 000 to 3 000 mm band with a good finish is sufficient and the cheapest honest route. If it is water-resistant or rainproof, start at 5 000 mm and taped seams. If it is waterproof with an immersion rating, specify 10 000 to 20 000 mm with welded seams and a qualified closure, and commission the article test as well as the fabric test.

Then check the decision against cost. Head figure is only one of four cost drivers in a waterproof bag: fabric, closure, seam method and validation. In most programmes the closure and the validation cost more than moving up one head band, so if you are choosing where to spend, spend it on the closure and on per-lot verification rather than on extra millimetres you will never use.

Finally, write the whole decision into the specification as one block: the head figure with its method and rate, the retention thresholds after abrasion and laundering, the seam method, the closure design, and the article-level test that proves the assembly. Our technical specification sheet template puts those five fields together so that the buyer, the laboratory and the production line are all working from one document.

If you send us your intended claim, your target retail band and your annual volume, we will return the band we recommend, the construction that matches it, and the cost difference between the two nearest options. That comparison is the fastest way to see whether an extra band of head is worth its money in your specific programme, and it costs you one email to find out.

Frequently Asked Questions

Q1: What hydrostatic head rating do I need for a waterproof bag?

A: For a bag sold with a waterproof claim and an immersion rating, specify 10 000 to 20 000 mm measured under ISO 811, with welded seams and a closure qualified by an immersion test. For a commuter or school bag that only sees rain, 5 000 mm is a sensible commercial floor. The right band depends on contact load, not on how hard it rains.

Q2: Is 3 000 mm hydrostatic head waterproof?

A: It is water-resistant rather than waterproof in the sense most buyers mean. Around 3 000 mm handles moderate sustained rain with no contact pressure, which suits a fashion tote or a light daypack. It will not survive being strapped down, set in standing water or pressed against a wet surface, so it should not carry a waterproof claim.

Q3: What is the difference between 5 000 mm and 10 000 mm hydrostatic head?

A: Ten thousand millimetres resists twice the standing water pressure, which matters mainly when the fabric is pressed against a wet surface or strapped down. For rain alone the difference is rarely visible in service. Buy 10 000 mm when the product will be set down in water, strapped to a rack, or sold into a wet climate with an expectation of real waterproof performance.

Q4: Is 20 000 mm hydrostatic head overkill for a dry bag?

A: Usually yes, unless the heavier laminate is being bought for abrasion life rather than for head. At 20 000 mm the fabric resists roughly two bar, while the closure and seams fail far below that. Money spent above this point is normally better spent on the closure, on weld quality control or on per-lot verification.

Q5: Does a higher coating weight always mean a higher hydrostatic head?

A: Up to a point. Penetration is governed by the weakest place in the coating film, so once the film is continuous and free of pinholes, extra thickness adds abrasion and puncture life rather than water resistance. That is why heavier coatings often show a large gain in abrasion performance and only a marginal gain in measured head.

Q6: How much hydrostatic head is lost after abrasion?

A: It depends on the coating, the substrate and the abrasion method, which is exactly why the specification should require a measured result rather than a rule of thumb. A common commercial approach is to accept no less than 70 percent of the original minimum after a defined abrasion cycle, tested on the finished construction rather than on flat fabric.

Q7: Should I test hydrostatic head after washing?

A: Yes, if the product will be laundered in service or if the label carries a waterproof claim. Finishes degrade with washing and some coatings can be affected by detergents and heat. Define a number of cycles, dry and condition the specimen, then retest and apply a retention threshold so the figure describes the product mid-life rather than only on day one.

Q8: Can I compare a mill certificate with my own laboratory result?

A: Only if both use the same method, the same rate of rise, the same conditioning and the same end point convention. ISO 811 at 10 cm per minute and AATCC 127 at a faster rate will not agree, and a visual third-drop end point carries operator variability. Fix those fields in your specification to make the comparison meaningful.

Q9: What is the hydrostatic head of a TPU laminated fabric?

A: TPU film laminates used in technical dry bags commonly fall in the 10 000 to 20 000 mm band and above, depending on film thickness and substrate. The more important property for bag construction is that TPU can be radio-frequency welded, which removes the needle holes that limit sewn and taped seams. Choose TPU for the weldability as much as for the head.

Q10: Why does my fabric pass the head test but the bag still leaks?

A: Because the head test evaluates a flat clamped specimen and never sees the seam, the needle holes, the closure or the geometry. In a finished bag, water takes the easiest path, which is usually a stitch line, a lifted tape edge or a roll-top folded loosely. Test the assembled article with an immersion or spray test alongside the fabric test.

Q11: What head figure should a hiking backpack specify?

A: Between 5 000 and 10 000 mm for the body fabric, paired with a spray rating rather than an immersion rating. A hiking pack has a sewn harness and many stitch lines, so an immersion claim invites returns. Specify the head figure for the fabric and an IPX3 or IPX4 spray rating for the assembled pack, and be honest about the difference on the label.

Q12: How often should hydrostatic head be tested in production?

A: Test every incoming fabric lot, tied to the roll number, rather than relying on a single mill certificate issued months earlier. Coating weight varies across and along a roll, and coatings age in storage. Per-lot testing with retained specimens is the cheapest way to catch a drift before it reaches the cutting table and becomes a finished-goods problem.

Q13: Does hydrostatic head relate to an IP rating?

A: Not directly. ISO 811 measures a flat fabric specimen while IEC 60529 evaluates the assembled article in a tank or under a nozzle. A high head figure is necessary but not sufficient for an immersion rating. Use the chart to select the fabric band, then commission the article-level test to confirm the rating you intend to print.

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