The Number on the Label — and What It Actually Tells You
If you've ever shopped for a tent, you've seen numbers like 3,000mm, 5,000mm, or 10,000mm listed under waterproofing specifications. These are hydrostatic head (HH) ratings — the standard measure of fabric waterproofing used across the outdoor industry. But what do these numbers actually mean? How are they measured? And how much HH do you actually need? This guide answers all of it.
What Is Hydrostatic Head?
Hydrostatic head is a measure of how much water pressure a fabric can withstand before water begins to pass through it. It's expressed in millimetres (mm) of water column — the height of a column of water that the fabric can support without leaking.
The test is straightforward: a fabric sample is clamped horizontally, and a tube of water is placed on top of it. The tube is filled with water until water begins to pass through the fabric. The height of the water column at that point — measured in millimetres — is the hydrostatic head rating.
A fabric rated 3,000mm HH can support a column of water 3 metres tall before leaking. A fabric rated 10,000mm HH can support a 10-metre column. The higher the number, the more water pressure the fabric resists.
The Test Standard
Hydrostatic head testing is governed by international standards, most commonly:
- ISO 811 — The international standard for hydrostatic pressure testing of fabrics. Water pressure is applied at a controlled rate (10cm H₂O per minute) and the pressure at which water penetrates three points on the fabric is recorded.
- EN 20811 — The European equivalent of ISO 811, producing comparable results.
- JIS L 1092 — The Japanese Industrial Standard, used by some Asian manufacturers. Results are broadly comparable to ISO 811.
It's worth noting that not all manufacturers test to the same standard or use independent testing laboratories. Self-certified HH ratings should be treated with more scepticism than independently verified ratings.
What the Numbers Mean in Practice
The HH rating tells you the static water pressure a fabric can withstand — but real-world rain creates dynamic conditions that are more demanding than a static laboratory test. Here's a practical guide to what different ratings mean in the field:
Under 1,500mm — Not Waterproof
Fabrics below 1,500mm HH are not considered waterproof for camping purposes. They may resist light drizzle briefly but will leak in any sustained rain. Found only in the cheapest budget tents and should be avoided for any serious camping use.
1,500–2,000mm — Shower Resistant
Adequate for very light, brief rain in calm conditions. Will leak in moderate rain, particularly under contact pressure (leaning against the tent wall, sleeping bag touching the fabric). Not suitable for camping in regions with reliable rainfall.
2,000–3,000mm — Light Rain
The minimum rating for a tent that can be described as genuinely waterproof in normal camping conditions. Suitable for occasional camping in mild climates with light to moderate rainfall. Will struggle in heavy or sustained rain, particularly as the coating ages.
3,000–5,000mm — Good All-Round Waterproofing
The sweet spot for most camping applications. A 3,000–5,000mm flysheet handles moderate to heavy rain reliably in new condition and maintains adequate performance as the coating ages. This is the minimum rating to look for in a quality camping tent.
5,000–10,000mm — Serious Waterproofing
Suitable for exposed camping, extended trips, and regions with heavy rainfall. Provides a meaningful performance margin above the minimum required, which is important as coatings degrade over time. The rating range for premium camping tents.
10,000mm+ — Expedition Grade
The highest ratings are found in expedition and mountaineering tents designed for extreme conditions — sustained heavy rain, driving sleet, and snow. For most camping applications, ratings above 10,000mm provide diminishing practical returns, though they do indicate high-quality fabric construction.
Different Parts of the Tent, Different Ratings
A complete tent has multiple fabric components, each with its own HH rating. Understanding which rating applies where is important:
- Flysheet — The outer waterproof layer. This is the primary weather barrier and the rating most commonly quoted in marketing. Look for 3,000mm minimum; 5,000mm+ for serious use.
- Groundsheet — The floor of the tent. Groundsheets face different stresses than flysheets — abrasion, puncture risk, and the weight of occupants pressing fabric into puddles. Groundsheet ratings should be higher than flysheet ratings: 5,000mm minimum, 10,000mm+ for quality tents.
- Inner tent — The inner tent is typically not waterproof — it's designed to be breathable to manage condensation. The inner tent relies on the flysheet for weather protection.
What HH Ratings Don't Tell You
Hydrostatic head is a useful but incomplete measure of tent waterproofing. Several critical factors are not captured by the HH rating alone:
Seam Sealing
A flysheet with a 10,000mm HH rating will still leak through unsealed seams. Every needle hole in a stitched seam bypasses the waterproof coating entirely. Seam sealing quality — whether seams are taped, welded, or sealed with compound — is as important as the fabric HH rating for real-world waterproofing performance.
Coating Durability
HH ratings are measured on new fabric. Waterproof coatings — particularly polyurethane (PU) coatings — degrade with UV exposure, repeated packing, and age. A tent rated 5,000mm when new may perform at 2,000–3,000mm after several seasons of use. Silicone coatings degrade more slowly than PU coatings and maintain their performance better over time.
DWR (Durable Water Repellent) Treatment
Most tent flysheets have a DWR surface treatment that causes water to bead and roll off rather than soaking into the fabric. DWR is not the same as the waterproof coating — it's a surface treatment that works in conjunction with it. When DWR degrades (which it does with use and washing), water no longer beads off the fabric surface. The fabric becomes saturated — a condition called "wetting out" — which adds weight and reduces breathability, though the underlying waterproof coating may still be intact. Re-applying DWR treatment restores beading performance.
Contact Pressure
The HH test applies pressure uniformly across a fabric sample with no contact on the other side. In real use, contact pressure — a sleeping bag touching the tent wall, a knee pressing against the fabric — concentrates pressure at a point and can cause leaking at ratings that would otherwise be adequate. This is why groundsheet ratings should be significantly higher than flysheet ratings, and why touching the inner tent wall during rain is inadvisable.
Fabric Construction
Two fabrics with identical HH ratings can have very different real-world performance if their base fabric construction differs. A tightly woven, high-denier base fabric with a 5,000mm coating will outperform a loosely woven, low-denier fabric with the same coating in abrasion resistance, tear strength, and long-term coating adhesion.
How to Use HH Ratings When Buying
Apply these principles when evaluating tent waterproofing specifications:
- Check both flysheet and groundsheet ratings — A tent that quotes only one rating is likely hiding a weakness in the other
- Look for seam sealing information — "Fully taped seams" is the gold standard; "critically taped" or no information is a red flag
- Consider the coating type — Silicone coatings (silpoly, silnylon) are more durable than PU coatings at equivalent HH ratings
- Factor in denier — Higher denier fabrics (70D, 150D) are more durable than lower denier fabrics (20D, 40D) at equivalent HH ratings
- Be sceptical of very high ratings on budget tents — A 10,000mm rating on a £50 tent almost certainly reflects poor-quality coating on poor-quality fabric that will degrade rapidly
Maintaining Your Tent's Waterproof Performance
Preserving HH performance over time requires simple but consistent maintenance:
- Always dry your tent completely before packing away — storing damp accelerates PU coating hydrolysis
- Store loosely in a cool, dry place away from UV exposure
- Re-apply DWR treatment when water no longer beads on the flysheet surface
- Re-seal seams annually or when tape begins to lift
- Clean with specialist tent cleaner rather than household detergents, which strip DWR and degrade coatings
The Right Rating for Your Camping
At Bestyle Camping Store, our inflatable tents are specified with flysheet and groundsheet HH ratings appropriate for serious camping use — not minimum-viable ratings chosen to hit a price point. Combined with fully taped seams and quality coating construction, our shelters deliver the waterproof performance the ratings promise, season after season. Browse our range and camp dry.