If you've been researching tents, one of the specs you're likely to have seen is HH (Hydrostatic Head) rating. So what exactly is a HH rating on a tent, and what's a good number to look for? This figure is actually pretty key when it comes to finding the best tent for your camping trip (it's actually also a spec used for waterproof jackets, but we're focusing on tents here). Read on for an explainer.
What is HH rating?
HH stands for Hydrostatic Head, given in millimetres. It's a measure of how waterproof a fabric is, based on the water pressure it can withstand – the higher the number, the more waterproof the fabric. You'll also sometimes see it called a water column rating, because it refers to the height of a column of water that the fabric can hold without letting water through. So a HH of 5000mm means the tent fabric could hold a column of water that's 5000mm tall.
What's a good HH rating for a tent?
A good tent will have a HH rating of 3000-4000mm. Some of the best backpacking tents will have higher ratings, because they're designed to withstand more extreme weather conditions, while a basic popup tent might have a lower HH rating, and sacrifice reliable waterproofing for a lower price point. Here's a quick guide:
1500mm: the bare minimum to look for – anything less than this is designed for nothing but the lightest showers
2000-3000mm: on the lower end, but probably fine for typical UK weather
4000mm: this rating should keep you reliably dry in strong winds and rain
5000-6000mm: premium / extreme tents, suitable for torrential rain and use all year round
Where does denier come into it?
Almost all tents will have a Hydrostatic Head rating listed, but there might also be some other specs included that are relevant when it comes to fabric quality. Key amongst these is the denier (D). This is a measure of thickness in fabrics – a higher denier means a thicker fabric.
"After size and weight, the flysheet denier should be the next deciding factor. A high D rating is the marker of a high quality tent," says Nick Roberts, Brand Manager at Khyam. "Some brands use a combination of specifications in their ratings, such as HDE or DD, which can be confusing when you are comparing products. Ignore an HDE count, DD count or anything else that is not simply D or Denier and request the D rating from the retailer if it is not readily available within the specification."
A higher denier and thicker fibres mean the tent fabric is more robust and better at regulating temperature, so the interior will stay cooler inside in summer, and warmer inside in winter. Here's Nick's guide to the denier (D) ratings to look for:
68D to 75D: thin and lightweight, this is the kind of denier count typically used for small backpacking or festival tents
150D: thicker and more robust, this is the minimum denier you should aim for when buying a premium tent or an air ten
210D: the denier count you'll find on most premium tents, although in reality the difference is minimal compared to a 150D tent
What about thread count?
You might also see thread count (T) listed. This refers to the number of horizontal and vertical threads per square inch of fabric. So a higher thread count equals a tighter weave, but this doesn’t necessarily equate to a higher quality or more waterproof fabric. Nick suggests taking into account denier too, to be sure that the higher T count isn't due to lots of thin fibres.
Nick adds: "The use of thread count instead of denier within a product’s technical specification can be misleading, giving the impression that a product with a high thread count but low denier count is higher quality than it is in reality."