A Hydrostatic Head (HH) rating in ultralight tents is a measure of how waterproof the fabric is.
It tells you how much water pressure (in millimetres of water) the material can withstand before it starts to leak. For example, a rating of 3,000mm HH means a 3-metre column of water would need to press on the fabric before water begins to pass through.
In practical terms:
- 1,000–2,000mm HH = light rain protection
- 2,000–5,000mm HH = solid all-weather camping protection
- 5,000mm+ HH = heavy rain, alpine, and extended storm conditions
For ultralight tents, higher HH usually means better wet-weather protection, but it can also add a bit of weight and reduce breathability depending on the fabric and coatings used.
In short, the higher the Hydrostatic Head rating, the better your tent will hold up in sustained rain and harsh backcountry conditions.