Understanding French degrees (°fH)

1 French degree (°fH or °f) = 10 mg of calcium carbonate (CaCO₃) per litre of water, i.e. 4 mg of calcium or 2.4 mg of magnesium. It is the official unit for measuring water hardness in Luxembourg, France, Belgium and several European countries.

The German degree (°dH or °d) is also widely used in Luxembourg. The conversion is simple: 1°dH ≈ 1.79°fH. Conversely: 1°fH ≈ 0.56°dH. For example, water at 25°fH is equivalent to about 14°dH.

Very soft water: 0–7°fH (little limescale, possible mild aggressiveness towards metal pipework). Soft water: 7–15°fH (ideal, little scaling). Moderately hard: 15–25°fH (light scaling, keep an eye on the water heater). Hard: 25–35°fH (significant scaling, water softener recommended). Very hard: >35°fH (rapid scaling, water softener essential).