Understanding French degrees (°fH)
How hardness is calculated, equivalences with the German °dH, and what the values really mean.
Definition
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 key takeaway: 1°fH = 10 mg of limescale per litre. Simply multiply the number of degrees by 10 to get the CaCO₃ concentration.
Conversion with the German °dH
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.
| °fH | °dH | Category |
|---|---|---|
| 7 | 3.9 | Very soft / soft |
| 15 | 8.4 | Soft / medium |
| 25 | 14.0 | Moderately hard |
| 35 | 19.6 | Hard / very hard |
| 45 | 25.2 | Very hard |
The thresholds to know
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).
Frequently asked questions
How do I convert °fH to °dH?
Multiply the value in °fH by 0.56 to get °dH. For example, 25°fH × 0.56 ≈ 14°dH.
At what hardness do I need a water softener?
Above 25°fH, scaling becomes significant and a softener is justified. Between 15 and 25°fH it remains optional, depending on your comfort and equipment.
Is water that is too soft a problem?
Very soft water (< 7°fH) can be mildly aggressive towards metal pipework, but it poses no health risk. It simply limits scaling.
Where can I find my water hardness in Luxembourg?
The value appears in the annual water quality report from your commune or supplier. You can also measure it with an over-the-counter test strip.
Does the French degree measure water quality?
No. It only measures the calcium and magnesium content (the limescale), not the potability or the presence of nitrates or pesticides, which require other analyses.