The hardness represents the amount of calcium and magnesium dissolved in the water.
 

Water hardness is generally expressed in French degrees (°f or °fH).
It is sometimes expressed in German degrees (°dH).

 1 French degree (°f or °fH) is equal to

• 4 mg/l of calcium
• 2.43 mg/l of magnesium
• 10 ppm or 10 mg/l of calcium carbonate
• 0.1 millimole per litre of calcium carbonate (mmol/l)
• 0.2 milliequivalent per litre of calcium carbonate (meq/l)
• 0.56 German degree (°dH)