The hardness of the water consists mainly of calcium and magnesium salts, expressed mostly in mmol / l or also in German degrees ° N. The conversion is 1 mmol / l = 5,6 ° N. According to the scale it may be water:

• very soft (up to 0,75 mmol / l)
• soft (0.75 to 1.5 mmol / l)
• medium hard (1.5 to 2.25 mmol / l)
• fairly hard (2.25 to 3 mmol / l)
• hard (3 to 4.5 mmol / l)