Quick Definition
°dH stands for Grad Deutsche Härte. It is the primary unit of measurement for water hardness used in Germany, Austria, and Switzerland. One German degree is defined as 10 milligrams of calcium oxide (CaO) per liter of water.
The Math: Converting °dH to mg/L
Because most of Europe and the scientific community uses mg/L (milligrams per liter of Calcium Carbonate), you will often need to convert German degrees into mg/L.
1 °dH = 17.848 mg/L (CaCO₃)
If a washing machine manual tells you to set your internal water softener to match a local water hardness of 14 °dH, that is equivalent to 250 mg/L—which is classified as very hard water!
Why do appliance manufacturers use °dH?
Historically, Germany has been the manufacturing powerhouse of Europe for white goods (brands like Bosch, Miele, and Siemens). Because these machines were designed and engineered in Germany, their internal software, salt dosing dials, and user manuals default to the German scale (°dH), even when sold in the UK or France.