Water hardness can be expressed in many different units including French degrees, German degrees, Clark degree, grains per gallon, mg/L CaCO3 (calcium carbonate), and ppm (parts per million). General conversions are below:
1 ppm = 1 mg/L CaCO3
1 ppm = 0.058 grains/US gallon
1 ppm = 0.07 Clark degrees
1 ppm = 0.10 French degrees
1 ppm = 0.056 German degrees
1 French degree = 1 hydrotimetric degree
1 Clark degree = 1 grain / Imperial gallon as calcium carbonate
1 French degree = 1 part / 100,000 calcium carbonate
1 German degree = 1 part / 100,000 calcium oxide
1 grain/US gallon = 17.1 ppm
1 grain/US gallon = 1.20 Clark degrees
1 grain/US gallon = 1.71 French degrees
1 grain/US gallon = 0.958 German degrees