Water Hardness Converter

Water quality reports give hardness in mg/L (ppm) as CaCO₃, softener makers work in grains per gallon, and European appliance and aquarium guides use German or French degrees. Type the value in the unit you have: 250 ppm is 14.60451 gpg, 2.497827 mmol/L, 14.007172 °dH or 24.978518 °fH, and it falls in the Very Hard band.

The band is the converter's own five-step scale in grains per gallon: soft below 1 gpg (17.1 mg/L), slightly hard to 3.5 (60), moderately hard to 7 (120), hard to 10.5 (180) and very hard above. The USGS uses four bands in mg/L instead: soft to 60, moderately hard 61 to 120, hard 121 to 180 and very hard above 180. There is no federal drinking water standard for hardness; the bands describe scale and soap use, not safety. The band card appears only when the two units differ.

Every unit goes through mg/L as CaCO₃, and result = value × factor(from) ÷ factor(to), rounded to six decimals:

• 1 gpg (grain per US gallon) = 17.118 mg/L

• 1 Clark degree (grain per imperial gallon) = 14.254 mg/L

• 1 °dH (German degree, 10 mg CaO per litre) = 17.848 mg/L

• 1 °fH (French degree) = 10.0086 mg/L

• 1 mmol/L = 100.087 mg/L

The French degree is taken as 0.1 mmol/L, about 0.09% more than the round 10 mg/L some references use. Values must be between 0 and 10,000. The tool converts total hardness only: it cannot separate calcium from magnesium, and a TDS meter reading is not a hardness value, because it counts every dissolved solid.