Hardness unit conversion. mmol/L ppm, mg/L dGH, °dH gpg °e, °Clark °fH mmol/L 1 0.009991 0.1783 0.171 0.1424 0.09991 ppm, mg/L 100.1 1 17.85 17.12 14.25 10 dGH, °dH 5.608 0.05603 1 0.9591 0.7986 0.5603 gpg 5.847 0.05842 1.043 1 0.8327 0.5842 °e, °Clark 7.022 0.07016 1.252 1.201 1 0.7016 °fH 10.01 0.1 1.785 1.712 1.425 1 Example 1: 1 mmol/L = 100.1 ppm; Example 2: 1 ppm = 0.056 dGH.