Classification hardness in mg/L hardness in mmol/L hardness in dGH/°dH hardness in gpg hardness in ppm Soft 0–60 0–0.60 0-3.37 0-3.50 less than 60 Moderately hard 61–120 0.61–1.20 3.38-6.74 3.56-7.01 60-120 Hard 121–180 1.21–1.80 6.75–10.11 7.06-10.51 120-180 Very hard ≥ 181 ≥ 1.81 ≥ 10.12 ≥ 10.57 > 181