Water Hardness Calculator
Convert water hardness measurements between common units (ppm, gpg, mmol/L, dGH, Clark degrees) and classify your water hardness on the USGS scale. Enter a value in any unit to see all conversions and the hardness classification.
Water hardness is a measure of dissolved calcium and magnesium minerals, typically expressed as an equivalent concentration of calcium carbonate (CaCO₃). Hard water causes scale buildup in pipes and appliances, reduces soap effectiveness, and can affect taste.
Hardness Units:
- ppm (mg/L as CaCO₃): The most universal unit. 1 ppm = 1 mg of CaCO₃ per liter.
- Grains per Gallon (gpg): Common in the US water treatment industry. 1 gpg = 17.12 ppm.
- mmol/L: SI unit. 1 mmol/L CaCO₃ = 100.09 ppm.
- German Degrees (°dH): Common in Europe. 1 °dH = 17.848 ppm = 10 mg/L CaO.
- Clark Degrees: British system. 1 °Clark = 14.254 ppm = 1 grain per imperial gallon.
USGS Classification:
- Soft: 0–60 ppm (0–3.5 gpg)
- Moderately Hard: 60–120 ppm (3.5–7.0 gpg)
- Hard: 120–180 ppm (7.0–10.5 gpg)
- Very Hard: >180 ppm (>10.5 gpg)
Most municipal water supplies in the US range from 60 to 300 ppm. Well water can be significantly harder. Water softeners typically use ion exchange to replace calcium and magnesium with sodium.