Water hardness converter
Hardness & home planning
Change the examples to match your water and system. Results update here in your browser.
Enter total hardness expressed as calcium carbonate, choose its unit, and read the equivalent values above. Do not enter total dissolved solids, alkalinity, or an electrical conductivity reading. If you have only a TDS meter, follow the hardness testing guide.
Conversion method
1 grain per US gallon ≈ 17.1 mg/L. Multiply gpg by 17.1 to obtain mg/L; divide mg/L by 17.1 to obtain gpg. The factor is rounded for household planning; a USGS report gives 17.12.
For dilute freshwater, ppm and mg/L are approximately equal. This approximation depends on water density; it is not intended for concentrated brine. See USGS water-quality units.
Read the category, then size
The USGS categories are soft through 60 mg/L, moderately hard through 120, hard through 180, and very hard above 180. The tool treats decimal results continuously: 60.5 is moderately hard. Classification uses the unrounded result, so a display rounded to a boundary can still fall just above it.
A hardness category does not select equipment. Continue to the size calculator with your daily treated-water demand and check service flow separately.