Water Hardness Calculator and Unit Converter
Convert water hardness between ppm (mg/L), grains per gallon, German degrees (dGH), and mmol/L.
Includes WHO soft, moderate, hard, and very hard classification scale.
Water hardness measures dissolved calcium and magnesium ions, expressed “as CaCO3” – a convention that allows direct comparison between calcium and magnesium contributions even though they have different molar masses. The most common unit in North America is ppm (parts per million), which for water equals mg/L as CaCO3.
Unit conversions (all relative to ppm as CaCO3):
- 1 grain per gallon (gpg) = 17.118 ppm. This is the unit used on US water softener packaging – a softener rated for 30,000 grains handles 30,000 grains of total hardness before regeneration.
- 1 German degree (dGH or dH) = 17.848 ppm. German degrees are based on calcium oxide rather than CaCO3, hence the slightly different factor. Common in European water quality reports and aquarium keeping.
- 1 mmol/L = 100.09 ppm. This is the SI unit, derived from the molar mass of CaCO3.
WHO hardness classification:
- 0-60 ppm: Soft
- 61-120 ppm: Moderately hard
- 121-180 ppm: Hard
- Above 180 ppm: Very hard
Aquarium note: most tropical freshwater fish prefer 50-150 ppm (3-9 dGH). African cichlids prefer harder water, 180-350 ppm. Soft water species (discus, cardinal tetras) want 10-80 ppm.
Hardness has no WHO health limit – very hard water is safe to drink and some studies link it to slightly lower cardiovascular risk. The problems are practical: scale buildup in pipes and appliances, reduced soap lathering, and spotted glassware.