Water Hardness Converter

Water hardness conversions use ppm (mg/L CaCO3) as the base unit.

Unit definitions (all relative to ppm CaCO3):

• 1 ppm = 1 mg/L CaCO3 (these are identical)
• 1 gpg (grains per gallon) = 17.118 ppm
• 1 dGH (German degrees) = 17.848 ppm
• 1 mmol/L = 100.09 ppm (molar mass of CaCO3)
• 1 Clark degree (English degrees) = 14.254 ppm
• 1 French degree (°fH) = 10 ppm

Water hardness scale (ppm):

• Soft: 0-60 ppm
• Moderately hard: 61-120 ppm
• Hard: 121-180 ppm
• Very hard: 180+ ppm

Common uses:

• Aquariums: dGH is the standard measure
• US water reports: ppm or gpg
• European water reports: dGH or French degrees
• Water softeners: gpg is most common in the US

Water hardness matters because dissolved calcium and magnesium cause the everyday annoyances of scale and poor lathering. Hard water leaves limescale in kettles and pipes, spots on glassware, and makes soap harder to foam, which is why people in hard-water areas reach for more detergent and descaler.

The unit you meet depends on where you are and what you’re buying. US water reports use ppm or grains per gallon, water softeners are almost always rated in grains (which is why converting ppm to gpg is the usual reason to be here), and parts of Europe use German (°dH) or French (°fH) degrees. To size a softener you multiply your hardness in grains by your daily water use, so getting the conversion right directly affects how often the unit regenerates. One reassurance: hard water isn’t a health problem, and the minerals are harmless to drink. It’s a nuisance and an appliance issue, not a safety one.