Water hardness is the total of dissolved calcium and magnesium. When heated, these minerals precipitate as limescale.
Units
- 1 °f (French) = 10 mg/L CaCO₃
- 1 °dH (German) = 1.79 °f
- 100 ppm CaCO₃ = 10 °f
Measuring
Drop test kits are the most practical field method; strips give a rough idea; laboratory analysis is precise. A TDS meter does not measure hardness.
When to soften
Above about 20 °f, homes see scale in boilers and appliances; boiler feed water should be near zero hardness. Use our softener calculator to size a system.
