The unit “mg/L” stands for “milligrams per litre” and is commonly used to express the concentration of calcium carbonate (CaCO₃) in water. Here's how it relates to water hardness:

1 mg/L of CaCO₃ means 1 milligram of calcium carbonate in 1 litre of water. Water hardness is primarily caused by calcium (Ca²⁺) and magnesium (Mg²⁺) ions in water. To quantify hardness, water testing often measures the concentration of these ions and expresses it as an equivalent amount of calcium carbonate in mg/L.

Converting mg/L to other hardness units:

• Degrees of Hardness (°dH): Water hardness can also be expressed in degrees German hardness. 1 °dH is approximately equal to 17.8 mg/L of CaCO₃.
• Parts Per Million (ppm): For water hardness, mg/L is numerically equivalent to ppm. So, 1 mg/L is the same as 1 ppm.

Example: If water hardness is measured as 200 mg/L CaCO₃, it means the concentration of calcium carbonate is 200 milligrams per liter of water. This value indicates relatively hard water, as higher concentrations of CaCO₃ result in increased hardness.