Hardness can be quantified by instrumental analysis. The total water hardness is the sum of the molar concentrations of Ca and Mg, in mol/L or mmol/L units. Although water hardness usually measures only the total concentrations of calcium and magnesium (the two most prevalent divalent metal ions), iron, aluminium, and manganese are also present at elevated levels in some locations. The presence of iron characteristically confers a brownish (rust-like) colour to the calcification, instead of white (the colour of most of the other compounds).
Water hardness is often not expressed as a molar concentration, but rather in various units, such as degrees of general hardness (dGH), German degrees (°dH), parts per million (ppm, mg/L, or American degrees), grains per gallon (mpg), English degrees (°e, e, or °Clark), or French degrees (°fH, °f or °HF; lowercase f is used to prevent confusion with degrees Fahrenheit). The table below shows conversion factors between the various units.
The various alternative units represent an equivalent mass of calcium oxide (CaO) or calcium carbonate (CaCO) that, when dissolved in a unit volume of pure water, would result in the same total molar concentration of Mg and Ca. The different conversion factors arise from the fact that equivalent masses of calcium oxide and calcium carbonates differ and that different mass and volume units are used. The units are as follows:
* Parts per million (ppm) is usually defined as 1 mg/L CaCO (the definition used below). It is equivalent to mg/L without chemical compound specified, and to American degree.
* Grain per gallon (gpg) is defined as 1 grain (64.8 mg) of calcium carbonate per U.S. gallon (3.79 litres), or 17.118 ppm.
* a mmol/L is equivalent to 100.09 mg/L CaCO or 40.08 mg/L Ca.
* A degree of General Hardness (dGH or German degree (°dH, deutsche Härte)) is defined as 10 mg/L CaO or 17.848 ppm.
* A Clark degree (°Clark) or English degrees (°e or e) is defined as one grain (64.8  mg) of CaCO per Imperial gallon (4.55 litres) of water, equivalent to 14.254 ppm.
* A French degree (°fH or °f) is defined as 10 mg/L CaCO, equivalent to 10 ppm.