It is possible to calculate the hardness of a water sample after determining Ca and Mg concentrations in water (SOP# 835). The method has a detection limit of 1 mg/L as CaCO3.
Method 314A (Hardness by Calculation) in Standard Methods for the Examination of Water and Wastewater, 16th Edition. Greenberg, A. E.; Trussell, R. R.; Clesceri, L. S.; eds. American Public Health Association; Washington DC; 1985. p. 209.