A simple and very inexpensive GH/KH fish water test kit from your local pet store can give you a decent analytical outlook on your water. GH is general (or total) hardness, and KH is alkalinity. Both as measured with respect to (or as) CaCO3.
One dH unit is equal to 17.848 mg/L (ppm).
A first approximation of total (general) hardness for fresh water is that often (on average) it is found to have about 70% of its hardness contribution from Ca++ ions, and 30% from Mg++ ions.
Hardness is the measure of combined MgCO3 (Magnesium Carbonate), and CaCO3 (Calcium Carbonate). But they are conflated into CaCO3 only, and reported as such (I.E., Total Hardness as CaCO3).
The molecular weight of CaCO3 is 100.0869
The molecular weight of the Ca++ ion is 40.078
The molecular weight of the Mg++ ion is 24.305
Therefore, the Ca++ ion percent of CaCO3 by weight is 40.078/100.0869 = 0.400432, and the inverse of this is 2.4973
And likewise the Mg++ ion percent of CaCO3 by weight is 24.305/100.0869 = 0.242839, and the inverse of this is 4.11796
So:
Total Hardness (as CaCO3) = 2.4973(Ca++) + 4.11796(Mg++)
Lets say that your water is hypothetically at a GH of 10 per the test kit.
This means that in terms of mg/L your hardness is 178.5 mg/L.
70% x 178.5 = 125 mg/L of the hardness is (on average) due to calcium ions.
30% x 178.5 = 53.6 mg/L of the hardness is (on average) due to magnesium ions.
2.4973(Ca++) = 125
Ca++ = 50 mg/L (if the water is average in nature)
4.11796(Mg++) = 53.6
Mg++ = 13 mg/L (if the water is average in nature)