Water will dissolve almost anything. In fact, tap water may contain dozens of chemicals in addition to water. Some of the dissolved material originates from the rocks in which the aquifer is located. Rocks such as limestone, chalk and calcareous sandstone add polyvalent positively charged ions of calcium and magnesium to the water. These cause "hardness," which is measured in parts per million (ppm) or grains per gallon (gpg). Converting ppm to gpg is simple, but requires a calculator.
Enter the water hardness value, in parts per million, into the calculator. Double-check the entry to ensure that it is accurate.
Divide the ppm hardness value by 17.1, the conversion factor for ppm to gpg. The result is the water hardness expressed in grains per gallon. For example, 180 ppm is equivalent to 10.526 gpg -- 180 / 17.1 = 10.526.
Round the result to one decimal place, the same degree of accuracy as the conversion factor, and state the units as gpg. For example, using the example in Step 2, write the result as 10.5 gpg.
TL;DR (Too Long; Didn't Read)
Parts per million may also be written as mg/l, or milligrams per liter.