By David Robinson • Updated March 24, 2022

The hardness of water is defined by the amount of dissolved calcium and magnesium ions it contains. These polyvalent cations originate when water passes through calcareous formations like limestone, and they influence how the water reacts with detergents, soaps and other chemicals. Hard water has high cation levels, whereas soft water contains comparatively low concentrations. Cation concentrations are commonly expressed either as grains per gallon (GPG) or milligrams per liter (mg/L).

Step 1 – Identify the mg/L Value

Start by determining the water’s hardness in mg/L. Enter the figure into a calculator and double‑check that the number is correct.

Step 2 – Convert to GPG

Divide the mg/L figure by 17.2, the standard conversion factor. The result, rounded to one decimal place, gives the hardness in GPG.

Step 3 – Verify the Result

To confirm accuracy, multiply the GPG value by 17.1. If the product is not close to the original mg/L figure, redo the calculation.

TL;DR

mg/L × 17.1 ≈ GPG; GPG ÷ 17.2 ≈ mg/L.

Note: 1 mg/L equals 1 ppm because a kilogram of water contains one million milligrams.