Quick Answer
Water hardness is the concentration of dissolved calcium and magnesium. It is measured in grains per gallon (gpg) or ppm/mg/L, with 1 gpg ≈ 17.1 ppm. General scale: under 1 gpg is soft, 1-3.5 moderately hard, 3.5-7 hard, over 7 very hard.
Problem
Hardness numbers appear on reports and water-softener marketing without a clear scale, making it hard to know where you stand.
Product Recommendation
A shower filter does not soften water. If hardness affects your whole home, research water softeners separately. For the shower specifically, ZOOSHEN filtration addresses chlorine/chloramine and sediment. See /water-science/hard-water-and-soap.