The Water Hardness Converter translates hardness stated in grains per gallon into parts per million by multiplying by 17.1 — ppm = GPG × 17.1 — and reports where the result falls on the standard hardness classification scale. It bridges the gap between the unit printed on US water-softener ratings and utility reports (GPG) and the unit used in laboratory analysis and most technical literature (ppm).
Grains per gallon (GPG) is a US customary unit: one grain of calcium-carbonate-equivalent hardness dissolved in one US gallon of water. Parts per million as CaCO₃ expresses the same hardness as an equivalent concentration of calcium carbonate in milligrams per liter. In both cases the number represents the combined effect of dissolved calcium and magnesium ions, the minerals responsible for scale deposits and reduced soap performance.
The conversion is used when sizing and programming water softeners, comparing a municipal water-quality report against lab results, and judging whether supply water will scale boilers, kettles, and plumbing. The accompanying classification follows the USGS scale: water is soft at 0–60 ppm, moderately hard at 61–120 ppm, hard at 121–180 ppm, and very hard above 180 ppm as CaCO₃.