Water Hardness Conversion: PPM to GPG Calculator
Convert water hardness between PPM, GPG, mg/L, and mmol/L with explanation of each unit.
The conversion
1 grain per gallon (GPG) = 17.1 parts per million (PPM). PPM and mg/L are identical for water hardness purposes. To convert: GPG = PPM ÷ 17.1. PPM = GPG × 17.1. For example, 220 PPM ÷ 17.1 = 12.9 GPG.
Which unit to use when
PPM (parts per million): Used in municipal water reports, EPA standards, and most consumer-facing water quality data. GPG (grains per gallon): Used by the water softener industry for sizing calculations. Every softener is rated in grains of capacity. mg/L: Identical to PPM for practical purposes. Used in scientific and international contexts. mmol/L: International scientific unit. 1 mmol/L = 100 PPM.
Hardness scale
Soft: 0-60 PPM (0-3.5 GPG). Moderately Hard: 61-120 PPM (3.6-7.0 GPG). Hard: 121-180 PPM (7.1-10.5 GPG). Very Hard: 181+ PPM (10.6+ GPG). These classifications come from the US Geological Survey. Look up your city for both PPM and GPG values.