What Are Grains Per Gallon (GPG)? Water Hardness Explained
Understanding the GPG measurement used in water softener sizing.
The definition
One grain per gallon (GPG) equals 17.1 milligrams per liter (mg/L) of calcium carbonate equivalent. It is the traditional US measurement for water hardness, still used universally in the water softener industry for equipment sizing. Municipal water reports typically use PPM or mg/L, which are equivalent to each other.
The hardness scale in GPG
0-1 GPG: Soft. 1-3.5 GPG: Slightly hard. 3.5-7 GPG: Moderately hard. 7-10.5 GPG: Hard. 10.5+ GPG: Very hard. Most softener recommendations begin at 7 GPG (120 PPM) where scale buildup becomes noticeable.
Why GPG matters for softeners
Every water softener is rated by grain capacity (e.g., 32,000 grains, 48,000 grains). This rating tells you how many grains of hardness the system can remove before needing to regenerate. See our sizing guide to calculate the right capacity for your household using your city's GPG value from CheckMyTap.