Water Hardness Table of Classifications
| grains per gallon (gpg) | milligrams per liter (mg/L) parts per million (ppm) | U.S. Geological Survey Classification | Water Quality Association Classification |
|---|---|---|---|
| < 1.0 | < 17.1 | soft | soft |
| 1 to 3.5 | 17.1 to 60 | soft | slightly hard |
| 3.5 to 7.0 | 60 – 120 | moderately hard | moderately hard |
| 7.0 to 10.5 | 120 – 180 | hard | hard |
| 10.5 + | 180 and over | very hard | very hard |
Table adapted from Water Treatment Fundamentals, prepared and distributed by the Water Quality Association
U.S. Geological Survey Water Hardness Explanation | Water Quality Association Scale Desposits from Hard Water
Water Hardness Map of the United States
Table adapted from and prepared by the United States Geological Survey
