Hard Water Map of the United States
Where hard water is worst in the US. Regional breakdown by state and geology.
The hard water belt
The hardest municipal water in the US runs through the Great Plains and Southwest: Arizona, Nevada, Utah, New Mexico, Texas (western), Indiana, Ohio, Wisconsin, Minnesota, Iowa, Kansas, and Nebraska. This band follows the limestone and dolomite bedrock formations that dissolve calcium and magnesium into groundwater and surface water.
The soft water coasts
The Pacific Northwest (Washington, Oregon) has the softest municipal water thanks to volcanic geology and heavy rainfall. The Northeast (New England, New York) is also generally soft due to granite and metamorphic rock. The Southeast (Georgia, Carolinas) has moderately soft water from Piedmont geology, though Florida is a major exception with very hard aquifer water.
Why it varies
Water hardness is determined by geology, not by latitude or coast. Limestone dissolves easily, releasing calcium. Granite barely dissolves at all. Cities on river systems that cross limestone formations (Ohio River, Missouri River) have hard water. Cities fed by mountain snowmelt or protected watersheds (Portland, Seattle, Memphis) have soft water. Look up your city for exact numbers.