Water Hardness Converter

Convert hardness between mg/L (ppm as CaCO₃) and grains per gallon (gpg), then see the USGS hardness class.

Last reviewed September 12, 2026 · Methodology

Planning estimate — not a water-safety determination. Verify health-related concerns with a certified lab/utility and verify final equipment, plumbing, certification and code requirements before purchase or installation. Safety & legal limits.

Conversion used

USGS field documentation uses 1 grain per gallon as equivalent to 17.1 mg/L of hardness as calcium carbonate. The USGS hardness classes are soft (0–60 mg/L), moderately hard (61–120), hard (121–180) and very hard (over 180).

Method note. These calculators are for planning. Product selection still needs manufacturer service-flow, pressure-drop, certified performance, plumbing and local-code checks.