Public data method
Water Hardness Methodology
WaterHardness.org combines public water system records, treated-water reports, USGS monitoring, and fallback estimates into local hardness reports. The source label is part of the result, not an afterthought.
Dataset updated March 9, 2026.
Calculation Workflow
Match ZIP codes to public water systems
ZIP pages are built from the public water systems matched to that ZIP code. When more than one system is present, the ZIP value is weighted toward larger known service populations when population data is available.
Prefer treated-water reports
Where direct treated-water data is available from state labs, Consumer Confidence Reports, or utility-reported sources, those values take priority over raw groundwater or regional estimates.
Normalize hardness units
Hardness is stored as PPM or mg/L as calcium carbonate. GPG values use the standard conversion of 17.1 PPM per 1 grain per gallon.
Label confidence and limitations
Every local page carries a source label so users can distinguish treated-water readings, USGS monitoring, county medians, state medians, and nearby estimates.
Hardness Classification
| Class | PPM as CaCO3 | GPG |
|---|---|---|
| Soft | 0-60 | 0-3.5 |
| Slightly hard | 61-120 | 3.6-7.0 |
| Moderately hard | 121-180 | 7.1-10.5 |
| Hard | 181-250 | 10.6-14.6 |
| Very hard | 251+ | 14.7+ |
Source Priority
The source hierarchy is designed to keep treated tap-water evidence ahead of broader environmental estimates.
- 1. Treated-water reports: state lab records, Consumer Confidence Reports, and utility published values.
- 2. USGS monitoring: direct water-quality observations when treated-water reports are not available.
- 3. Local fallback estimates: nearby grid cells, county medians, and state medians used only when stronger data is unavailable.
Important Limits
A ZIP code is not the same as a water utility service boundary. A city can contain multiple systems, and a home can be on a private well even when nearby homes use municipal water. Local plumbing, treatment, blending, and seasonal source changes can also change the actual tap reading.
Use the lookup as a public starting point. Test your tap water before sizing a softener or making equipment decisions.