USGS hardness classes
The U.S. Geological Survey classifies 0–60 mg/L as soft, 61–120 as moderately hard, 121–180 as hard and more than 180 mg/L as very hard. Hardness is normally reported as calcium carbonate (CaCO₃).
| mg/L as CaCO₃ | Approx. gpg | USGS class |
|---|---|---|
| 0–60 | 0–3.5 | Soft |
| 61–120 | 3.6–7.0 | Moderately hard |
| 121–180 | 7.1–10.5 | Hard |
| >180 | >10.5 | Very hard |
Convert ppm to grains
1 grain per gallon is approximately 17.1 mg/L as CaCO₃. So 171 mg/L is about 10 gpg.
What hardness does — and does not — tell you
Hardness is useful for softener sizing. It does not tell you whether water contains lead, PFAS, arsenic, nitrate, bacteria or other contaminants. Those require the relevant utility report or laboratory testing.
Next step
Run your numbers through the Water System Planner, then verify any product against its actual service-flow, pressure-drop, capacity and certification data.