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Water hardness explained: mg/L, grains per gallon and what the numbers mean
Hard water is water with a high amount of dissolved calcium and magnesium. [1] Those minerals are harmless to drink, but they leave limescale on glass, faucets and showerheads and build up inside water heaters and appliances.
How hardness is measured
Hardness is reported in two units. Laboratories and water quality reports use milligrams per liter (mg/L), the same as parts per million. Water treatment professionals often use grains per gallon (gpg). The scale below shows both. [2]
| Classification | mg/L (ppm) | Grains per gallon |
|---|---|---|
| Soft | 0–17 | 0–1.0 |
| Slightly hard | 17–60 | 1.0–3.5 |
| Moderately hard | 60–120 | 3.5–7.0 |
| Hard | 120–180 | 7.0–10.5 |
| Very hard | 180 and over | 10.5 and over |
How hardness is reduced
A softener uses ion exchange: calcium and magnesium are exchanged for sodium on a resin, and the resin is recharged with salt. [2] Softening adds sodium at about 8 mg/L for each grain per gallon of hardness. People on a restricted sodium diet should account for that and talk to their physician. [2]
In short
- Hardness = dissolved calcium and magnesium.
- Above 120 mg/L (7 gpg) is classified as hard.
- Ion exchange reduces hardness and adds a small amount of sodium.
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