Water-hardness scale (gpg ↔ ppm)
Water hardness is measured in grains per gallon (gpg) or parts per million / mg/L (ppm) as calcium carbonate. The conversion is fixed: ppm ÷ 17.1 = gpg. The classification below (soft → very hard) is a widely used convention. Hardness sets how often a softener regenerates — size it with the softener sizing tool. Hardness is not a safety measure — test potability with a certified lab.
| Classification | Grains per gallon (gpg) | ppm (≈ gpg × 17.1) |
|---|---|---|
| Soft | 0.0–1.0 | 0–17 |
| Slightly Hard | 1.0–3.5 | 17–60 |
| Moderately Hard | 3.5–7.0 | 60–120 |
| Hard | 7.0–10.5 | 120–180 |
| Very Hard | 10.5–10.5+ | 180–180+ |
The ppm ÷ 17.1 = gpg conversion is fixed; the classification is a common convention. Hardness sets softener regeneration frequency — it is not a safety test. Test potability with a certified lab.