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What do water hardness numbers mean?
Hardness is mostly dissolved calcium and magnesium. Reports give it in three units that are easy to mix up.
The units
- mg/L as CaCO3: milligrams per liter, expressed as calcium carbonate. This is the usual unit in lab results and utility reports.
- ppm: parts per million. For water, EPA treats ppm as equivalent to mg/L.
- gpg: grains per gallon. Softener makers size equipment in grains. To convert, divide mg/L by about 17.1 (so 120 mg/L ≈ 7 gpg and 180 mg/L ≈ 10.5 gpg). EPA rounds the second one to 10 gpg.
The USGS scale
| mg/L as CaCO3 | ≈ gpg | USGS class |
|---|---|---|
| 0–60 | 0–3.5 | Soft |
| 61–120 | 3.6–7 | Moderately hard |
| 121–180 | 7.1–10.5 | Hard |
| >180 | >10.5 | Very hard |
EPA's softening thresholds
EPA WaterSense says water above 120 mg/L (7 gpg) “is generally considered hard and may benefit from softening,” and that softeners “are often recommended when hardness exceeds 180 mg/L or 10 gpg.”
Is hard water a health problem?
USGS describes hardness as “not a health concern, but it can be a nuisance.” The problems are scale in plumbing and water heaters, spots on glassware, and soap that doesn't lather or rinse well. USGS also notes the World Health Organization's view that drinking water can supply some dietary calcium and magnesium.
Your number, specifically
USGS warns that its national maps aren't meant for decisions about an individual home. Use your utility's report or a test of your own tap. See how to read your water report, then use the decision guide.
Sources
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