Water Hardness Calculator (ppm CaCO₃, °dH, gpg)
By Santaji GadepatilinUpdated November 4, 2026
Water hardness comes mainly from dissolved calcium and magnesium, and is reported as the equivalent amount of calcium carbonate. This water hardness calculator converts Ca and Mg results from a lab report into mg/L (ppm) as CaCO₃, German and French degrees and grains per gallon, and rates the water from soft to very hard.
How to use the water hardness calculator
Enter the calcium and magnesium concentrations in mg/L from your water test. The calculator returns total hardness as mg/L CaCO₃ (the same as ppm in dilute water) with a soft-to-very-hard rating, plus German degrees, French degrees, grains per gallon and mmol/L. If you only have a total hardness result, clear Ca and Mg and enter it directly to convert between units.
Water hardness formula
Hardness (mg/L CaCO₃) = 2.497 × Ca + 4.118 × Mg
°dH = ppm ÷ 17.848 · °fH = ppm ÷ 10 · gpg = ppm ÷ 17.118 · mmol/L = ppm ÷ 100.09
Example
A well water with 60 mg/L calcium and 15 mg/L magnesium:
- From calcium: 2.497 × 60 = 149.8 mg/L
- From magnesium: 4.118 × 15 = 61.8 mg/L
- Total hardness = 211.6 mg/L as CaCO₃ — very hard
- = 11.86 °dH = 21.16 °fH = 12.36 gpg
Hardness classification
| Hardness as CaCO₃ | Rating | German degrees | Grains per gallon |
|---|---|---|---|
| 0–60 mg/L | Soft | 0.0–3.4 °dH | 0.0–3.5 gpg |
| 61–120 mg/L | Moderately hard | 3.4–6.7 °dH | 3.6–7.0 gpg |
| 121–180 mg/L | Hard | 6.8–10.1 °dH | 7.1–10.5 gpg |
| 181–… mg/L | Very hard | 10.1+ °dH | 10.6+ gpg |
Hardness for typical Ca and Mg levels
| Calcium | Magnesium | Total hardness | German degrees |
|---|---|---|---|
| 10 mg/L | 2 mg/L | 33 mg/L | 1.9 °dH |
| 20 mg/L | 5 mg/L | 71 mg/L | 4.0 °dH |
| 40 mg/L | 10 mg/L | 141 mg/L | 7.9 °dH |
| 60 mg/L | 15 mg/L | 212 mg/L | 11.9 °dH |
| 80 mg/L | 20 mg/L | 282 mg/L | 15.8 °dH |
| 100 mg/L | 30 mg/L | 373 mg/L | 20.9 °dH |
| 150 mg/L | 40 mg/L | 539 mg/L | 30.2 °dH |
Notes
- Lab titrations with EDTA report total hardness directly as mg/L CaCO₃.
- To convert between mg/L and other concentration units, use the ppm calculator.
Frequently asked questions
How is total hardness calculated from calcium and magnesium?
Hardness (mg/L as CaCO₃) = 2.497 × Ca + 4.118 × Mg, with Ca and Mg in mg/L. The factors are the molar mass of CaCO₃ (100.09) divided by the molar masses of Ca (40.08) and Mg (24.31).
What is a good water hardness?
Below 60 mg/L is soft, 61–120 moderately hard, 121–180 hard and above 180 very hard (USGS classification). Hardness is not a health concern, but above about 120 mg/L scale builds up in kettles, boilers and pipes.
How do I convert ppm hardness to °dH or grains per gallon?
1 °dH = 17.848 mg/L CaCO₃, 1 °fH = 10 mg/L CaCO₃ and 1 grain per US gallon = 17.118 mg/L CaCO₃. Divide the ppm value by these factors.
What is the difference between temporary and permanent hardness?
Temporary (carbonate) hardness is the part balanced by bicarbonate and is removed by boiling, which precipitates CaCO₃ scale. Permanent hardness is balanced by sulfate or chloride and stays after boiling; it needs ion exchange or reverse osmosis.