What is water hardness?
Water hardness is the parameter used to measure the dissolved minerals — primarily calcium (Ca²⁺) and magnesium (Mg²⁺) — in water. Based on the value of water hardness, water is classified as soft, moderately hard, hard, or very hard. Hard water is common in areas where gypsum, limestone, and chalk are found in the ground. The hardness of water is measured in mg/L (milligrams per liter), ppm (parts per million), mmol/L (millimoles per liter), or gpg (grains per US gallon).
How to calculate the hardness of water
The formula to calculate water hardness (as mg/L of CaCO₃) is:
Hardness = 2.497 × Ca + 4.118 × Mg
where Ca is the calcium concentration and Mg is the magnesium concentration, both in mg/L.
The coefficients 2.497 and 4.118 are derived from the molar masses of calcium and magnesium relative to calcium carbonate (CaCO₃, molar mass 100.09 g/mol):
- Ca factor: 100.09 / 40.08 × 1 = 2.497 (molar mass of CaCO₃ / molar mass of Ca)
- Mg factor: 100.09 / 24.31 × 1 = 4.118 (molar mass of CaCO₃ / molar mass of Mg)
Water hardness classification
| Hardness (mg/L as CaCO₃) | Hardness (gpg) | Classification |
|---|---|---|
| 0 – 60 | 0 – 3.5 | Soft |
| 61 – 120 | 3.5 – 7.0 | Moderately Hard |
| 121 – 180 | 7.0 – 10.5 | Hard |
| > 180 | > 10.5 | Very Hard |
Unit systems
This calculator supports two measurement systems:
- Metric — enter calcium and magnesium concentrations in mg/L. Results are shown in mg/L (as CaCO₃), ppm, mmol/L, and gpg.
- American — enter concentrations in mg/US gallon (the calculator converts to mg/L internally using 1 US gal = 3.78541 L). Results display grains per gallon (gpg) as the primary unit.
Example: Using the water hardness calculator
Estimate the water hardness for a supply with a calcium concentration of 31 mg/L and a magnesium concentration of 27 mg/L:
Hardness = 2.497 × 31 + 4.118 × 27
= 77.407 + 111.186
= 188.59 mg/L as CaCO₃
Based on the hardness scale, this water is classified as Very Hard (hardness > 180 mg/L as CaCO₃).
Types of hardness
There are two types of water hardness — temporary and permanent:
- Temporary hardness — caused by dissolved bicarbonates such as calcium bicarbonate Ca(HCO₃)₂ and magnesium bicarbonate Mg(HCO₃)₂. It can be removed by boiling the water or adding lime (calcium hydroxide).
- Permanent hardness — caused by calcium sulfate (CaSO₄), calcium chloride (CaCl₂), magnesium sulfate (MgSO₄), or magnesium chloride (MgCl₂). It cannot be removed by boiling alone and requires an ion-exchange column or chemical treatment.
FAQs
Is hard water safe to drink?
Yes. Hard water is generally safe — calcium and magnesium are essential minerals with known health benefits. However, very hard water can cause scale buildup in pipes, boilers, kettles, and water heaters, reducing their efficiency and lifespan.
What units does the calculator use?
The calculator accepts calcium and magnesium concentrations in mg/L (metric) or mg/US gal (American). Results are always shown in mg/L as CaCO₃, ppm, mmol/L, and gpg so you can use whichever unit your water test kit reports.
What is grains per gallon (gpg)?
Grains per gallon (gpg) is the standard US unit for water hardness. One grain equals 64.8 mg, and 1 gpg corresponds to approximately 17.12 mg/L of CaCO₃. Most American home water-softener systems are rated and set in gpg.
How do I measure calcium and magnesium in my water?
Use a water test kit or digital photometer available at home-improvement stores or online. Many municipalities also publish annual water quality reports that include calcium and magnesium levels.