About the Water Hardness Calculator
This water hardness calculator converts a hardness reading between every common unit: ppm or mg/L as calcium carbonate, grains per US gallon (gpg), German degrees (°dH), French degrees (°fH), English or Clark degrees (°e), millimoles per litre and milliequivalents per litre. It can also calculate total hardness from separate calcium and magnesium results on a lab report, and it classifies the water using the US Geological Survey scale.
It is handy when setting a water softener (which usually asks for gpg), comparing a test strip in °dH with a utility report in mg/L, dosing aquarium or brewing water, or checking dishwasher and coffee machine settings. All conversions are exact ratios based on calcium carbonate, so results match what water labs and softener manufacturers use.
Total hardness here means calcium plus magnesium expressed as CaCO₃. Iron and other polyvalent ions contribute slightly in reality but are ignored, as in standard laboratory practice.
With the default inputs, the hardness (mg/l as caco₃ = ppm) is 171.2 ppm. Change any value above to recalculate instantly.
How to use the water hardness calculator
- 1Choose whether you have a hardness reading or separate calcium and magnesium values.
- 2Enter the reading and pick its unit, or enter Ca and Mg in mg/L.
- 3Read the hardness in every unit and the USGS classification.
- 4Use the gpg value for softener settings and °dH for many dishwashers and test kits.
Formula and method
All hardness units express the same quantity — dissolved calcium and magnesium — as an equivalent amount of calcium carbonate. One grain per US gallon is 64.79891 mg in 3.785 L, or 17.118 mg/L. One German degree is 10 mg/L of calcium oxide, equal to 17.848 mg/L of CaCO₃. One French degree is 10 mg/L CaCO₃, and one English degree is one grain per imperial gallon (14.254 mg/L). One millimole of CaCO₃ weighs 100.09 mg.
To build hardness from ion concentrations, each ion is converted to its CaCO₃ equivalent by the ratio of molar masses: 100.09 ÷ 40.08 = 2.497 for calcium and 100.09 ÷ 24.31 = 4.118 for magnesium. This is the calculation method in Standard Methods 2340 B.
- ppm
- Milligrams per litre expressed as CaCO₃
- gpg
- Grains per US gallon
- °dH, °fH, °e
- German, French and English degrees of hardness
- [Ca], [Mg]
- Calcium and magnesium concentrations, mg/L
Worked examples
10 grains per gallon to ppm
Ten grains per US gallon × 17.118 = 171.2 mg/L as CaCO₃, which the USGS classes as hard (121–180 mg/L). That equals about 9.6 °dH, 17.1 °fH or 1.71 mmol/L.
Test strip reading of 14 °dH
Fourteen German degrees × 17.848 = 249.9 mg/L as CaCO₃, about 14.6 gpg or 2.5 mmol/L — very hard water by the USGS scale.
Lab report with Ca 40 mg/L and Mg 12 mg/L
Calcium contributes 40 × 2.497 = 99.9 mg/L and magnesium 12 × 4.118 = 49.4 mg/L as CaCO₃, for a total hardness of about 149 mg/L (8.7 gpg), which is hard water.
Frequently asked questions
How do I convert gpg to ppm?+
Multiply grains per gallon by 17.1. For example 7 gpg × 17.118 ≈ 120 ppm (mg/L as CaCO₃). To go from ppm to gpg, divide by 17.1.
What level of water hardness is considered hard?+
The USGS classifies 0–60 mg/L as CaCO₃ as soft, 61–120 as moderately hard, 121–180 as hard and more than 180 mg/L as very hard. Softener makers often call anything above about 7 gpg hard.
What is a German degree of hardness (°dH)?+
One °dH corresponds to 10 mg of calcium oxide per litre, which is equivalent to 17.85 mg/L of calcium carbonate or 0.178 mmol/L. It is widely used on European test kits, dishwashers and aquarium products.
Is ppm the same as mg/L for water hardness?+
For dilute water samples, yes: one litre weighs about one kilogram, so 1 mg/L ≈ 1 part per million. Hardness in ppm is always stated as calcium carbonate equivalent, not as raw calcium.
Is hard water bad for health?+
Hard water is not considered a health risk and calcium and magnesium are dietary minerals. Its main drawbacks are limescale in pipes, kettles and heaters, spotting on dishes and reduced soap lathering.