Water Hardness Converter
The water hardness converter expresses hardness reported as calcium carbonate in milligrams per litre, grains per US gallon and millimoles per litre.
How to use this calculator
- Enter a non-negative hardness value expressed as calcium carbonate in mg/L.
- Check the units and definitions before comparing results with another source.
- Change one input at a time when comparing scenarios, and keep the reporting period and assumptions consistent.
Formula used by the water hardness converter calculator
gpg ≈ mg/L as CaCO₃ ÷ 17.118; mmol/L ≈ mg/L ÷ 100.0869
- gpg ≈ mg/L as CaCO₃ ÷ 17.118; mmol/L ≈ mg/L ÷ 100.0869
- This is a unit conversion, not an analysis of water quality. Regional classifications and treatment recommendations vary.
- Inputs are checked for required values and valid ranges; displayed results are rounded for readability.
Worked examples
Worked example
Water hardness is reported as 120 mg/L as CaCO₃.
That is about 7.01 grains per US gallon and 1.20 mmol/L as CaCO₃.
That is about 7.01 grains per US gallon and 1.20 mmol/L as CaCO₃.
Understanding the result
The water hardness converter expresses hardness reported as calcium carbonate in milligrams per litre, grains per US gallon and millimoles per litre.
This is a unit conversion, not an analysis of water quality. Regional classifications and treatment recommendations vary.
Important notes
This is a unit conversion, not an analysis of water quality. Regional classifications and treatment recommendations vary.
The result applies to the numbers and assumptions entered here; verify specifications for safety-critical or regulated uses.
Frequently asked questions
What does CaCO₃ mean here?
It is the equivalent reporting basis commonly used to express total hardness, not a claim that all hardness is physically calcium carbonate.
Does this determine water safety?
No. Hardness conversion does not test microbiological, chemical or other drinking-water parameters.
How do I use this calculator?
Enter a non-negative hardness value expressed as calcium carbonate in mg/L.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.