Water Hardness Conversion Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
To convert a water hardness value, it is reduced to a common unit (ppm of CaCO₃) then converted back into the new unit. Water at 25 French degrees (°f), considered moderately hard, represents 250 ppm of CaCO₃.
Explanation
Water hardness, which reflects its calcium and magnesium ion content (responsible for limescale), is expressed on different scales depending on the country, complicating the comparison of a water analysis or an appliance manual from abroad. The French degree (°f), used in France, corresponds directly to 10 mg per liter of calcium carbonate (CaCO₃) — the reference unit for this calculator. The German degree (°dH), used in Germany and Central Europe, is historically defined from calcium oxide (CaO) rather than the carbonate, giving a different factor (1 °dH = 17.848 mg/L of CaCO₃), due to the distinct molar mass of these two chemical compounds. Ppm (parts per million), equivalent to mg per liter for a dilute aqueous solution, serves as a direct international reference in CaCO₃, while the US grain per gallon (gpg) remains the unit used in the United States, notably on water softeners and their technical manuals (1 gpg = 17.118 mg/L of CaCO₃) — a concentration of the same nature as the one handled by our molar concentration calculator, reduced here to a scale specific to water hardness rather than a general chemistry unit. In France, water is generally described as soft below 15 °f, moderately hard between 15 and 25 °f, hard between 25 and 42 °f, and very hard beyond 42 °f — indicative benchmarks that vary slightly by source and use (drinking water, water heater, washing machine). This dissolved mineral content also has a generally small influence on water's density compared with pure water. If your water softener's manual is in gpg, our water softener salt dosage calculator picks up directly from a known hardness to help plan regenerations.
Example: 25 French degrees in ppm of CaCO₃
Inputs
Value: 25. Starting unit: French degree (°f). Target unit: ppm of CaCO₃.
Calculation
25 °f × 10 (French degree factor) = 250 ppm of CaCO₃.
Result
This water, at 25 °f, contains the equivalent of 250 ppm of calcium carbonate.
Frequently asked questions
Why don't the French degree and the German degree share the same conversion factor?
Because they are historically defined from two different calcium compounds: the French degree is based directly on calcium carbonate (CaCO₃), while the German degree is based on calcium oxide (CaO), which has a different molar mass. This difference in chemical definition, not an arbitrary scale difference, explains the gap between the two conversion factors.
What does "hard" water actually mean?
Hard water contains a high concentration of calcium and magnesium ions, which promotes limescale buildup in pipes, water heaters, and appliances, and reduces the sudsing efficiency of soaps and detergents. It isn't harmful to health and is sometimes even valued for its mineral content, but it has concrete practical and maintenance consequences for household equipment.
Why does my US water softener manual use a different unit than my French water analysis?
Because equipment manufacturers generally follow the hardness convention used in their home country: American appliances use grains per gallon (gpg), while French water analyses use French degrees (°f). This calculator lets you match the two scales directly without manual recalculation.