Water Hardness to PPM

Methodology & Conversion Standards

This tool reverse-engineers international water hardness measurements, such as German Degrees (°dH), French Degrees (°fH), and Millimoles (mmol/L), back into the universal scientific baseline of Parts Per Million (PPM) or milligrams per liter (mg/L).

The Chemistry Behind the Conversions

Understanding why these specific multipliers exist requires looking at the chemical compounds they represent:

• PPM (mg/L): Expressed as a Calcium Carbonate ($CaCO_3$) equivalent.
• Grains per Gallon (GPG): Predominantly used in the USA. Multiplier to PPM is 17.118.
• German Degrees (°dH or dGH): Expressed as 10 mg/L of Calcium Oxide ($CaO$). Because the molar mass of $CaCO_3$ is roughly 1.78 times that of $CaO$, the conversion factor to PPM is 17.848.
• French Degrees (°fH): Defined directly as 10 mg/L of $CaCO_3$, making the conversion a simple factor of 10.
• Millimoles (mmol/L): Based on the molar mass of $CaCO_3$ (100.086 g/mol).

The Mathematical Formulas

Depending on the starting unit selected, the total PPM is derived using the following constants:

USGS Water Hardness Scale

Once the baseline PPM is calculated, result is categorized using the official United States Geological Survey (USGS) classification: