Water Hardness Calculator — ppm, gpg, °dH, °fH Converter
Enter your water hardness in any unit and convert instantly to all other common scales — mg/L (ppm), grains per gallon, °dH, °fH, °e and mmol/L — with WHO/EPA classification as soft, moderately hard, hard or very hard.
Unit
Hard
- 1
Conversion factor to ppm
1 ppm = 1 ppm = 1 - 2
Hardness in ppm (mg/L as CaCO₃)
150 × 1 = 150
How does this calculator work?
Water hardness is measured as mg/L CaCO₃ (= ppm). One grain per gallon = 17.12 ppm; one °dH = 17.85 ppm; one °fH = 10 ppm. WHO classification: 0–60 soft, 61–120 moderately hard, 121–180 hard, > 180 very hard. Enter any unit and all others are calculated automatically.
Formula
How this is calculated
Water hardness measures the total concentration of dissolved calcium and magnesium ions, expressed as an equivalent mass of calcium carbonate (CaCO₃) per litre. The reference unit is milligrams per litre (mg/L), which equals parts per million (ppm) at the densities typical of drinking water. All other hardness scales are simple multiples of this baseline: one US grain per gallon (gpg) = 17.118 mg/L; one German degree (°dH) = 17.848 mg/L; one French degree (°fH) = 10 mg/L; one English or Clark degree (°e) = 14.254 mg/L; one millimole per litre = 100.09 mg/L (the molar mass of CaCO₃).
The WHO and US EPA classify drinking-water hardness by the mg/L value: 0–60 is soft, 61–120 is moderately hard, 121–180 is hard, and above 180 is very hard. Hard and very hard water causes limescale deposits in kettles, pipes and water heaters but carries no established health risk — the WHO does not set a health-based guideline value for hardness. Soft water can taste flat and may be slightly more corrosive to metal plumbing.
This calculator converts any one input unit to all others in a single step. The conversions assume hardness is already expressed as equivalent CaCO₃, which is standard for "total hardness" measurements. Direct calcium or magnesium ion concentrations in mg/L require separate conversion by their atomic weights (Ca: 40.08, Mg: 24.31) before applying these factors.
Frequently asked questions
Temporary hardness is due to dissolved calcium (and magnesium) bicarbonate. Boiling decomposes bicarbonate into insoluble carbonate, which precipitates as limescale — removing the temporary hardness. Permanent hardness comes from calcium and magnesium sulfates and chlorides, which do not precipitate on boiling and require ion-exchange softening or chemical treatment.
Yes. All hardness levels up to around 500 mg/L are generally safe. Hard water may taste slightly bitter and causes scale on appliances; very soft water can taste flat and be marginally more aggressive to copper piping. Calcium and magnesium from hard water can even contribute positively to dietary mineral intake.
Dip-and-read test strips (0–200 ppm range) give a quick colour-match result. Drop-count titration kits are more precise. Electronic TDS meters measure total dissolved solids, not hardness specifically, so they are less reliable for this purpose. Labs and water utilities use EDTA titration for certified readings.
Also known as
TG we-Calculate Editorial Team. (2026). Water Hardness Calculator — ppm, gpg, °dH, °fH Converter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/water-hardness-calculator
TG we-Calculate Editorial Team. "Water Hardness Calculator — ppm, gpg, °dH, °fH Converter." TG we-Calculate. 2026. https://we-calculate.com/calculator/water-hardness-calculator.
TG we-Calculate Editorial Team, "Water Hardness Calculator — ppm, gpg, °dH, °fH Converter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/water-hardness-calculator
@misc{wecalculate_water_hardness_calculator, title = {Water Hardness Calculator — ppm, gpg, °dH, °fH Converter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/water-hardness-calculator}}, year = {2026}, note = {TG we-Calculate} }
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