Water Hardness Calculator
Water hardness converted between every scale — ppm as CaCO₃, grains per gallon, German, French and English degrees.
Hardness is the total calcium and magnesium, conventionally expressed as an equivalent concentration of calcium carbonate in mg/L (which equals ppm).
How the water hardness calculator works
Hardness is the total calcium and magnesium, conventionally expressed as an equivalent concentration of calcium carbonate in mg/L (which equals ppm).
The other scales are simple multiples: one grain per US gallon is 17.1 ppm, one German degree (°dH) is 17.85 ppm, one French degree 10 ppm and one English (Clark) degree 14.3 ppm. Softener sizing usually wants grains per gallon; laboratory results come as ppm.
Formula: 1 gpg = 17.1 ppm; 1 °dH = 17.85 ppm; 1 °fH = 10 ppm
Worked examples
| Inputs | Hardness | Note |
|---|---|---|
| 250 ppm | 250 ppm as CaCO₃ | very hard — 14.6 gpg |
| 10 grains per gallon | 171 ppm as CaCO₃ | 171 ppm — hard |
| 8 °dH | 142.8 ppm as CaCO₃ | 143 ppm — hard |
FAQFrequently asked questions
What is hard water?
Water high in dissolved calcium and magnesium. Above 180 ppm as calcium carbonate is conventionally "very hard".
How do I convert ppm to grains per gallon?
Divide by 17.1.
Which scale will my lab report use?
ppm or mg/L as CaCO₃ in most English-speaking countries; German degrees in central Europe; French degrees in France and much of Africa.
Is hard water bad for you?
No — it is harmless to drink, and there is some evidence it is mildly beneficial. The problems are scale in appliances and poor lathering.
How much salt does a softener use?
It depends on the grain load: hardness in grains per gallon times daily water use. A hard-water household of four typically regenerates every few days.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.