ConverterHub

Water Hardness Converter

Convert water hardness between common units.

Result

178.48 ppm
10.43 gpg
10.00 °dH
1.78 mmol/L

How it is calculated

Water hardness is expressed in several units, and the tool converts through one reference, ppm as CaCO3 (mg per litre). The factors used are: 1 gpg (grains per US gallon) = 17.118 ppm, 1 °dH (German degree) = 17.848 ppm, 1 mmol/L = 100.0869 ppm. So ppm = value × factor of the unit you picked, then each output is ppm ÷ its own factor. The unit defaults to ppm, and an unknown unit is treated as ppm. Negative values give no result. All four results are shown to two decimals. The tool converts units only; it does not say whether the water counts as soft or hard, as those ranges vary between sources.

Worked example

Take 10 °dH. ppm = 10 × 17.848 = 178.48. In grains per gallon: 178.48 ÷ 17.118 = 10.43. In °dH it is of course 10.00. In mmol/L: 178.48 ÷ 100.0869 = 1.78. The tool shows 178.48 ppm, 10.43 gpg, 10.00 °dH and 1.78 mmol/L.

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Common mistakes

  • Leaving the unit on ppm while typing a °dH value. 10 ppm is very soft water, while 10 °dH is moderately hard; pick the unit that matches your number.
  • Confusing gpg with °dH. They are different scales (17.118 vs 17.848 ppm per unit), so 10 gpg is not 10 °dH.
  • Mixing mmol/L of calcium alone with total hardness. This conversion assumes the value is total hardness expressed as CaCO3.
  • Expecting a hard/soft verdict. The tool only converts units; classification thresholds depend on the source.
  • Rounding the factors yourself. Small differences in the factor, like 17.8 instead of 17.848, add up at larger values.

About this calculator

This tool converts water hardness between the four units labs and manufacturers use: ppm / mg/L as CaCO₃, grains per gallon (gpg) (1 gpg = 17.118 ppm), German degrees (°dH) (1 °dH = 17.848 ppm), and millimoles per liter (mmol/L) of calcium carbonate. Water hardness reflects the dissolved calcium and magnesium content, which affects soap lathering, scale buildup, and the settings needed on a water softener.

Homeowners reading a water utility report, water treatment professionals sizing a softener, and appliance manufacturers specifying compatible water conditions use it to translate a hardness figure from one region's or lab's preferred unit into another. Enter a value in any one unit to see it converted to the rest.

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