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Water Hardness Converter: GPG to PPM

Water tests, softener manuals and utility reports use different hardness units. Convert between grains per gallon, ppm (mg/L as calcium carbonate), German and French degrees and mmol/L, and see where your water sits on the hardness scale.

Results

10 gpg equals171.2 ppmHard water (Water Quality Association scale)
ppm / mg/L as CaCO₃
171.2 ppm
German degrees (°dH)
9.591 °dH
French degrees (°f)
17.12 °f
mmol/L
1.71 mmol/L
Hardness classes
Classgpgppm (mg/L)
Soft0–10–17.1
Slightly hard1–3.517.1–60
Moderately hard3.5–760–120
Hard7–10.5120–180
Very hardover 10.5over 180

How it works

  1. Enter the hardness figure from your water test or utility report.
  2. Choose its unit — ppm and mg/L are the same thing for water.
  3. Read the converted value in grains per gallon (or ppm) and every other unit, plus the hardness class.

Formula

1 gpg = 17.118 ppm (mg/L as CaCO₃)
gpg = ppm ÷ 17.118
1 °dH = 17.848 ppm = 1.043 gpg
1 °f = 10 ppm
1 mmol/L = 100.09 ppm

A grain is 64.79891 mg and a U.S. gallon is 3.785411784 L, so one grain per gallon is 17.118 mg/L. Hardness is expressed “as calcium carbonate”, so calcium and magnesium are counted together. A German degree is 10 mg/L of calcium oxide, which equals 17.848 mg/L as CaCO₃.

Example

A city water quality report lists total hardness as 250 mg/L as CaCO₃, but the softener asks for grains per gallon.

Inputs

Hardness
250
Unit
ppm or mg/L

Result

250 ppm equals14.6 gpgVery hard water (Water Quality Association scale)
Load this example into the calculator →

Where the hardness classes come from

The scale follows the Water Quality Association classification: under 1 gpg (17.1 mg/L) is soft, 1–3.5 gpg slightly hard, 3.5–7 gpg moderately hard, 7–10.5 gpg hard and above 10.5 gpg (180 mg/L) very hard. The U.S. Geological Survey uses similar bands.

Hardness is mainly a scale and soap problem, not a health rating. Use it to decide whether softening is worthwhile and to program or size a softener.

Frequently asked questions

How many ppm is 1 grain per gallon?

One grain per U.S. gallon equals 17.118 ppm (mg/L as calcium carbonate). Softener manufacturers often round it to 17.1.

Are ppm and mg/L the same?

For fresh water, yes. One liter of water weighs about one kilogram, so one milligram per liter is one part per million.

What number do I enter into my water softener?

Most U.S. softeners ask for hardness in grains per gallon. If your water also contains iron, many manufacturers suggest adding about 4 gpg for each ppm of iron.

What is a good water hardness level?

Water under about 3.5 gpg (60 mg/L) rarely causes noticeable scale. Above 7 gpg (120 mg/L) scale on fixtures and water heaters becomes common, which is where softeners are usually considered.