Water Hardness Converter (gpg, ppm, mg/L, °dH)

Convert water hardness between grains per gallon, ppm, mg/L and German degrees, and see the USGS hardness classification.

Reviewed Oct 21, 20261 cited source (EPA, CDC, USGS, NSF and university extension)How we calculate

Results update as you type.

Grains per gallon

10 gpg

That is hard water on the USGS scale.

mg/L as CaCO₃
171.2 mg/L
ppm
171.2 ppm
German degrees
9.59 °dH
USGS classification
Hard

Estimate only. Not 100% accurate or guaranteed. Confirm with a water test, your utility or a qualified pro before you buy or act.

How it works

Water reports and test strips use different units. Softener manuals usually want grains per gallon; utility reports often list mg/L or ppm as calcium carbonate.

All conversions go through mg/L as CaCO₃ using exact unit definitions, so results are consistent in every direction.

The formula

  • 1 gpg = 64.79891 mg ÷ 3.785412 L ≈ 17.12 mg/L as CaCO₃
  • 1 °dH = 10 mg/L CaO ≈ 17.85 mg/L as CaCO₃
  • ppm ≈ mg/L for drinking water
  • USGS: 0–60 soft, 61–120 moderately hard, 121–180 hard, >180 mg/L very hard

How to use this calculator

  1. Enter the hardness number from your test or water report.
  2. Choose the unit it is reported in.
  3. Read the value in every other unit and its USGS class.

Frequently asked questions

Is ppm the same as mg/L?

For drinking water, yes in practice: one liter of water weighs about one kilogram, so 1 mg/L ≈ 1 ppm.

What hardness is considered hard?

USGS classifies 121–180 mg/L as hard and above 180 mg/L as very hard.

Sources

  1. USGS: Hardness of Water: Soft/moderately hard/hard/very hard classes in mg/L as CaCO₃

Sources last checked Oct 21, 2026Written and reviewed by the Tapped Out team against the sources above.How we calculate

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