What this converter does
This converter turns water hardness in ppm (mg/L as calcium carbonate) into grains per gallon and back. It also covers German, French and Clark degrees — every pair uses an exact fixed factor. Type a hardness value, pick your scales, and read the result instantly.
One grain per gallon equals about 17.1 ppm. Softeners are usually sized in gpg in the US, so converting from a ppm lab result is a common first step.
The units it covers
Hardness is a concentration of calcium carbonate, so every scale converts through ppm (mg/L as CaCO₃) with a fixed ratio.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Part per million | ppm | 1 | mg/L as CaCO₃ — the base |
| Grain per gallon | gpg | 17.118 | US softeners & test kits |
| German degree | °dH | 17.848 | Germany & central Europe |
| French degree | °fH | 10 | France & food industry |
| English (Clark) degree | °e | 14.254 | UK legacy scale |
The formula
Each scale has a fixed value in ppm CaCO₃, so converting between any two means going through ppm once:
result = value × factor_from ÷ factor_toWhere:
- value = the hardness you typed, in the “from” scale
- factor_from = the “from” scale’s value in ppm
- factor_to = the “to” scale’s value in ppm
Worked example
Convert 171 ppm to grains per gallon.
1 gpg = 17.1181 ppm171 ÷ 17.1181 = 9.99 gpgSo 171 ppm is about 10 grains per gallon — hard water needing softening.
The units in this example
Milligrams of calcium carbonate per litre of water — the standard lab measure of hardness.
- 1 ppm = 0.0584 gpg
- 1 ppm = 1 mg/L CaCO₃
- 17.1 ppm = 1 gpg
- 1 ppm = 0.1 °fH
The US hardness unit used on water-softener labels and home test kits.
- 1 gpg = 17.1 ppm
- 1 gpg = 1.71 °fH
- 1 gpg ≈ 0.959 °dH
- 7 gpg ≈ 120 ppm (hard)