GPG to PPM — water hardness converter
One grain per gallon (gpg) of hardness equals 17.118 mg/L, and mg/L and ppm are identical for hardness — so to convert gpg to ppm you multiply by 17.118. For example, 10 gpg × 17.118 = 171 ppm, which is hard water. Softener labels use gpg; water reports use ppm.
Softener labels talk in grains per gallon, while water reports talk in ppm — this converts back the other way.
Convert gpg to ppm
Enter a number to convert; the result shows its band and how it ranks among 2,977 US counties.
Chart
gpg to ppm chart
| gpg | ppm | Band |
|---|---|---|
| 1 | 17 | Soft |
| 2 | 34 | Soft |
| 3 | 51 | Soft |
| 5 | 86 | Moderately hard |
| 7 | 120 | Moderately hard |
| 10 | 171 | Hard |
| 12 | 205 | Very hard |
| 15 | 257 | Very hard |
| 20 | 342 | Very hard |
| 25 | 428 | Very hard |
| 30 | 514 | Very hard |
Formula
The formula, and why
ppm = gpg × 17.118. The grain per gallon is one grain of calcium carbonate — 64.79891 mg — dissolved in one US gallon — 3.785412 L: 64.79891 ÷ 3.785412 = 17.118 mg/L. Charts often round this to 17.1; we compute at 17.118 and round only the displayed result, so a 17.1-based chart can differ by about 0.1%.
Reverse: PPM to GPG · mg/L→gpg · gpg→°dH · °dH→ppm · all-units calculator
Questions
Frequently asked
- How many ppm is 1 gpg?
- 17.118 ppm — one grain per gallon equals 17.118 mg/L, and mg/L equals ppm.
- Is gpg larger than ppm?
- Yes — one gpg is 17.118 ppm, so gpg numbers are much smaller for the same water.
- What is 7 gpg in ppm?
- 7 × 17.118 = 120 ppm, right on the moderately-hard / hard line.
By Marko Visic, MPharm, BSc Physics. Source: Conversion factors per USGS/NIST unit definitions; band data from USGS/EPA WQP. Data last verified . Methodology.