PPM to GPG — water hardness converter
Water-hardness ppm and mg/L are the same number, and one grain per gallon (gpg) equals 17.118 mg/L — so to convert ppm to gpg you divide by 17.118. For example, 200 ppm ÷ 17.118 = 11.7 gpg, which is very hard water. Softeners are set in gpg; tests report ppm.
Water tests report hardness in ppm, but softeners are set in grains per gallon — so this is the conversion people reach for most.
Convert ppm to gpg
Enter a number to convert; the result shows its band and how it ranks among 2,977 US counties.
Chart
ppm to gpg chart
| ppm | gpg | Band |
|---|---|---|
| 10 | 0.6 | Soft |
| 25 | 1.5 | Soft |
| 50 | 2.9 | Soft |
| 100 | 5.8 | Moderately hard |
| 120 | 7 | Hard |
| 150 | 8.8 | Hard |
| 200 | 11.7 | Very hard |
| 250 | 14.6 | Very hard |
| 300 | 17.5 | Very hard |
| 400 | 23.4 | Very hard |
| 500 | 29.2 | Very hard |
| 750 | 43.8 | Very hard |
| 1000 | 58.4 | Very hard |
Formula
The formula, and why
gpg = ppm ÷ 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: GPG to PPM · mg/L→gpg · gpg→°dH · °dH→ppm · all-units calculator
Questions
Frequently asked
- How many gpg is 1 ppm?
- About 0.058 gpg — one ppm is 1 ÷ 17.118 grains per gallon.
- Is ppm the same as mg/L?
- Yes. For water hardness, ppm and mg/L as calcium carbonate are the same number; only the label differs.
- What is 250 ppm in gpg?
- 250 ÷ 17.118 = 14.6 gpg, which is very hard on the USGS scale.
By Marko Visic, MPharm, BSc Physics. Source: Conversion factors per USGS/NIST unit definitions; band data from USGS/EPA WQP. Data last verified . Methodology.