Water hardness in the US is measured either in ppm (parts per million) or gpg (grains per gallon).
Parts per million, also referred to as American degrees, is a water hardness unit equal to 1 mg/L CaCO3 dissolved in water.
Grains per gallon is also a water hardness unit defined as 1 grain of CaCO3 dissolved in water.
We have created two useful resources for ppm to gph conversion:
- ppm To gpg Water Hardness Calculator. Just insert ppm, and the calculator will dynamically determine the equivalent gpg water hardness. Example: You can convert 100 ppm to grains per gallon. Result: 5.84 gpg.
- ppm To gpg Water Hardness Chart. We have converted 1 to 1000 ppm into gpg, and summarized the results in the chart below. We have denoted soft water (0-60 ppm) with this green color, moderately hard water (61-120 ppm) with this light blue color, hard water (121-180 ppm) with this blue color, and very hard water (181+ ppm) with this dark blue color.
In order to convert ppm to GPG, you can use this converter:
If you want to use this conversion in reverse – convert GPG to PPM – you can check a similar GPG to PPM calculator here.
How To Convert PPM To GPG?
The common denominator between ppm and gpg is a reference to water hardness measured as a quantity of CaCO3 dissolved in water.
Here is a formula that enables you to calculate gpg from ppm:
gpg = ppm/17.118
In essence, if you divide parts per million by 17.118, you get grains per gallon. Factor 17.118 comes from the volumetric and mass difference between the water hardness units.
Thus every 1 gpg is equal to 17.118 ppm (1 gpg = 17.118 ppm).
Water hardness calculations are most relevant for water softener systems.
Water Hardness PPM To GPG Chart
| Parts Per Million (PPM): | Grains Per Gallon (GPG): |
| 1 ppm | 0.06 gpg |
| 2 ppm | 0.12 gpg |
| 3 ppm | 0.18 gpg |
| 4 ppm | 0.23 gpg |
| 5 ppm | 0.29 gpg |
| 6 ppm | 0.35 gpg |
| 7 ppm | 0.41 gpg |
| 8 ppm | 0.47 gpg |
| 9 ppm | 0.53 gpg |
| 10 ppm | 0.58 gpg |
| 15 ppm | 0.88 gpg |
| 20 ppm | 1.17 gpg |
| 25 ppm | 1.46 gpg |
| 30 ppm | 1.75 gpg |
| 35 ppm | 2.04 gpg |
| 40 ppm | 2.34 gpg |
| 45 ppm | 2.63 gpg |
| 50 ppm | 2.92 gpg |
| 55 ppm | 3.21 gpg |
| 60 ppm | 3.51 gpg |
| 65 ppm | 3.80 gpg |
| 70 ppm | 4.09 gpg |
| 75 ppm | 4.38 gpg |
| 80 ppm | 4.67 gpg |
| 85 ppm | 4.97 gpg |
| 90 ppm | 5.26 gpg |
| 95 ppm | 5.55 gpg |
| 100 ppm | 5.84 gpg |
| 110 ppm | 6.43 gpg |
| 120 ppm | 7.01 gpg |
| 130 ppm | 7.59 gpg |
| 140 ppm | 8.18 gpg |
| 150 ppm | 8.76 gpg |
| 160 ppm | 9.35 gpg |
| 170 ppm | 9.93 gpg |
| 180 ppm | 10.52 gpg |
| 190 ppm | 11.10 gpg |
| 200 ppm | 11.68 gpg |
| 210 ppm | 12.27 gpg |
| 220 ppm | 12.85 gpg |
| 230 ppm | 13.44 gpg |
| 240 ppm | 14.02 gpg |
| 250 ppm | 14.60 gpg |
| 260 ppm | 15.19 gpg |
| 270 ppm | 15.77 gpg |
| 280 ppm | 16.36 gpg |
| 290 ppm | 16.94 gpg |
| 300 ppm | 17.53 gpg |
| 310 ppm | 18.11 gpg |
| 320 ppm | 18.69 gpg |
| 330 ppm | 19.28 gpg |
| 340 ppm | 19.86 gpg |
| 350 ppm | 20.45 gpg |
| 360 ppm | 21.03 gpg |
| 370 ppm | 21.61 gpg |
| 380 ppm | 22.20 gpg |
| 390 ppm | 22.78 gpg |
| 400 ppm | 23.37 gpg |
| 410 ppm | 23.95 gpg |
| 420 ppm | 24.54 gpg |
| 430 ppm | 25.12 gpg |
| 440 ppm | 25.70 gpg |
| 450 ppm | 26.29 gpg |
| 460 ppm | 26.87 gpg |
| 470 ppm | 27.46 gpg |
| 480 ppm | 28.04 gpg |
| 490 ppm | 28.62 gpg |
| 500 ppm | 29.21 gpg |
| 550 ppm | 32.13 gpg |
| 600 ppm | 35.05 gpg |
| 650 ppm | 37.97 gpg |
| 700 ppm | 40.89 gpg |
| 750 ppm | 43.81 gpg |
| 800 ppm | 46.73 gpg |
| 850 ppm | 49.66 gpg |
| 900 ppm | 52.58 gpg |
| 950 ppm | 55.50 gpg |
| 1000 ppm | 58.42 gpg |
You can get quite a lot of the ppm to grains per gallon conversion from this chart.
Example 1: What is 450 ppm hardness in grains per gallon? You can just check the chart; 450 ppm water hardness is equal to 26.29 grains per gallon.
Example 2: What is 400 ppm hardness in grains per gallon? Same thing – check the chart – and you see that 400 ppm hardness is equal to 23.37 gpg.
Hope this helps. If you need any assistance or advice, you can use the comment section below, give us some numbers, and we can help you out.