GPG To PPM Calculator: Convert Grains Per Gallon To PPM (Water Hardness)

When figuring out water hardness, we will sometimes need to convert GPG to PPM. Both of these units determine how hard your water is. To help you out with the conversion, we have created the GPG to PPM calculator, complete with the GPG to PPM chart (with 0.1 to 50 GPG converted into PPM).

Note: In the conversion chart below, we have denoted soft water (0-3.5 gpg) with this green color, moderately hard water (3.56-7.01 gpg) with this light blue color, hard water (7.06-10.51 gpg) with this blue color, and very hard water (10.57+ gpg) with this dark blue color.

high gpg vs low gpg water hardness or ppm water hardness
Effect of high 20 GPG water hardness (right) vs low 3 GPG water hardness (left).

Let’s first look at the units of water hardness we are dealing with here:

  • GPG or Grains Per Gallon is the water hardness when 1 grain of calcium carbonate (64.8 mg of CaCO3) is dissolved in 1 US gallon of water. A US gallon is equal to 3.785412 liters of water.
  • PPM or Parts Per Million is the water hardness when 1 mg of calcium carbonate (CaCO3) is dissolved in 1 liter of water. It is equal to mg/L unit for hard water.

As we can see, GPG is basically equal to 64.8 mg of CaCO3 per 3.785412 liters of water (US gallon). PPM is equal to 1 mg of CaCO3 per 1 liter of water. Based on this, we can use this formula to convert hard water GPG to PPM:

PPM = GPG × 17.118

Example: Let’s say that we have 25 GPG water hardness and would like to know how much is those 25 GPG in PPM. We just use the equation above like this:

PPM (25 GPG) = 25 × 17.118 = 427.95 PPM

As we can see, 25 GPG is equal to 427.95 PPM of water hardness.

Essentially, 1 GPG is equal to 17.118 PPM. For PPM to GPG conversion, you can check this similar PPM to GPG calculator and chart.

This was just one example. To help you out with the conversion, you can simply use the calculator or consult the GPG to PPM chart below the calculator:

GPG To PPM Calculator

Here you just insert the GPG, and the calculator will automatically convert that to PPM. You can slide left to right to see how water hardness classification (“Soft”, “Moderately Hard”, “Hard”, “Very Hard” water) changes:


Let’s do one quick example to see how this calculator works. We want to convert 10 GPG to PPM. Just slide the slider to ’10’ and you get the result: 10 GPG is equal to 171.18 PPM.

Here is the full GPG to PPM chart with 0.1 GPG to 50 GPG water hardness converted into PPM units:

GPG To PPM Conversion Chart

Grains Per Gallon (GPG): Parts Per Million (PPM):
0.1 gpg 1.71 ppm
0.2 gpg 3.42 ppm
0.3 gpg 5.14 ppm
0.4 gpg 6.85 ppm
0.5 gpg 8.56 ppm
0.6 gpg 10.27 ppm
0.7 gpg 11.98 ppm
0.8 gpg 13.69 ppm
0.9 gpg 15.41 ppm
1 gpg 17.12 ppm
1.5 gpg 25.68 ppm
2 gpg 34.24 ppm
2.5 gpg 42.79 ppm
3 gpg 51.35 ppm
3.5 gpg 59.91 ppm
4 gpg 68.47 ppm
4.5 gpg 77.03 ppm
5 gpg 85.59 ppm
5.5 gpg 94.15 ppm
6 gpg 102.71 ppm
6.5 gpg 111.27 ppm
7 gpg 119.83 ppm
7.5 gpg 128.39 ppm
8 gpg 136.94 ppm
8.5 gpg 145.50 ppm
9 gpg 154.06 ppm
9.5 gpg 162.62 ppm
10 gpg 171.18 ppm
11 gpg 188.30 ppm
12 gpg 205.42 ppm
13 gpg 222.53 ppm
14 gpg 239.65 ppm
15 gpg 256.77 ppm
16 gpg 273.89 ppm
17 gpg 291.01 ppm
18 gpg 308.12 ppm
19 gpg 325.24 ppm
20 gpg 342.36 ppm
21 gpg 359.48 ppm
22 gpg 376.60 ppm
23 gpg 393.71 ppm
24 gpg 410.83 ppm
25 gpg 427.95 ppm
26 gpg 445.07 ppm
27 gpg 462.19 ppm
28 gpg 479.30 ppm
29 gpg 496.42 ppm
30 gpg 513.54 ppm
35 gpg 599.13 ppm
40 gpg 684.72 ppm
45 gpg 770.31 ppm
50 gpg 1855.90 ppm

As you can see, the colors in the chart will mean different things; green for soft water, light blue for moderately hard water, blue for hard water, and dark blue for very hard water. This water hardness classification is based on these GPG and PPM water hardness scales:

  • Below 3.5 GPG, we have soft water (that’s below 60 PPM).
  • Between 3.56 and 7.01 GPG (that’s roughly between 61 and 120 PPM), we have moderately hard water.
  • Between 7.06 and 10.51 GPG (that’s roughly between 121 and 189 PPM), we have hard water.
  • At 10.57 GPG and above (that’s roughly 180 PPM and above), we have very hard water.

We hope that this illustrates well how to convert grains per gallon to parts per million hard water units. If you have any questions regarding this conversion, or would like for us to help you out, you can use the comment section below.

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