Your water softener hardness level is based on the concentration of calcium carbonate in your water supply. However, there are two different units used for quantifying this concentration.

One, grains per gallon, is based on the US imperial system and the other, parts per million, has its origin in the metric system.

Because these two units tend to be used interchangeably this can cause a lot of confusion, for example when you are trying to figure out the conversion from ppm to gpg to correctly set up your water softener.

It is however not as hard as it looks, below we provide all the info needed (including a handy unit conversion calculator) to convert between grains per gallon, parts per million, and milligrams per liter.

How To Convert Grains Per Gallon To Parts Per Million

1 grain64.8 milligram
1 gallon (US)3.785 liter
1 grain per gallon17.12 milligram per liter
1 milligram per liter1 part per million

If we take the values above we can derive that 1 grain (64.8 milligrams) per US Gallon (3.785 liter) equals 64.8/3.785 = 17.12 milligrams per liter (or parts per million).

So if we want to go from grains per gallon to parts per million we have to multiply our input value with 17.12 to get our answer. However, using the calculator below will do all the work for you!

Water Hardness Conversion Calculator

The calculator below can be used to convert between grains per gallon (gpg), parts per million (ppm), and milligrams per liter (mg/L).

Simply input the value of your starting unit in the corresponding box, it will automatically convert to the other units.

Grains per Gallon
Parts per Milion
Milligrams per Liter

Now that you have your water hardness value you might be wondering whether you need a water softener, using the water hardness chart and table below you can see whether your water is considered hard or not.

Water Hardness Unit Conversion Chart Between Grains Per Gallon (gpg) and Parts Per Million (ppm) or Milligrams Per Liter (mg/L)
gpgppm or mg/L
Soft Water0-3.40-59
Moderately Hard3.5-6.960-119
Hard Water7.0-10.4120- 179
Very Hard Water10.5-15180-260
Extremely Hard15+260+