Understanding Water Hardness: GPG and PPM Explained
"Hard water" gets thrown around a lot, but the actual measurement behind it is straightforward once you know what the numbers mean. Here is what GPG and PPM actually measure, and what counts as hard, very hard, or soft.
What Makes Water "Hard"
Water hardness comes from dissolved calcium and magnesium, minerals the water picks up as it moves through soil and rock before reaching a well or municipal supply. In Eastern Idaho, that typically means groundwater moving through basalt rock, which dissolves these minerals along the way. The more calcium and magnesium in the water, the "harder" it is, and the more scale, spots, and mineral buildup it tends to leave behind.
GPG vs. PPM: The Same Thing, Different Units
Grains per gallon (GPG) and parts per million (PPM, equivalent to milligrams per liter) both measure the same dissolved mineral content, just in different units. One grain per gallon equals about 17.1 parts per million. Municipal water quality reports often use PPM or mg/L, while water softener sizing in the industry is usually done in GPG, which is why you will see both units depending on the source.
The Hardness Scale
| Classification | GPG | PPM (mg/L) |
|---|---|---|
| Soft | 0 - 1 | 0 - 17 |
| Slightly hard | 1 - 3.5 | 17 - 60 |
| Moderately hard | 3.5 - 7 | 60 - 120 |
| Hard | 7 - 10.5 | 120 - 180 |
| Very hard | 10.5+ | 180+ |
This is a standard water-hardness classification used across the water treatment industry, not a figure specific to any one home or system.
Where Eastern Idaho Falls on the Scale
Water across this region is commonly cited in the hard to very hard range, a result of the shared aquifer geology rather than anything specific to one home. Because hardness is not a contaminant the EPA requires water systems to report, exact figures are not always published. See sourced details for Idaho Falls, or check the Idaho DEQ's lookup tool for your own address.
Why This Number Matters for Your Softener
Hardness in GPG is one of the two key numbers used to size a water softener correctly, alongside your household's daily water use. See how the two combine to determine the right system size for your home.
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