Converting Water Hardness Units: gpg vs ppm

Why hardness units appear when you shop

A water report that states how hard your water is uses one or both of two units: grains per gallon (gpg) and parts per million (ppm). Penn State Extension's water softening page explains that ppm is also written as milligrams per liter (mg/l).

On a softener listing, the grain figure you see names the system's capacity, not the hardness of your water. A title such as "48,000 grain" is the seller's label for resin capacity. To compare listings, read the grain figure as stated; the link between your own hardness reading and a capacity is the maker's sizing guidance, not this site's. See what grain capacity means and how to read a water softener listing.

What grains per gallon and parts per million mean

Parts per million means one unit of calcium carbonate dissolved in one million units of water. Penn State states that ppm equals milligrams per liter, so the two are the same expression of dissolved calcium and magnesium.

Grains per gallon is a hardness unit used in North America. Penn State states that one gpg equals approximately 17 mg/l or ppm. Both units describe the same water, written as calcium carbonate, so a conversion is a simple unit change rather than a change in the reading.

The classification table below pairs the two units, so you can see where a reading falls. The factor remains approximate because a water report may round its figures; the maker's page is the place to read how the manufacturer wants the unit handled. Compare systems on stated details in the main buying guide.

How to convert between gpg and ppm

To convert gpg to ppm, multiply the gpg figure by approximately 17. To convert ppm to gpg, divide the ppm figure by approximately 17.

For example, a reading of 7.0 gpg multiplies to about 119 ppm. A reading of 180 ppm divides to about 10.6 gpg. The conversion is straightforward, but it does not change what the units tell you; both refer to the same concentration of calcium carbonate.

When you read a water report, note which unit it uses. Some reports list both, while others list only one. If the report gives ppm, convert to gpg so you can compare with the Penn State classification and with the grain figures on a listing. This conversion is for your own reading; it never replaces the maker's guidance for choosing a capacity.

How hardness levels are classified

Penn State groups hardness readings into five classes. The same ranges appear in gpg and ppm. This table is reproduced from the Penn State water softening page.

Reading your home's hardness

A water report from a city supplier or a well test is one way to learn your hardness. A hardness kit, sold separately, is another. The kit gives a reading in gpg or ppm. This site does not provide hardness figures for any water supply; it only explains the units and how to read a listing's stated details.

For a private well, the EPA explains that well owners are responsible for their own water quality; the EPA private wells page in the sources below is the place to read about well water. For city water, your supplier's annual water quality report is a source of information about your supply. The EPA's drinking water pages are also listed in the sources.

Once you have a hardness reading, convert it to gpg if needed, then look at listings by stated grain capacity. See how many grains do I need and water softener size by household for background on how capacity relates to a home.

Matching units to listing terms

A listing title may say "40,000 grain" or "48,000 grain"; these are capacity labels, not hardness readings. The same system might appear with a gallon figure or a grain figure in the title. The grain figure is the one that describes the resin capacity.

When you compare listings, read the grain capacity as stated. A larger number does not mean a better pick for every home. The right capacity depends on your own water's hardness, your household's water use, and the maker's sizing table. This site compares stated details only and does not rank on price.

If a listing names iron in the title, that is a separate stated feature. Iron in water is a factor you may want to read about when choosing a system; see the buying guide for iron.

Well water and other sources

For homes on a private well, the EPA's private wells page explains that the well owner is responsible for water testing and treatment. The EPA recommends annual private well testing for coliform bacteria, nitrates, total dissolved solids, and pH; it does not list hardness as a required annual item, but the page's table includes hardness as a possible reason to test. Read the EPA page directly for the full guidance.

For city water, your supplier provides an annual water quality report, which may include hardness. The EPA's Ground Water and Drinking Water page lists resources for reading water quality reports. This site does not interpret these reports; it only helps you convert the units you see.

Matching a guide to your situation

With the unit conversion in hand, read a listing's stated details: type, grain capacity, regeneration, flow rate and iron. Matching guides are below.

What to pick for your situation

Questions

What is the difference between gpg and ppm?

Both units express the same hardness. Grains per gallon is a North American hardness unit; parts per million is a concentration unit also written as milligrams per liter. Penn State states that one gpg equals approximately 17 ppm.

How do I convert a ppm reading to gpg?

Divide the ppm figure by approximately 17. A reading of 120 ppm, the top of Penn State's moderately hard class, divides to about 7.0 gpg, which matches the table's threshold.

How do I convert a gpg reading to ppm?

Multiply the gpg figure by approximately 17. The classification table shows the paired ranges, so a reading near 7.0 gpg lands near 120 ppm.

Which unit do softener listings use?

A listing title uses grains for the system's capacity, such as 48,000 grains. That figure is not your water's hardness; it is the seller's label for the resin capacity.

Do I need to convert before choosing a softener?

The conversion helps you read your own water report and compare it with the classification table. Choosing a capacity is the maker's sizing guidance plus your own hardness reading; this site compares stated listing details only and never gives a capacity for your water.

Where can I find a hardness reading for my water?

A hardness kit is a separate purchase you can use at home. For a private well, the EPA private wells page in the sources explains well owner responsibilities; for city water, your supplier's report is a place to read about your water.