What Is the Hardness of Water? gpg, ppm and mg/L Explained

Water hardness is the amount of calcium and magnesium dissolved in your water. Penn State says it is written two ways. One is parts per million of calcium carbonate, which is the same as milligrams per liter. The other is grains per gallon. One grain per gallon equals about 17 mg/L.

You will see ppm, mg/L and gpg on lab reports and spec sheets. Two of them are the same unit under different names.

Penn State says parts per million is also equal to milligrams per liter. So a report that says 120 ppm and a report that says 120 mg/L are saying the same thing.

The third unit is the odd one. Penn State says a grain per gallon is used only as a hardness unit. It equals about 17 mg/L, or 17 ppm.

Nebraska Extension prints the conversion with one more decimal place. Its table starts soft water at 0 to 17.1 mg/L, which it lists as 0 to 1 grain per gallon.

Both figures are published by an extension service. Use about 17 for quick math, and expect a lab to use the longer figure.

Hardness is not one mineral. Penn State says the ppm figure stands for the calcium and magnesium dissolved in the water, expressed as calcium carbonate.

Nebraska Extension explains why the reading gets changed. Hardness ions usually pair up with sulfate, chloride, carbonate or bicarbonate. The amounts are then changed to match calcium carbonate. That keeps one lab result readable next to another.

So the unit says calcium carbonate, but the thing in your pipes is calcium and magnesium. Penn State says using that equal figure simplifies the math.

Two extension services publish the same bands, so you can read your result without guessing.

Penn State lists soft as less than 1.0 grain per gallon, which is less than 17 ppm. Slightly hard runs 1.0 to 3.5 gpg, or 17 to 60 ppm. Moderately hard runs 3.5 to 7.0 gpg, or 60 to 120 ppm. Hard runs 7.0 to 10.5 gpg, or 120 to 180 ppm. Very hard is greater than 10.5 gpg, or greater than 180 ppm.

Nebraska Extension prints the same bands in its own table. It credits the Water Quality Association, and says the classification is used by the U.S. Department of the Interior as well.

That means your reading has a name, whichever unit it came in.

A band is a label. This is the line where it turns into a decision.

Minnesota Health says there is no requirement to soften your water. It calls the choice a personal one. It then says that above 7 grains per gallon, or 120 mg/L, you might need a softener to keep appliances running well.

Penn State lands in the same place from the other side. It says levels below 7.0 grains per gallon will probably not cause major scaling and soap film.

Look back at the table and you can see why. Seven grains per gallon is exactly where the band called hard begins.

This surprises people, so it is worth saying plainly.

Nebraska Extension says the EPA sets two kinds of standard. Primary ones cover health. Secondary ones cover taste, odor, color and staining. It then says neither kind covers water hardness.

It gives its home state as the example. Nebraska's health department runs drinking water standards there. It does not regulate hardness in public supplies.

Nebraska Extension also says public suppliers are not required to manage hardness in the water they send you. Some do it anyway, and in some cases treating for something else lowers hardness as a side effect.

So a high hardness reading is never a violation. It is information about your plumbing bill, not your safety.

Hardness has a second use. It tells you how much sodium a softener would add.

Nebraska Extension says the trade adds about 8 mg/L of sodium for each grain per gallon of hardness removed. It gives the worked example. Water at 10 grains per gallon holds about 80 mg/L of sodium once it has been softened.

So the harder your water, the more sodium goes in during softening. That matters to anyone counting sodium, and it is a reason some people bypass the kitchen tap.

Hardness is also one of the four inputs that set how often a softener cleans itself.

Penn State prints a worked example. It uses a capacity of 20,000 grains per cleaning cycle. It assumes 75 gallons of water per person per day, 10 grains per gallon of raw hardness, and 4 people in the house.

The math runs like this. 75 gallons at 10 gpg for 4 people uses 3,000 grains a day. Divide 20,000 by 3,000 and you get a cycle of about 6 to 7 days.

Change the hardness and everything downstream moves. That is why a spec sheet cannot tell you what size you need until you know your own number.