The city report prints hardness in milligrams per liter. The test strip reads it in grains per gallon. The homeowner has never heard of either. One conversion turns the report's number into the strip's number, and the strip's number into a sentence the homeowner understands.
Hardness has no federal limit. Nobody will make the city fix it. That is why it is the plumber's number.
Short answer
Hardness is dissolved calcium and magnesium, the minerals water picks up from rock and soil. A grain per gallon (gpg) is 17.1 milligrams per liter of those minerals, measured as calcium carbonate. To turn a city report's number into grains, divide by 17.1. A report that says 200 milligrams per liter is saying 11.7 grains.
A grain is an old unit of weight: 64.8 milligrams, about one grain of barley. A US gallon is 3.785 liters. Spread 64.8 milligrams of mineral through 3.785 liters of water and you have 17.1 milligrams in every liter. One grain per gallon.
Water treatment equipment is sized and sold in grains. Test strips read in grains. Only the city report uses milligrams per liter (some print ppm, parts per million, which is the same thing). The plumber is the one who converts.
The scale
The U.S. Geological Survey publishes the classification most of the industry uses. Here it is in both units, with what the homeowner usually notices at each step.
| Classification | Milligrams per liter | Grains per gallon | What the homeowner notices |
|---|---|---|---|
| Soft | 0 to 60 | 0 to 3.5 | Little or no scale. Soap lathers easily. |
| Moderately hard | 61 to 120 | 3.6 to 7 | Spots on glassware. Some scale over time. |
| Hard | 121 to 180 | 7.1 to 10.5 | Scale in the water heater, film on the shower door, soap that will not lather. |
| Very hard | Above 180 | Above 10.5 | All of the above, faster. White crust on faucets and showerheads. |
Source: U.S. Geological Survey, Water Science School, Hardness of Water. The grains column is the milligrams column divided by 17.1.
Three real cities
| City | What the report prints | In grains | Where it lands |
|---|---|---|---|
| Sachse, Texas | 96.0 to 200 mg/L (2025 report, page 3) | 5.6 to 11.7 | Moderately hard to very hard, depending on the day |
| Dallas, Texas | 120 to 155 mg/L (City of Dallas) | 7.0 to 9.1 | Hard. Roger's own field readings in Dallas homes run 8 to 12. |
| Carlsbad, California, imported water | 165 to 291 mg/L (district report, page 2, Skinner plant) | 9.6 to 17.0 | Hard to very hard |
| Carlsbad, California, desalinated water | 43.7 to 79.6 mg/L (district report, page 2) | 2.6 to 4.7 | Soft to moderately hard |
Carlsbad blends the two, so the report can only give a range and the house's own number comes from the tap. That is the whole reason for the sixty-second water test. See water hardness by city for the figures each utility publishes.
What it does
Hardness is not a health problem. It is an equipment problem, and the equipment is the plumber's. A study by Battelle for the Water Quality Research Foundation, begun in 2009, ran water heaters, showerheads and faucets on hard water and on softened water side by side.
On unsoftened water, scale built up at about 528 grams a year. On softened water, about 7 grams a year. Efficiency on softened water stayed essentially constant through the test, and none of the hard-water water heaters lasted to the end of it.
Showerheads on unsoftened water clogged after an average of 3,203 gallons of flow.
Faucets on unsoftened water clogged after the equivalent of 19 days of flow, at an estimated 50 gallons of hot water a day.
Source: WaterWorld, "Battelle Benefits," 23 April 2010, reporting the Battelle study for the Water Quality Research Foundation. The article does not state the hardness level tested, so these figures show the direction and the scale of the damage, not a number for any one house.
Where it goes wrong
A report that says 200 is saying 11.7 grains, not 200. A plumber who skips the division tells a homeowner their water is seventeen times harder than it is. Divide by 17.1, every time.
Water quality reports list dozens of rows and the numbers sit close together. Chlorate is not hardness. Total dissolved solids is not hardness. Find the row that says hardness, or "Total Hardness as CaCO3," and read that one.
The report gives a range for the system, sometimes from two different plants. Only the strip at this kitchen tap gives this house's grains today.
Watch it done
Questions
One grain per gallon is 17.1 milligrams per liter of dissolved calcium and magnesium, measured as calcium carbonate. A grain is 64.8 milligrams, a US gallon is 3.785 liters, and 64.8 divided by 3.785 is 17.1.
Divide by 17.1. A report that prints 120 milligrams per liter is 7.0 grains. One that prints 200 is 11.7 grains. Parts per million and milligrams per liter are the same number for water, so the same division works for both.
Yes. On the U.S. Geological Survey scale, 7.1 to 10.5 grains is hard and above 10.5 is very hard. Dallas reports 7.0 to 9.1 grains, and Roger's field readings in Dallas homes run 8 to 12.
There is no federal limit on hardness and no single number that answers it. The USGS scale above says how hard the water is. What a particular house needs depends on its readings, its equipment and what the homeowner is living with, and that decision is what Day Two of the HALO Water Tour teaches.
Yes. In the Battelle study for the Water Quality Research Foundation, water heaters on unsoftened water built up about 528 grams of scale a year against about 7 grams on softened water, and none of the hard-water heaters lasted to the end of the test.
Sources
Checked in October 2026. Reports are reissued every year, so confirm with the source before you rely on a number.
Written by Roger Wakefield, Texas Master Plumber
45 years in the trade and LEED AP. Published . More about Roger or visit rogerwakefield.com.
Keep going
Three consecutive days, in person, twenty seats a city: Carlsbad October 27-29 and Dallas November 3-5, 2026. A certification exam closes every day, and the HALO Certified Water Quality Expert credential is awarded on day three.