On the USGS water hardness scale, water is soft up to 60 mg/L (3.5 gpg), moderately hard from 61 to 120 mg/L (3.6 to 7 gpg), hard from 121 to 180 mg/L (7.1 to 10.5 gpg) and very hard above 180 mg/L. mg/L and ppm are the same number for water. Find your figure in the tables below to see its class and the value in every other unit.
On this page
This page is a lookup. If you want the background first (what hardness is, where it comes from and why it scales up kettles), start with our guide to what hard water is.
0 → 180+ mg/L as calcium carbonate
USGS Water Science School classes. The bar runs to 300 mg/L.
Water hardness scale in gpg, with every other unit
Softener valves and test strips sold in the US mostly use grains per gallon (gpg). Here is each common reading converted and classed.
| gpg | mg/L or ppm | °dH | °f | °e (Clark) | mmol/L | USGS class |
|---|---|---|---|---|---|---|
| 1 | 17 | 1.0 | 1.7 | 1.2 | 0.17 | Soft |
| 2 | 34 | 1.9 | 3.4 | 2.4 | 0.34 | Soft |
| 3 | 51 | 2.9 | 5.1 | 3.6 | 0.51 | Soft |
| 4 | 68 | 3.8 | 6.8 | 4.8 | 0.68 | Moderately hard |
| 5 | 86 | 4.8 | 8.6 | 6.0 | 0.86 | Moderately hard |
| 6 | 103 | 5.8 | 10.3 | 7.2 | 1.03 | Moderately hard |
| 7 | 120 | 6.7 | 12.0 | 8.4 | 1.20 | Moderately hard |
| 8 | 137 | 7.7 | 13.7 | 9.6 | 1.37 | Hard |
| 9 | 154 | 8.6 | 15.4 | 10.8 | 1.54 | Hard |
| 10 | 171 | 9.6 | 17.1 | 12.0 | 1.71 | Hard |
| 11 | 188 | 10.6 | 18.8 | 13.2 | 1.88 | Very hard |
| 12 | 205 | 11.5 | 20.5 | 14.4 | 2.05 | Very hard |
| 14 | 240 | 13.4 | 24.0 | 16.8 | 2.39 | Very hard |
| 16 | 274 | 15.3 | 27.4 | 19.2 | 2.74 | Very hard |
| 18 | 308 | 17.3 | 30.8 | 21.6 | 3.08 | Very hard |
| 20 | 342 | 19.2 | 34.2 | 24.0 | 3.42 | Very hard |
| 25 | 428 | 24.0 | 42.8 | 30.0 | 4.28 | Very hard |
| 30 | 514 | 28.8 | 51.4 | 36.0 | 5.13 | Very hard |
Water hardness scale in ppm and mg/L
US water quality reports usually give hardness in mg/L or ppm. These round numbers turn up most often:
| ppm / mg/L | gpg | °dH | °e (Clark) | USGS class |
|---|---|---|---|---|
| 25 | 1.5 | 1.4 | 1.8 | Soft |
| 50 | 2.9 | 2.8 | 3.5 | Soft |
| 75 | 4.4 | 4.2 | 5.3 | Moderately hard |
| 100 | 5.8 | 5.6 | 7.0 | Moderately hard |
| 150 | 8.8 | 8.4 | 10.5 | Hard |
| 200 | 11.7 | 11.2 | 14.0 | Very hard |
| 250 | 14.6 | 14.0 | 17.5 | Very hard |
| 300 | 17.5 | 16.8 | 21.0 | Very hard |
| 350 | 20.4 | 19.6 | 24.6 | Very hard |
| 400 | 23.4 | 22.4 | 28.1 | Very hard |
Your number isn’t in either table? The converter does any value, and our gpg to ppm guide explains the maths:
This converter needs JavaScript. Divide mg/L by 17.118 for gpg, or multiply gpg by 17.118 for mg/L.
What each level means in practice
The labels are only useful if you know what to expect at home. This is what the sources behind the scale describe at each step (for a side-by-side view, see hard water vs soft water).
Soft (0–60 mg/L)
Soap lathers easily and feels slippery, because there’s little calcium or magnesium to react with it, as the USGS explains. Kettles and shower heads stay mostly clear. Softening makes no sense here.
Moderately hard (61–120 mg/L)
You’ll see some white marks on taps and glass, and a light crust in the kettle over months. Most households get by with occasional descaling and the right hardness setting on the dishwasher. Austin, Texas, which treats its water with lime softening at the plant, delivers water in this band: about 93 mg/L on average.
Hard (121–180 mg/L)
Scale builds on heating elements and in shower heads, glasses spot in the dishwasher and soap leaves more scum. The USGS points to lower efficiency in electric water heaters as scale builds. This is where many households start to weigh up a softener.
Very hard (over 180 mg/L)
The USGS notes that utilities generally try to keep water out of this category, because it scales equipment and increases soap use. Above about 250 mg/L the crust comes back within weeks of cleaning. Las Vegas, at 280 mg/L, and San Antonio, at 15 to 20 gpg, are well inside it.
Other hardness scales you may see
The USGS scale is the one most US sources quote, but it isn’t the only one. Some state health departments and UK water companies draw the lines elsewhere, so a report that calls your water “moderately hard” may be using a different scale from the softener dealer who calls it “hard”.
| Label | USGS | Vermont Dept. of Health |
|---|---|---|
| Soft | 0–60 | under 75 |
| Moderately hard | 61–120 | 76–150 |
| Hard | 121–180 | 151–300 |
| Very hard | over 180 | over 300 |
UK water companies often add Clark degrees, an old British unit (one grain of calcium carbonate per imperial gallon). Anglian Water’s fact sheet, for example, gives 250 mg/L as an example of “hard” water and puts it at 17.4 °Clark. Because an imperial gallon is bigger than a US gallon, Clark degrees run about 20% higher than gpg for the same water.
Reading the scale correctly: four traps
“Hardness” vs “calcium”. Some reports list calcium on its own, in mg/L of calcium rather than of calcium carbonate. Calcium alone is a smaller number and leaves out magnesium. If that’s all you have, multiplying calcium by 2.5 converts it to calcium carbonate, but the result still excludes magnesium, so treat it as a minimum.
A range, not one number. Many utilities publish a low, a high and an average. For appliance and softener settings, the high figure is the safer choice, because a setting that’s too low lets hard water through.
gpg vs Clark. A UK appliance asking for Clark degrees and a US softener asking for gpg want different numbers for the same water. Convert rather than copying.
Total dissolved solids. TDS includes sodium, chloride and other salts. It’s not hardness, and a TDS meter can’t tell you your hardness.
Frequently asked questions
What is a good water hardness level?
There’s no health target: the EPA sets no limit for hardness. For appliances and soap, soft to moderately hard water (under about 120 mg/L, or 7 gpg) causes the least trouble.
Is 7 gpg hard water?
7 gpg is 120 mg/L, the top of the USGS “moderately hard” band. One grain more and it’s “hard”.
Is 300 ppm hard water?
Yes. 300 ppm is 17.5 gpg, very hard on the USGS scale and hard on Vermont’s.
Is 15 gpg very hard?
Yes. 15 gpg is 257 mg/L, well over the 180 mg/L line for very hard water.
Are ppm and mg/L the same?
For drinking water, yes. One litre of water weighs about one kilogram, so one milligram per litre is one part per million.
