Water Hardness Scale: From Soft to Very Hard
The standard scale: USGS classifies water by its hardness as calcium carbonate: 0 to 60 mg/L soft, 61 to 120 moderately hard, 121 to 180 hard, and over 180 very hard (USGS). WHO uses the same breakpoints (WHO). The softener industry often uses a five-level chart in grains per gallon: under 1 soft, 1 to 3.5 slightly hard, 3.5 to 7 moderately hard, 7 to 10.5 hard, and over 10.5 very hard (Pentair). One mg/L equals one ppm, and 17.1 mg/L equals one grain per gallon.
The USGS and WHO Scale
Water hardness is the amount of dissolved calcium and magnesium in water, reported as the equivalent amount of calcium carbonate (CaCO3). That is why lab reports and city water reports say "mg/L as CaCO3." The US Geological Survey uses four classes:
| USGS Class | mg/L (ppm) as CaCO3 | Grains per Gallon (approx.) |
|---|---|---|
| Soft | 0 to 60 | 0 to 3.5 |
| Moderately hard | 61 to 120 | 3.6 to 7.0 |
| Hard | 121 to 180 | 7.1 to 10.5 |
| Very hard | Over 180 | Over 10.5 |
Classes from USGS Water Science School; grains per gallon calculated at 17.1 mg/L per grain.
The World Health Organization's background document on hardness uses the same four breakpoints: below 60 mg/L generally considered soft, 60 to 120 moderately hard, 120 to 180 hard, and more than 180 very hard. This is the scale most US utilities, extension services and our own city guides use.
The Grains per Gallon Chart
Water softener makers and dealers usually talk in grains per gallon (gpg), because softeners are sized in grains. Many use a five-level chart that adds a "slightly hard" band. Pentair's water hardness reference chart, for example (Pentair):
| Description | Grains per Gallon | PPM |
|---|---|---|
| Soft | Less than 1.0 | Less than 17.1 |
| Slightly hard | 1.0 to 3.5 | 17.1 to 60 |
| Moderately hard | 3.5 to 7.0 | 60 to 120 |
| Hard | 7.0 to 10.5 | 120 to 180 |
| Very hard | 10.5 and over | 180 and over |
The upper three bands match USGS exactly. The difference is at the bottom: USGS calls everything under 60 mg/L "soft," while this chart reserves "soft" for water under about 17 mg/L, which is close to what a working water softener produces. So if a softener dealer calls your 50 mg/L water "slightly hard" and your utility calls it "soft," both are using a legitimate scale.
Combined Hardness Chart
| mg/L (ppm) | gpg | °dH (German) | °fH (French) | mmol/L | USGS Class |
|---|---|---|---|---|---|
| 17 | 1.0 | 1.0 | 1.7 | 0.17 | Soft |
| 60 | 3.5 | 3.4 | 6 | 0.6 | Soft (upper limit) |
| 120 | 7.0 | 6.7 | 12 | 1.2 | Moderately hard (upper limit) |
| 150 | 8.8 | 8.4 | 15 | 1.5 | Hard |
| 180 | 10.5 | 10.1 | 18 | 1.8 | Hard (upper limit) |
| 250 | 14.6 | 14.0 | 25 | 2.5 | Very hard |
| 350 | 20.5 | 19.7 | 35 | 3.5 | Very hard |
Calculated from mg/L as CaCO3 using the conversion factors below, rounded.
Converting Between Units
Everything converts through mg/L as calcium carbonate:
- mg/L and ppm: the same for practical purposes. 1 mg/L = 1 ppm.
- Grains per gallon (gpg): divide mg/L by 17.1. Multiply gpg by 17.1 to get mg/L. (A grain is a unit of weight; 1 grain per US gallon is about 17.1 mg per liter.)
- German degrees (°dH): 1 °dH is 10 mg/L of calcium oxide, equal to about 17.8 mg/L as CaCO3. Divide mg/L by 17.8. Common on European appliances such as dishwashers and coffee machines.
- French degrees (°fH): 1 °fH = 10 mg/L as CaCO3. Divide mg/L by 10.
- mmol/L: calcium carbonate has a molar mass of about 100 g/mol, so 1 mmol/L is about 100 mg/L as CaCO3. Divide mg/L by 100.
Watch the "as" part. Some reports list calcium (Ca) and magnesium (Mg) separately in mg/L rather than total hardness as CaCO3. To convert, multiply calcium by 2.5 and magnesium by 4.1 and add them. Those factors come from the ratio of calcium carbonate's molar mass to each metal's (about 100/40 and 100/24.3). Our testing guide covers reading results.
What Each Level Means at Home
| Level | What You Typically Notice | Usual Fix |
|---|---|---|
| Soft (under 60 mg/L) | Easy lather, little scale | No hardness treatment needed |
| Moderately hard (61 to 120) | Some kettle scale and spotting over time | Descale kettles and shower heads; rinse aid in the dishwasher |
| Hard (121 to 180) | Regular scale, soap scum, spotty glasses, coated-feeling hair | A water softener is worth considering; chelating shampoo for hair |
| Very hard (over 180) | Heavy scale, faster appliance wear, tankless heater problems | A water softener is the most effective fix |
Hard water is a nuisance, not a health risk: USGS calls hardness "not a health concern." See is hard water bad for you? for the research.
Useful Thresholds
- About 120 mg/L (7 gpg): when a softener may help. The Minnesota Department of Health says that if hardness is greater than 7 grains per gallon or 120 mg/L, you might need a water softener to keep appliances running well (MDH).
- 200 mg/L (about 11.7 gpg): tankless water heaters. Rinnai's installation manual gives a total hardness guideline of up to 200 mg/L and says hard water must be treated (Rinnai). See tankless scale prevention.
- Iron adds to your setting. When programming a softener, manufacturers typically add about 5 gpg for every 1 ppm of iron (compensated hardness). See our Fleck review for an example.
- Sodium after softening: about 8 mg/L of sodium is added per grain removed. See is softened water safe to drink?
Real Examples From US Cities
From official water quality reports summarized in our city guides:
| City | Reported Hardness | Where It Falls |
|---|---|---|
| Seattle | 0.5 to 1.7 gpg | Soft |
| Los Angeles (aqueduct water) | 103 mg/L, about 6 gpg | Moderately hard |
| Dallas | 120 to 155 mg/L, about 7 to 9 gpg | Moderately hard to hard |
| Chicago (Lake Michigan) | 143 mg/L, 8.4 gpg | Hard |
| Tampa | 211 mg/L, about 12.3 gpg | Very hard |
| Phoenix | 172 to 302 ppm, 10 to 17.6 gpg | Hard to very hard |
Details and sources: Los Angeles, Dallas, Chicago, Phoenix and the city guides hub for Seattle, Tampa and more. Hardness can vary within a city and through the year, so check the report for your area.
How to Find Your Number
- City water: look up your utility's annual water quality report (Consumer Confidence Report). Hardness is often listed under "secondary" or "other" parameters.
- Quick home test: hardness test strips give a range in a minute.
- Precise number for sizing a softener: a drop-count titration kit such as the Hach 5-B, where each drop equals 1 gpg.
- Well water: use a certified lab, which also checks iron, manganese and health-related contaminants.
Compare options in our test kit guide, or start with is my water hard or soft?
Check Hardness Test Strips on Amazon
Frequently Asked Questions
USGS and WHO classify water as soft (0 to 60 mg/L as calcium carbonate), moderately hard (61 to 120), hard (121 to 180) and very hard (over 180). The softener industry also uses a grains per gallon chart: under 1 soft, 1 to 3.5 slightly hard, 3.5 to 7 moderately hard, 7 to 10.5 hard, over 10.5 very hard.
Under 60 mg/L (3.5 gpg) is soft by the USGS scale and causes little scale. The Minnesota Department of Health says a softener may help when hardness is above 120 mg/L (7 gpg). Hardness is not a health concern at any of these levels.
By the USGS scale, 121 to 180 ppm is hard and over 180 ppm is very hard. 61 to 120 ppm is moderately hard.
Divide ppm (mg/L as calcium carbonate) by 17.1. For example, 171 ppm is 10 grains per gallon.
About 171 mg/L. That is hard on both the USGS scale (121 to 180 mg/L) and the grains per gallon chart (7 to 10.5 gpg), close to the very hard line at 10.5 gpg.
German degrees of hardness. 1 degree dH equals 10 mg/L of calcium oxide, or about 17.8 mg/L as calcium carbonate. It is common on European appliance settings.
They use different bottom bands. USGS calls everything under 60 mg/L soft, while many softener charts call only water under about 17 mg/L soft and 17 to 60 slightly hard. The upper bands are the same.
Last updated: October 2026. Sources: USGS, WHO Hardness in Drinking-water, Pentair water hardness chart, Minnesota Department of Health, Rinnai installation manual.