Water hardness scale
Water hardness is classified on a four-band scale: soft (0–60 mg/L as CaCO₃), moderately hard (61–120), hard (121–180), and very hard (over 180) — the USGS definition this site uses. Enter your number below to see your band and how it compares with 2,977 measured US counties.
Hardness is a single number — the calcium and magnesium dissolved in your water, reported in milligrams per litre (mg/L) as calcium carbonate. The scale below turns that number into a plain band, from soft to very hard. Enter your reading to place it, or read on for the full unit table, how US counties compare, and where your number comes from.
How hard is your water?
Enter your hardness number in any unit — see your band, your position on the USGS scale, and how you compare with 2,977 measured US counties.
Bands
The scale in every unit
Labs, water utilities, softener manuals, and imported charts all describe the same quantity in different units. Here are the four USGS bands in each — mg/L (identical to ppm), grains per gallon, and German degrees (°dH) — with how many measured US counties land in each band.
| Band | mg/L as CaCO₃ | ppm | gpg | °dH | US counties |
|---|---|---|---|---|---|
| Soft | 0–60 | 0–60 | 0–3.5 | 0–3.4 | 862 |
| Moderately hard | 61–120 | 61–120 | 3.5–7 | 3.4–6.7 | 474 |
| Hard | 121–180 | 121–180 | 7–10.5 | 6.7–10.1 | 393 |
| Very hard | over 180 | over 180 | over 10.5 | over 10.1 | 1,248 |
ppm ≡ mg/L in dilute water; gpg = mg/L ÷ 17.118; °dH = mg/L ÷ 17.848. County counts: 2,977 gated US counties (USGS/EPA Water Quality Portal).
Distribution
Where the US falls on the scale
Across the 2,977 US counties with enough public data to map, 55% land in the hard or very-hard bands and only 29% are soft — nationally, hard water is the rule, not the exception. The bar below shows the split; the darkest slice is the 1,248 very-hard counties. See where they are on the national water hardness map.
Scales
USGS vs WQA vs state-health scales
Not every body draws the lines in the same place. This site uses the federal USGS bands, because they are measurement-derived and match our county data — but a softener dealer or a state health page may class your water differently. The table reconciles the common scales, each row with its source, so a number that reads “hard” in one place and “moderately hard” in another makes sense.
| Scale | Soft | Slightly hard | Moderately hard | Hard | Very hard | Source |
|---|---|---|---|---|---|---|
| USGS (used here) | 0–60 | — | 61–120 | 121–180 | over 180 | USGS Water Science School |
| WQA | under 17.1 | 17.1–60 | 60–120 | 120–180 | over 180 | Water Quality Association classification (as published in WQA/Pentair reference charts) |
| Vermont Dept. of Health | — | — | — | ≥ 150 mg/L classed “hard” | healthvermont.gov | |
All thresholds in mg/L as CaCO₃ (÷17.118 for gpg). Rows reflect each body's published classification, not a paraphrase.
Your number
Reading your number
Your hardness number comes from one of a few places. Your utility's annual water quality report — the CCR — sometimes prints it, though many list only the calcium and magnesium you add together to get it. A home test strip or drop-kit gives a quick reading in a few minutes; a lab titration is the precise reference method behind most reported values. Or skip the test entirely: our ZIP-code lookup returns the measured median for your county straight from public USGS/EPA data.
However you get the number, the unit matters — labs report mg/L (the same figure as ppm), water softeners are set in grains per gallon, and European charts use °dH. The hardness converter moves between them, and the band table above tells you where you land. If your reading falls in the hard or very-hard range, the everyday signs of hard water — spotting on glasses, scale on the kettle, stiff laundry — usually follow, and the home test guide walks through confirming it yourself.
Questions
Frequently asked
- What is a normal water hardness level?
- There is no federal hardness standard — the EPA sets no maximum for hardness. Across 2,977 measured US counties the median is 140 mg/L as CaCO₃, which is “hard” on the USGS scale, so a typical US tap sits on the harder side of the four-band scale.
- Is there an ideal water hardness?
- There is no health-based ideal and no EPA limit. Water in the moderately-hard 60–120 mg/L range is often cited in industry references as producing the fewest scale and lathering complaints, but that is an aesthetic and appliance consideration, not a health recommendation.
- Is 25 ppm hard water?
- No. 25 ppm equals about 25 mg/L as CaCO₃ (ppm and mg/L are equivalent in water), which is “soft” — it sits in the 0–60 mg/L band, well below the 120 mg/L hard threshold. Enter any number in the classifier above to place it.
- What’s the difference between the USGS and WQA scales?
- The USGS scale used here splits hardness into four bands at 60, 120, and 180 mg/L. The Water Quality Association adds a fifth “slightly hard” band with a 17.1 mg/L cutoff; this site standardizes on the federal USGS bands because they are measurement-derived and match our county data.
- Is hardness measured in ppm or gpg?
- Both, plus mg/L. Hardness is reported in mg/L as CaCO₃ (identical to ppm in dilute water) and in grains per gallon (gpg). 1 gpg = 17.118 mg/L, so divide mg/L by 17.118 to get gpg. See the mg/L to gpg converter for exact conversions.
By Marko Visic, MPharm, BSc Physics. Source: USGS/EPA Water Quality Portal (2,977 gated counties). Data last verified . Methodology.
Cite this page: “Water Hardness Scale.” Hard Water Map. Retrieved 2026-07-22, https://hardwatermap.com/water-hardness-scale/