Hard water is water with a lot of dissolved calcium and magnesium, picked up as it moves through rock such as limestone and chalk. The USGS calls water hard above 120 mg/L (7 grains per gallon) and very hard above 180 mg/L (10.5 gpg). It is not a health concern, but it leaves scale in kettles and water heaters and makes soap harder to rinse off.
On this page
- The hard water scale in every unit
- Where hard water comes from
- How much mineral goes through your pipes in a year
- Signs you have hard water
- Is hard water bad for you?
- How to find out how hard your water is
- Why one report says “hard” and another says “moderately hard”
- What you can do about hard water
- Frequently asked questions
That’s the short version. Below you’ll find the scale in every unit you’re likely to meet, the chemistry behind the white crust, how much mineral actually flows through a home in a year, and how to find the figure for your own tap.
The hard water scale in every unit
Hardness is measured as the amount of calcium carbonate that the calcium and magnesium in the water would be equivalent to. US reports give it in mg/L or ppm (the same thing for water), softeners use grains per gallon, and appliances made for Europe use German, French or Clark degrees. The table puts the USGS boundaries in all of them.
| USGS class | mg/L or ppm | gpg | °dH (German) | °f (French) | °e (Clark) | mmol/L |
|---|---|---|---|---|---|---|
| Soft | 0–60 | 0–3.5 | 0–3.4 | 0–6 | 0–4.2 | 0–0.6 |
| Moderately hard | 61–120 | 3.6–7.0 | 3.4–6.7 | 6.1–12 | 4.3–8.4 | 0.6–1.2 |
| Hard | 121–180 | 7.1–10.5 | 6.8–10.1 | 12.1–18 | 8.5–12.6 | 1.2–1.8 |
| Very hard | over 180 | over 10.5 | over 10.1 | over 18 | over 12.6 | over 1.8 |
For every reading from 1 to 30 gpg, see the full water hardness scale.
Got a number in a different unit? Type it in here and the converter shows every other unit and the class.
This converter needs JavaScript. The table above and the factors under it let you convert by hand.
The same tool has its own page: the water hardness converter. For the maths behind it, see grains per gallon to ppm.
Where hard water comes from
Rain falls almost free of minerals. It turns hard on the way to your tap. Groundwater that sits in limestone, chalk or dolomite dissolves some of the rock, and rivers pick up minerals over the hundreds of miles they travel. The USGS puts it simply: hardness is the amount of dissolved calcium and magnesium in the water.
That’s why hardness follows geology rather than city limits. In Texas, San Antonio’s limestone aquifer gives 15 to 20 gpg, while Austin, which draws lake water and softens it at the plant, averages 5.4 gpg. Las Vegas, which draws most of its water from the Colorado River via Lake Mead, sits at 280 mg/L, or 16 gpg. Anglian Water, in the chalk country of eastern England, describes its whole region as hard to very hard for the same reason: the water runs through rock that is largely calcium carbonate.
Temporary and permanent hardness
Chemists split hardness into two parts, and the difference explains why kettles scale up and cold taps mostly don’t.
Temporary hardness is calcium and magnesium paired with bicarbonate. Heat drives carbon dioxide out of the water, and the calcium drops out as solid calcium carbonate: limescale.
Permanent hardness is calcium and magnesium paired with sulfate or chloride. Boiling doesn’t remove it, and it doesn’t form the same crust. Your report’s “total hardness” figure is the two added together, which is why two towns with the same total can scale kettles at different rates. If yours has already crusted up, here’s how to descale a kettle.
How much mineral goes through your pipes in a year
Hardness figures look small because they’re per litre. Multiply by a household’s water use and the numbers stop looking small. This table assumes 80 gallons per person per day, the bottom of the USGS range for indoor use, and counts the dissolved hardness (as calcium carbonate) that passes through the house in a year.
| Hardness | USGS class | 1 person | 2 people | 4 people |
|---|---|---|---|---|
| 50 mg/L (2.9 gpg) | Soft | 5.5 kg | 11.1 kg | 22.1 kg |
| 100 mg/L (5.8 gpg) | Moderately hard | 11.1 kg | 22.1 kg | 44.2 kg |
| 150 mg/L (8.8 gpg) | Hard | 16.6 kg | 33.2 kg | 66.3 kg |
| 200 mg/L (11.7 gpg) | Very hard | 22.1 kg | 44.2 kg | 88.4 kg |
| 280 mg/L (16.4 gpg) | Very hard | 30.9 kg | 61.9 kg | 123.8 kg |
| 350 mg/L (20.4 gpg) | Very hard | 38.7 kg | 77.4 kg | 154.7 kg |
Most of that leaves down the drain still dissolved. The part that causes trouble is what drops out where water is heated or evaporates: the water heater, the kettle, the dishwasher, the shower head and the tap spout where drops dry. Even a small fraction of 124 kg is a lot of scale, which is why a family of four on very hard water notices it within weeks.
Signs you have hard water
- A white or grey crust inside the kettle and around tap spouts.
- Shower heads that spray unevenly as nozzles block.
- Spots and a cloudy film on glasses straight out of the dishwasher.
- Soap and shampoo that lather poorly and take longer to rinse off. The USGS explains that calcium and magnesium react with soap to form scum that clings to your skin.
- A scum on tea or coffee. Anglian Water notes that scale reacts with tannins to leave a brown film; it’s harmless, just unappealing.
Any one of these can have other causes. Together, they’re a strong hint, and the only way to be sure is to look up the number or test it.
Is hard water bad for you?
No. The USGS describes hardness as “not a health concern” but a nuisance, and the EPA has no legal limit for it. The Vermont Department of Health says the same and adds a sense of scale: an 8-ounce glass of moderately hard water holds roughly 50 to 75 mg of calcium, against about 300 mg in a glass of milk. Anglian Water tells its customers hard water is safe to drink.
The cost is to your home rather than your health. The USGS points to scale that lowers the efficiency of electric water heaters, pipes that narrow over time, more soap and detergent per wash, and spotted glassware. Vermont adds that hardness scales up the membranes of reverse osmosis units and the bulbs of UV systems, so other water treatment works less well on hard water.
Softening changes one thing worth knowing about. A salt-based softener swaps calcium and magnesium for sodium, adding about 0.46 mg of sodium for every mg of hardness removed, according to Vermont’s health department. That’s why water companies such as Wessex Water suggest keeping the kitchen tap unsoftened for drinking and cooking. If you’re on a sodium-restricted diet, ask your doctor before drinking softened water.
How to find out how hard your water is
On mains water in the US
Every community water system has to send customers an annual water quality report, the Consumer Confidence Report, by July 1. Search your utility’s name with “water quality report”, or use the EPA’s CCR finder. Hardness usually sits in a table of “secondary” or “other” parameters. Watch the unit: many reports give mg/L, some give gpg, and a few give both. Our water hardness lookup and water hardness by city list the cities we’ve checked in the utility’s own document.
On mains water in the UK
Most water companies have a postcode checker on their website. They often quote mg/L of calcium carbonate alongside Clark, French or German degrees for setting appliances.
On a private well
Nobody tests it for you. Vermont’s health department recommends testing a private well or spring for hardness every five years, as part of a wider chemical test. Test strips give a quick reading in gpg or ppm; a lab test is more precise and can catch iron, which matters if you’re sizing a softener.
The soap test
Shake a few drops of pure liquid soap (not detergent) in a bottle of tap water. Lots of lasting suds suggests soft water; a cloudy, flat result suggests hard. It’s a rough guide, useful only to decide whether a proper test is worth doing.
Why one report says “hard” and another says “moderately hard”
There’s no single official scale. The USGS, state health departments and UK water companies draw the lines in different places, so the same water can carry different labels:
| Label | USGS | Vermont Dept. of Health | Anglian Water (reference points) |
|---|---|---|---|
| Soft | 0–60 | under 75 | 75 |
| Moderately hard | 61–120 | 76–150 | – |
| Hard | 121–180 | 151–300 | 250 |
| Very hard | over 180 | over 300 | 350 |
Take 160 mg/L. It’s “hard” for the USGS and “hard” in Vermont, but only just in Vermont’s lower band and close to the top of the USGS one. The number matters more than the label, and it’s the number your softener, dishwasher or coffee machine will ask for.
What you can do about hard water
Live with it and descale. At moderate hardness, many households just clean scale off now and then. Our guide on how to remove limescale covers kettles, shower heads, taps and water heaters.
Set your appliances. Dishwashers with a built-in softener need the hardness entered and the salt topped up. Wessex Water reminds customers to use the unit their manual asks for, which is often German or Clark degrees.
Install a water softener. It removes calcium and magnesium for the whole house by ion exchange. Not sure it’s worth it? Run through our do I need a water softener checklist. Size it to your hardness and household, not to a showroom recommendation: our guide to what size water softener you need shows the sum, and the guide to how much salt a softener uses shows what it costs to run.
Consider a conditioner. Salt-free conditioners don’t remove the minerals; Wessex Water describes them as changing the minerals so they’re less likely to form scale when heated. Results vary more than with a softener, so look for independent test data before buying.
Frequently asked questions
Does boiling water make it soft?
Partly. Boiling removes temporary hardness by turning it into scale, which is exactly what coats the inside of your kettle. Permanent hardness stays in the water.
Is 10 gpg hard water?
Yes. 10 gpg is 171 mg/L, inside the USGS “hard” band of 121 to 180 mg/L. Above 10.5 gpg it becomes very hard.
What is the difference between hard and soft water?
Only the amount of calcium and magnesium. Soft water has little, so soap lathers easily and feels slippery; hard water has more, so soap forms scum and heated surfaces collect scale. Full comparison: hard water vs soft water.
Is hard water safe to drink?
Yes, according to the USGS, the Vermont Department of Health and UK water companies such as Anglian Water. There’s no health-based limit for hardness.
Can hard water damage pipes?
Over many years scale can narrow pipes and reduce pressure, the USGS notes, mainly on hot water lines. The LVVWD adds a point in its favour: hard water is less corrosive to pipes than very soft water.
Does a softener make water soft enough to drink?
Softened water is drinkable for most people, but it carries extra sodium. Many water companies suggest keeping one unsoftened tap in the kitchen.
Sources
- USGS Water Science School: Hardness of Water (updated Aug 24, 2026)
- USGS FAQ: Do you have information about water hardness in the United States?
- Vermont Department of Health: What You Need to Know About Hardness in Drinking Water
- Anglian Water: Water quality fact sheet, hard water (PDF)
- Wessex Water: Limescale
- US EPA: Consumer Confidence Reports
