What Is Hard Water? Calcium, Magnesium, Hardness Units and How to Find Your Number
Hard water is water carrying a relatively high concentration of dissolved calcium and magnesium, almost always reported as an equivalent amount of calcium carbonate (CaCO3). The term describes a chemical property rather than something you can see: you cannot tell by looking at a glass of water whether it counts as hard, though scale building up on a kettle element or soap that will not lather easily are common signs further down the line. Limescale is the chalky, often white or grey deposit that forms when some of that dissolved calcium comes out of solution, typically where the water is heated or left to evaporate.
- Hardness specifically measures dissolved calcium and magnesium, not every dissolved substance in the water; that broader measure has its own name, total dissolved solids (TDS), and its own article.
- Utilities and labs report hardness in several different units depending on the country, including mg/L as CaCO3, grains per gallon, and degrees German, French or English, and they all describe the same underlying mineral concentration.
- Hardness comes in two chemical forms, temporary and permanent, and only one of them goes away when you boil the water.
- A water-quality report from your own supplier is the most reliable way to find your household's actual number, because hardness varies by region and sometimes street by street within the same town.
What dissolved calcium and magnesium actually do
When geologists and water utilities talk about hardness, they mean the combined concentration of calcium and magnesium ions dissolved in the water, expressed as the equivalent weight of calcium carbonate that would contain the same amount of those two metals (source: USGS Water Resources — Explanation of Water Hardness). A small amount of other dissolved metals can contribute as well, but calcium and magnesium are what the measurement is built around.
It helps to separate hardness from the wider idea of total dissolved solids. TDS adds up everything dissolved in the water, from sodium and chloride to sulfate and whatever trace minerals happen to be present, and it has its own meter readings and guidelines, covered in a separate look at what TDS actually measures. Hardness is a narrower, specific slice of that total: two particular minerals, measured on their own scale, because they are the ones responsible for scale formation and the feel of the water against skin and hair.
Where hard water comes from
The mineral content starts in the ground long before it reaches a tap. Rain absorbs a small amount of carbon dioxide as it falls, which makes it mildly acidic. As that slightly acidic water percolates through limestone, chalk or gypsum bedrock, it dissolves calcium and magnesium carbonates and sulfates out of the rock and carries them into the groundwater (source: Wikipedia — Hard water). Regions sitting on that kind of geology end up with naturally mineral-rich supplies.
Catchments built on harder, more impervious rock such as granite behave differently: there is far less calcium or magnesium available to dissolve, so the water that collects there stays naturally soft. That geological difference is the reason hardness can vary sharply between two towns only a short drive apart, and why a national average figure tells you very little about your own supply.
How hard water is measured: units and conversions
A single hardness reading can arrive dressed in several different units, depending on where the report comes from. mg/L as CaCO3 is the most widely used reference unit internationally and is numerically identical to parts per million (ppm). US utility reports commonly use grains per gallon (gpg) instead. Older or European documentation may show degrees German (°dH), French (°fH) or English/Clark (°e). All of them are just different ways of expressing the same underlying calcium-plus-magnesium concentration, so converting between them changes the number on the page, not the water itself.
Hardness unit conversion table
| Unit | Equivalent to 1 mmol/L Ca+Mg | Where you typically see it |
|---|---|---|
| mg/L as CaCO3 (ppm) | 100.1 | International reference unit; most lab and utility reports |
| Grains per gallon (gpg) | 5.847 | US water-softener and utility literature |
| Degrees German (°dH) | 5.608 | Germany, Austria, Switzerland, some appliance manuals |
| Degrees French (°fH) | 10.01 | France and some other continental European reports |
| Degrees English / Clark (°e) | 7.022 | Older UK references |
(Conversion figures per source: Wikipedia — Hard water.) If your report already states a unit, there is usually no need to convert anything: the classification bands further down use mg/L as CaCO3, so that is the one worth converting to if your own reading arrives in gpg or degrees.
Temporary and permanent hardness are not the same problem
Total hardness on a report is a single number, but it is actually made up of two chemically different contributions, and the distinction matters for what you can do about each one. Temporary (carbonate) hardness comes from dissolved calcium and magnesium bicarbonates. Heating the water drives off dissolved carbon dioxide and precipitates calcium carbonate out of solution, which is exactly why kettles, boilers and hot-water pipes are where visible scale tends to build up first; boiling the water or lime softening both reduce this type (source: Wikipedia — Hard water).
Permanent (non-carbonate) hardness is caused mainly by calcium and magnesium sulfates and chlorides. These stay fully dissolved even when the water is heated, so boiling does nothing to reduce them. Addressing permanent hardness means removing the calcium and magnesium ions themselves, which is what an ion-exchange water softener is built to do. In practice, most tap water contains some of both, which is why "just boil it" only ever solves part of the scaling problem rather than all of it.
How hard is "hard"? The classification bands
Once you have your own figure in mg/L as CaCO3, the commonly used classification sets four bands: soft is 0–60, moderately hard is 61–120, hard is 121–180, and very hard is anything above 180 (source: USGS Water Resources — Explanation of Water Hardness). Other organizations publish their own versions of this scale with slightly different cut-off points, so treat the bands above as a reference figure rather than the only correct answer.
It is worth being precise about where hardness sits in US regulation, because the wording on product pages and forums often overstates it. The EPA's secondary standards are non-enforced aesthetic guidelines, not health-based limits, and hardness itself does not appear in that table as its own regulated characteristic. It shows up only indirectly, as a noticeable effect once total dissolved solids pass the secondary guideline of 500 mg/L (source: EPA — Secondary Drinking Water Standards). In other words, federal guidance treats hardness as a nuisance characteristic that affects taste, appliances and plumbing, not as a contaminant with a mandatory health limit. Whether that nuisance framing understates or overstates the practical downsides for a household is a separate question, answered in more depth in a closer look at whether hard water is actually bad for you.
Finding your own number
The most reliable figure is the one your own supplier publishes. In the US, every community water system issues an annual water quality report, often called a Consumer Confidence Report, and hardness is typically listed there alongside calcium, magnesium and TDS. Outside the US, the local supplier's equivalent report, or a call to their customer service line, is the same first stop, and it will reflect your actual neighborhood rather than a national average.
When no report is available, or when you want to check your own household plumbing directly, a home test strip or a handheld meter is the usual alternative. Either category will give you a working number without needing a lab, though the method and the brand you choose are a separate decision from understanding what the number means.
Where shower and tap filters fit — and where they don't
It is worth being clear about what a carbon or KDF filter actually changes, because the names on some of these products can suggest more than the specification sheet backs up. The Filtered Showerhead's own product page describes a 20-stage cartridge built around KDF-55 and calcium sulfite media, with a 1/2" BSP thread and roughly three months of use per cartridge, and states that it filters chlorine and heavy metals. Nothing on that page claims that the cartridge softens the water or reduces dissolved calcium and magnesium.
That distinction is worth keeping in mind whatever filter you are looking at: carbon and KDF media are built to deal with chlorine, taste, odour and particulates, and that is a genuinely useful thing for a shower or a kitchen tap to do. If limescale itself, rather than chlorine taste or metal particulates, is the problem you are trying to solve, the relevant product category is a water softener or conditioner, which works through ion exchange rather than carbon filtration; it is a different purchase for a different job, named here neutrally rather than as a pitch. For point-of-entry treatment that covers a whole house rather than a single shower or tap, the whole-house filtration guide walks through how that category works.
With your own number in hand and the right unit converted, you can read any water-quality report with some confidence: whether you are dealing mostly with the temporary kind that a kettle complains about, or the permanent kind that only a softener addresses, and which category of product is actually built to help with each. If visible scale on taps and tiles is already the pressing issue, removing hard water stains without damaging the finish underneath is covered separately.