The hardness at which water produces limescale measures 200 mg/L as CaCO₃ of total hardness, the level World Health Organisation Guidelines 2017 identify as scale-forming, per Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does (read 20 September 2026).
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Drinking Water Inspectorate (DWI) enforces the UK drinking water quality regulations in England and Wales. Figures it classifies or regulates on this page cover England and Wales only.
What this page can tell you, and what it cannot
How much scale forms in a house depends on the water that arrives at its own stop tap, and that figure is published by the company that supplies it rather than by the regulator. Most of them publish it only one address at a time. So the first thing this page does is send a reader to check whether we hold theirs.
What it will not do is turn that figure into a verdict. No authority here publishes a hardness at which scale becomes a problem, and a level invented by a website is not a substitute for one.
Why the regulator is quoted rather than summarised
The passages below are printed in full and in the regulator's own words. Which things are named in which sentence is the whole of what they establish, and a summary is exactly the form in which that gets lost: two lists written a paragraph apart become one list the moment somebody shortens them.
Everything on this page that is not inside quotation marks is either a figure with its own source beside it, or a sentence about how the page was put together.
Scale, and the marks that are not scale
Hard water does two different things, and they are not the same thing seen twice. One is a deposit that builds up where water is heated. The other is what soap leaves behind on a cold surface, which can look similar where water is left standing and has a different cause.
The distinction decides what this site is able to say about any particular thing in a house. Where the regulator names something in the first sentence, this site can say so; where it names it only in the second, this site says that instead, and does not quietly promote it.
The hardness at which water produces scale
DWI relays a figure from World Health Organisation Guidelines 2017, and states it like this:
The World Health Organisation Guidelines 2017 identified that water with a hardness of value of 200 mg/l or higher (measured as CaCO3 calcium carbonate) will produce scale and soft water with a value of 100 mg/l (as CaCO3 calcium carbonate) or less will be more corrosive to pipework.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
DWI classifies 200 mg/L as CaCO₃ as Moderately hard (150–200).
What the regulator says hard water does
Scaling is defined as deposits of calcium and magnesium that build up on heating elements over time.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
“Temporary hardness” is water hardness due to the presence of calcium and magnesium carbonates and bicarbonates. These can be removed from the water by boiling, forming scale.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Hard water causes scaling in hot water systems, kettles, electric irons and domestic appliances.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Scaling of heating elements shortens their life and makes appliances less efficient.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Hard water produces less lather from soap, washing up liquid and washing powders. It also leaves ‘tide marks’ on basins, sinks, baths and toilets and a scum on the surface of hot drinks, especially tea brewed in the cup with a teabag (due to the air and oils in the tea).
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Some activated carbon jug filters also contain ion exchange resin beads which alter the “temporary” hardness of water so that the filtered water has a lower tendency to form scum on the top of hot drinks like tea and they may minimise scale build up in kettles. These devices do not alter the permanent hardness of the water.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Where water is softened in this way by base exchange softening it is important to provide an unsoftened outlet for drinking purposes. Installation of a softener just before the hot water tank or boiler is a more economical method for preventing precipitation of hardness salts (limescale) than softening the whole supply.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Some devices are sold on the basis that they produce a magnetic field which reduces scaling by altering the shape of the crystals from needle like to rhomboid – which means they are less adherent to the heating elements in boilers. These devices do not soften the water. The science behind them is based on continuous water flow and they were designed originally for large industrial water systems, not for the household situation, where water tends not to flow for up to eight hours in 24 hours (at night). If you are considering purchasing such a device it is recommended you do so only on a sale or return basis, and that you request data on performance in the home setting.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — read 20 September 2026
Your own water
How much scale forms starts from the hardness of the water arriving at your own stop tap, and this site holds a measured figure for 39 supply zones, published by 2 of 18 water suppliers. Most postcodes resolve to a supplier that publishes none, and the checker says so rather than estimating one.
Check what we hold for your postcode
Lichfield
The figure published for this area is 167.4–280.5 mg/L as CaCO₃, which is partly below that level and partly at or above it.
This supply runs 167.4–280.5 mg/L as CaCO₃, varying by supply zone. That spans DWI’s Moderately hard (150–200) to Hard (200–300) bands, so no single band describes it.
Hardness figure: South Staffordshire Water — supply-zone water quality reports — dated 31 December 2025, read 20 September 2026
What nobody publishes, and what we will not write
At what hardness does limescale become a problem?
What was looked for: a hardness above which a United Kingdom authority says scale becomes a problem, or a rate at which it says scale forms. Where: the Drinking Water Inspectorate's own consumer page on water hardness, which is the page that publishes the five-band scale and the hardness at which water will produce scale.
What it says instead: “The Drinking Water Directive and the UK drinking water quality regulations do not specify standards for hardness, calcium or magnesium.”
This site states none — not at 280.5 mg/L as CaCO₃, the hardest figure on this page, and not at any other figure.
What would change this: a UK regulator, a UK standards body or a UK statutory guideline stating a hardness at or above which scale is treated as a problem. The World Health Organisation figure the regulator relays is about scale forming in water, not about a level at which anybody should act, and this site states it as the former.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — dated 20 September 2026, read 20 September 2026
What does limescale cost?
What was looked for: a figure for what scale costs — an efficiency penalty as a percentage, an appliance life in years, or a price for descaling or for replacing what scale shortened. Where: the Drinking Water Inspectorate's own consumer page on water hardness, which is where the effect is stated, and this project's own committed price observations, of which there are none for this country.
What it says instead: “Scaling of heating elements shortens their life and makes appliances less efficient.”
What would change this: a UK regulator, a UK energy body or a named study measuring the efficiency penalty or the appliance life, plus dated prices for what any of that would be multiplied by. Both are outstanding, so every line here states the effect in the regulator's words and attaches no number to it.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — dated 20 September 2026, read 20 September 2026
How do you remove it?
What was looked for: a method for removing scale from a kettle, a toilet or any other appliance, published by a United Kingdom authority. Where: the Drinking Water Inspectorate's own consumer page on water hardness, which is the UK regulator's consumer page on the subject and the page that answers the softening question directly.
What it says instead: “Please read our Domestic water filters and softeners leaflet”
What would change this: the leaflet that sentence points at, which no task in this split has read, or another UK authority publishing a descaling method. Until one of those is read and dated, this site describes what scale is and where an area's water sits, and publishes no method of its own.
Source: Drinking Water Inspectorate — Water Hardness / Hard Water, including what it says hard water does — dated 20 September 2026, read 20 September 2026