Is limescale bad for you?
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Part of our hard water guide
There is a white crust in the kettle and a film on your tea. Is that a problem for you, or only for the kettle?
Short answer: The UK drinking water regulations set no standard for hardness, calcium or magnesium, says the Drinking Water Inspectorate (DWI). The Scottish regulator calls limescale and tea scum harmless, and Which? calls it harmless to consume. The cost falls on appliances, because the DWI says water at 200 mg/l or more will produce scale.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
Is limescale bad for you?
No UK official source we read says it is. The table sets out what each body says, and we stop there. To see how hard your own tap is, a 20-in-1 test strip gives a hardness reading in under a minute.
| Who | What it says |
|---|---|
| Drinking Water Inspectorate (DWI) | The Drinking Water Directive and the UK drinking water quality regulations do not specify standards for hardness, calcium or magnesium. It adds that there is some limited evidence of a link between hardness and cardiovascular health, possibly through the calcium and magnesium, and recommends that companies which soften water keep total hardness at 150 mg/l or more. |
| World Health Organization (WHO), 2011 background document | Some evidence suggests a protective effect of magnesium or hardness on deaths from cardiovascular disease, but the evidence is being debated and cause has not been established. It found too few data to suggest minimum or maximum mineral concentrations, so it proposed no guideline values. |
| Drinking Water Quality Regulator for Scotland (DWQR) | Says limescale in kettles and on fittings, and the slight scum on hot drinks, is harmless. |
| Which? (consumer organisation) | Calls limescale harmless to consume. Floating flakes and, in bad cases, a slightly bitter aftertaste are the downsides it names. |
The cautions in these pages are mostly about soft and softened water. The DWI says most water softeners swap hardness for sodium, so the kitchen drinking tap should stay unsoftened, because too much sodium can be a problem for premature babies and people on a low-salt diet. It adds that artificially softened water may be aggressive to plumbing and leach copper and lead. WHO says unstabilised soft water tends to corrode pipes, which can bring metals into the water. Our guides on drinking softened water and lead pipes take those points further.
Is limescale in a kettle bad for you?
Which? says no, and the Scottish regulator says the same about kettle scale and tea scum. Which? lists a floating flake in your cup, a possible slightly bitter aftertaste, and a kettle that takes longer and uses more energy to boil as the practical downsides. It advises descaling a kettle every month or so, more often for a glass kettle in a hard water area, then rinsing well and boiling clean water before drinking. Our kettle limescale guide shows what each hardness band looks like in a kettle. In the sources we read there is no official guidance on hygiene and old scale, so we say nothing on it.
What one reader found
A poster on a UK student forum said they grew up with parents who drink only bottled water and even put it in the kettle to avoid limescale. Staying at a boyfriend's mum's house, they noticed "her kettle has SO much limescale" and wondered whether it was harmful. Their boyfriend said it was fine, and a search said the same. They asked the forum whether the worry might be a placebo. Replies said limescale is not harmful, and one suggested a cup of vinegar to shift it. The official lines above are the ones to rely on, and the thread is one person's experience. Shared on The Student Room.
What is limescale and what causes it?
A hard, chalky deposit that is mainly calcium carbonate, which Which? says is usually off-white but can look grey, pink or reddish-brown. Hardness comes first and heat second.
The DWI says hardness is caused by dissolved calcium and magnesium and varies with local geology. WHO says the main natural sources are sedimentary rocks, most commonly limestone and chalk. Heat then does the rest. The DWI describes "temporary" hardness as calcium and magnesium carbonates and bicarbonates, which boiling removes from the water by forming scale. It defines scaling as deposits of calcium and magnesium that build up on heating elements over time. WHO adds that scale in heated water also depends on factors such as pH and alkalinity.
How hard is hard?
The DWI calls 200 to 300 mg/l hard and more than 300 mg/l very hard, measured as calcium carbonate, on a scale of five bands. The scale and corrosion lines come from the DWI, which cites the WHO's 2017 guidelines.
| DWI band | mg/l calcium carbonate | What the DWI says |
|---|---|---|
| Soft | Up to 100 | At 100 mg/l or less, water will be more corrosive to pipework |
| Slightly hard | 100 to 150 | No separate statement |
| Moderately hard | 150 to 200 | No separate statement |
| Hard | 200 to 300 | At 200 mg/l or higher, water will produce scale |
| Very hard | More than 300 | No separate statement (above the 200 mg/l scale figure, our note) |
What does limescale do to kettles and appliances?
It coats heating surfaces. The DWI says scaling affects hot water systems, kettles, electric irons and domestic appliances, shortens the life of heating elements and makes appliances less efficient. It also lists less lather from soap, tide marks on basins and baths, and scum on tea. WHO says scale in heated water applications coats surfaces and reduces the efficiency of heat exchangers, and that soft or softened water scales minimally. Which? says a scaled kettle takes longer and uses more energy to boil. In the sources we read there is no official figure for how much extra energy scale uses, so we give none.
Does the area hardness figure match what comes out of your tap?
Not necessarily. The area figure is a typical value, and your tap may differ. The DWI says hardness varies with local geology and that its map of typical hardness in England and Wales does not show areas with local variation. For an accurate measurement it says to ask your water company. The DWQR says most drinking water in Scotland is classified as soft, because chalk and limestone are not common there. We found no figure for a named town in the sources we read, so our guides to how hard your water is and to Scottish water are the next stop.
What happens inside the home matters too. The DWI says domestic softeners replace calcium and magnesium with sodium and that an unsoftened drinking outlet should be kept, so taps in one house can differ. It says some jug filters contain ion exchange resin that alters temporary hardness, and Which? says filtered water has fewer minerals. A figure for the area does not tell you what comes out of your own tap, and a strip dipped in a glass from that tap gives you a reading to set beside it.
“The area figure is only a typical value, so I would never assume it matches your own tap, especially if you have a softener or a filter jug in the house. A quick check from the tap you actually use is a sensible way to see where you really stand before you decide what to do about the kettle.”
How can you check the hardness at your own tap?
Fill a clean glass or cup from the tap, dip one strip, wait 30 to 60 seconds and match the hardness pad to the chart. You can also scan the QR code on the chart, or enter the colours at our free results checker, with no app or account. The hardness pad steps are 0, 25, 50, 100, 250 and 425 mg/l. The portal gives them without saying whether they are read as calcium carbonate, the unit of the DWI bands, so this table is our own reading of those steps against them.
| Pad step (mg/l) | DWI band it falls in |
|---|---|
| 0, 25, 50 and 100 | Soft, which is up to 100 |
| 250 | Hard, which is 200 to 300 |
| 425 | Very hard, which is over 300 |
There is no step between 100 and 250, so water in the slightly hard and moderately hard bands has no step of its own, and you match to the nearest. There is no UK standard for hardness, so no reading is a pass or a fail, and a strip result is not a legal sample. The strip cannot read lead at the UK limit, so for that question see our lead pipes guide. Our guide to testing hardness at home covers the method in more detail.
The area figure is an average, and your own tap may differ, because the DWI says its map does not show areas with local variation and that a domestic softener or jug filter can change what reaches a tap. The hardness pad is the one that matters here. A match at 250 or 425 indicates a reading above the 200 mg/l figure the DWI gives for scale, and a reading that drops a step or more after you fit a softener or filter indicates less hardness reaching that tap. Best pack: 20 strips, enough to test your kitchen tap and another tap now, and again after any change such as a jug filter or a softener, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your limescale questions
What is limescale?
A chalky deposit that is mainly calcium carbonate, as Which? describes it. The DWI defines scaling as calcium and magnesium deposits that build up on heating elements.
Is limescale bad for you?
The DWI says the UK regulations set no standards for hardness, calcium or magnesium, and the DWQR calls limescale harmless. See the table above for each body's wording.
Is limescale in kettle bad for you?
Which? calls it harmless to consume, and the Scottish regulator says kettle limescale and tea scum are harmless. Which? names flakes in the cup and a slower kettle as the downsides.
Is limescale harmful?
To appliances, the DWI says it shortens the life of heating elements and makes appliances less efficient. On health, the official lines are in the table above.
What causes limescale?
Calcium and magnesium dissolved from rock such as chalk or limestone, which form scale when water is heated. The DWI describes the carbonate part of hardness as the part that boiling removes as scale.
Sources
- Drinking Water Inspectorate, Water Hardness / Hard Water (undated page; no standards for hardness, calcium or magnesium; limited evidence on cardiovascular health; 150 mg/l minimum when softening; bands; WHO 2017 figure of 200 mg/l for scale and 100 mg/l for corrosion; scaling and its effects; temporary hardness; softener and jug filter points; ask your water company; read 29 October 2026).
- World Health Organization, Hardness (undated page; read 29 October 2026), its chapter 12 fact sheet page (extracted from the fourth edition with addenda published June 2026; we read only the page description, not the fact sheet text; read 29 October 2026), and Hardness in Drinking-water, background document, 2011 (sources of hardness, scale and heat exchangers, corrosion of soft water, cardiovascular evidence debated, no guideline values proposed; read 29 October 2026).
- Drinking Water Quality Regulator for Scotland, Water Hardness (undated page; most Scottish water soft; limescale and tea scum harmless; read 29 October 2026).
- Which?, How to descale a kettle (page dated 20 October 2026; composition of limescale, harmless to consume, effect on kettles, descaling frequency, filtered water; read 29 October 2026).
- The Student Room, forum thread on limescale in a kettle (one poster's account, paraphrased from the post as supplied to us; the page returned an access error when we tried to fetch it; seen 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, optional rapid coliform test read after 24 to 48 hours, from £3.99, free 2nd Class delivery on every order, 30-day money-back guarantee) and results portal (hardness pad steps 0, 25, 50, 100, 250 and 425 mg/l). The pad-step table is WaterKit's own reading of the colour chart steps.
Contains public sector information licensed under the Open Government Licence v3.0.