Does boiling tap water make it cleaner?
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Part of our tap water guide
The kettle clicks off and the water looks exactly as it did a minute ago. So what has boiling actually changed, and what has it left behind? You can check some of it at your own tap with a 20-in-1 test strip.
Short answer: of germs, yes, but not of much else. The Drinking Water Inspectorate (DWI) says boiling kills or inactivates bacteria, viruses and parasites, and that stored boiled water should be thrown away after 24 hours. United Utilities says boiling will not remove lead, and the DWI says boiling removes temporary hardness as scale.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
Does boiling tap water make it cleaner?
Of germs, yes. For the rest, the sources we read give a mixed answer, with chlorine and temporary hardness reduced and lead left in. The Drinking Water Inspectorate says heating water is one of the best ways to kill or inactivate bacteria, viruses or parasites, and that boiling also works against parasites such as Cryptosporidium. The World Health Organization says a rolling boil is sufficient to inactivate pathogenic bacteria, viruses and protozoa. Germs are only part of what a glass of tap water holds, and you can check some of the rest at your own tap with a 20-in-1 test strip. The table sets out what we found.
| What is in the water | What the sources say boiling does | Who says so |
|---|---|---|
| Bacteria, viruses, parasites | Killed or inactivated | DWI, WHO |
| Chlorine | Can help remove most of it for people sensitive to the taste | United Utilities |
| Temporary hardness (carbonates) | Can be removed by boiling, forming scale | DWI |
| Lead | Boiling will not remove lead | United Utilities |
| Other metals and chemicals | The sources we read do not say boiling removes them, and none of the sources we read gives a list | None found |
Do I need to boil UK tap water?
Not unless a notice tells you to. The DWI says UK tap water does not need boiling, and that on rare occasions systems fail, usually because of weather emergencies or engineering works. Its Drinking Water 2025 report puts overall compliance for England at 99.97%, across 24 water companies supplying 59.74 million consumers. For a private supply, the DWI says a notice comes from your local authority.
What should I do if I get a boil water notice?
Follow your own notice, because it sets the area and what you can use unboiled. The DWI's general advice is below.
- Boil, then cool. Bring the water to a rolling boil, take it off the heat and let it cool naturally. The DWI warns that boiling water can scald.
- Store it. Keep it in a clean container in the fridge and throw it away if it is not used within 24 hours.
- Use it for what goes in your mouth. The DWI lists drinking, brushing teeth, ice cubes, food preparation, cleaning feeding equipment and pets. It says you can still shower and bathe if the water does not get into your mouth.
- Stopping altogether. Very occasionally the instruction is to stop using the water, and the DWI says to use bottled water or an alternative supply from your water company.
Does boiling water reduce hardness?
Only the temporary part. The DWI defines temporary hardness as calcium and magnesium carbonates and bicarbonates, and says boiling can remove them, forming scale. That is the crust in your kettle. The DWI says scale on heating elements shortens their life and makes appliances less efficient. Its page does not say that boiling removes the rest of the hardness, and none of the sources we read gives a figure for how much a kettle takes out. Our kettle limescale guide covers the crust, and our guide to how hard your water is covers the local figure. The DWI also says its hardness map is typical and that local variation is not shown, so asking your water company gives a more exact figure.
Why might my tap water differ from the area figure?
The area figure is an average. Your tap sits at the end of the company's pipes and the start of your own, and the sources we read give several reasons it can differ.
- Distribution pipes. The DWI reports that 447 of the 2025 compliance failures in England were related to consumer distribution systems.
- Chlorine. The DWI says properties near a treatment works may get a little more than those several miles away. Companies typically keep residual chlorine to 0.5 mg/l or less. United Utilities adds that taste can be stronger when flow rises at busy times or after a nearby main is replaced. Our chlorine smell guide has more.
- Standing water. The DWI says to draw off a washing-up bowlful after several hours of no use. It says bathroom taps and hot water usually come from a loft storage tank and are not as fresh as mains water. See our loft tank guide.
- Your own plumbing and appliances. The DWI says a TCP-like taste can come from appliance hoses, some plastic kettles and anti-splash tap devices.
- Lead pipework. United Utilities says water can pick up lead in a property's pipes or its connecting pipe, and that homes built before 1970 may still have lead pipework. Our lead pipes guide covers how to look.
- Private supplies. A water company's figures do not cover them. The DWI's Drinking Water 2025 key facts for private supplies list 34,102 supplies serving 818,213 people, with 3,119 sources without a risk assessment.
“If you want to know why your tap might not match the area figure, check it yourself, because a lot sits between the works and your glass, from the pipes to your own plumbing. I would treat the area figure as a starting point and use a strip at the kitchen tap to see what its pads show for your own water.”
A figure for the area does not tell you what comes out of your own tap. Boiling is not a stand-in for knowing it. A 20-in-1 strip at the kitchen tap gives a reading on some of it, within the chart steps set out below.
What can a test strip show before and after boiling?
A chlorine reading, a hardness reading and a bacteria presence screen, each with limits. WaterKit has not published a before-and-after result for boiling, so we make no claim about what you will see. Fill a clean glass from the cold kitchen tap and dip one strip, then do the same with a sample you boiled and cooled. Read each after 30 to 60 seconds against the chart, or scan the QR code on the chart to open the free results checker, with no app or account. The table is our reading of the chart, not a claim from the DWI.
| Pad | Chart steps (mg/l) | What it can indicate here |
|---|---|---|
| Free and total chlorine | 0, 0.5, 1, 3, 5, 10, 20 | A fall from 0.5 or 1 to the 0 colour would indicate the level is below the first step. The DWI says a jug of tap water in the fridge reduces the chlorine smell and taste, so in our reading a drop alone does not show boiling was the cause |
| Hardness | 0, 25, 50, 100, 250, 425 | The steps are wide, and we have not confirmed whether they are read as calcium carbonate, the unit the DWI uses. A change of less than one step would not show |
| Lead | First step above the UK limit | The strip cannot read lead at the UK limit, so a clear pad is not a pass, and United Utilities says boiling does not remove lead. Use our lead pipes guide above |
The optional rapid coliform test is a presence screen on a small sample, read after 24 to 48 hours, and a positive result indicates coliform bacteria in that sample. It is not a count, and we do not claim the test for parasites such as Cryptosporidium. A strip result is not a legal sample, and if your water company or council has issued a notice, follow the notice.
The area figure is an average, and your own tap can differ because of the pipes between the works and your glass, water that has stood in your plumbing, and lead pipework in an older home. The chlorine and hardness pads are the ones that matter here, since a fall in chlorine to the 0 colour would indicate the level is below the first step, and a move of one hardness step would indicate a change in the hardness reading. Best pack: 20 strips, enough for a cold-tap reading, a few before-and-after samples and a repeat at another tap, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your boiled water questions
Is boiled water better than tap water?
Boiled water is tap water that has been heated, and we found no UK official source in those we read that ranks one above the other. The DWI says boiling deals with germs, and United Utilities says it will not remove lead.
Is boiled water safe to drink?
The DWI says that during a notice, bringing water to the boil kills or deactivates any bacteria or viruses that were in it. It says that in normal circumstances UK tap water does not need boiling.
Does boiling water filter it?
No. None of the sources we read describe boiling as filtering. Scale from temporary hardness forms in the kettle, but the DWI does not call that filtering.
Is boiled water the same as filtered water?
No. The DWI says activated carbon filters are used to remove taste and odour, and its hardness page says some jug filters also have resin that alters temporary hardness. Our boiled versus distilled water guide covers another process people confuse with boiling.
Can bacteria grow in cold water?
For a jug of drinking water, none of the sources we read answers this directly, though the DWI advises throwing away unused jug water after 24 hours. For building water systems, the Health and Safety Executive says 20 to 45°C suits legionella growth, and its hot and cold water systems page advises keeping cold water below 20°C where possible.
Does boiling tap water make it taste better?
For chlorine, it may. United Utilities says boiling can help remove most of it for people sensitive to the taste, and the DWI says a covered jug in the fridge reduces it. Our chlorine removal guide goes further.
Does UV light kill bacteria in water?
The DWI describes UV as a common way to disinfect private supplies. It says it works only if a sufficient dose reaches water low in turbidity, and that unlike chlorination it inactivates Cryptosporidium. It says UV units for ponds and fish tanks are not suitable for supplies for human consumption, and that a lamp can look the same when its output has fallen, so a lit lamp does not necessarily mean the required dose is being delivered.
Sources
- Drinking Water Inspectorate, Receiving a Boil Water Notice (what boiling kills, how to boil, storage, uses, private supplies; undated page; read 29 October 2026).
- Drinking Water Inspectorate, Water hardness and hard water (temporary hardness and boiling, scale, hardness map, jug filter resin; undated page; read 29 October 2026).
- Drinking Water Inspectorate, Chlorine (residual levels, treatment works distance, fridge jug, standing water, loft tanks, TCP-like taste; undated page; read 29 October 2026).
- Drinking Water Inspectorate, Domestic water filters and softeners (activated carbon filters; undated page; read 29 October 2026).
- Drinking Water Inspectorate, UV disinfection (private supplies; undated page; read 29 October 2026).
- Drinking Water Inspectorate, Drinking Water 2025 (England), compliance with water quality standards (99.97% overall; 447 failures related to consumer distribution systems; read 29 October 2026).
- Drinking Water Inspectorate, Drinking Water 2025 (England), water supplies and testing (24 companies, 59.74 million consumers; private supply key facts; read 29 October 2026).
- Health and Safety Executive, Hot and cold water systems (cold water below 20°C) and What Legionnaires' disease is (20 to 45°C suitable for growth; undated pages; read 29 October 2026).
- World Health Organization, Boil water (WHO/FWC/WSH/15.02; we read the overview page, not the full technical brief; read 29 October 2026).
- United Utilities, Information about lead pipes (boiling does not remove lead; pipes in properties; homes built before 1970) and Chlorine taste or smell (boiling and chlorine; causes of stronger taste; undated pages; read 29 October 2026). These are one company's statements.
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, optional rapid coliform test read after 24 to 48 hours, 20 strips £5.99 or £10.99 with a bacteria test, free 2nd Class delivery on every order) and results portal. The strip-chart table is WaterKit's own reading of the colour chart steps.
Contains public sector information licensed under the Open Government Licence v3.0.