Water
Emergency water purification: boiling, filters and tablets
How to make doubtful water safer to drink in an emergency. When to boil, what filters and tablets can and cannot do, and when not to drink water at all.
Food & water · Free tool
Water Treatment Calculator
Work out how to make doubtful water safer to drink by boiling, household bleach, tablets or a filter, with the bleach dose in drops and millilitres and a wait timer.
Last reviewed
How to use it (4 steps)
- Check for a notice from your water company first. If there is one, follow it.
- Choose how many litres you want to treat and whether the water is clear or cloudy.
- Pick a method. For bleach, choose the strength printed on the label.
- Follow the steps, start the wait timer, and print or download the card to keep with your supplies.
Follow your water company’s notice first. If you have a boil water notice, or have been told to stop using your water, do exactly what it says. This tool is for when you have no safe water and no instructions.
Never mix bleach with ammonia or any other cleaner. It can give off toxic fumes.
How to treat it
Method: Boil
1 min at a rolling boil
- Bring 5 litres to a rolling boil.
- Keep it boiling for 1 minute. Above 6,500 feet (about 1,981 metres), boil for 3 minutes.
- Take it off the heat and let it cool naturally.
- Store it covered, in a clean container in the fridge. Throw away any you have not used within 24 hours.
Take care: boiling water can cause scalds.
Start this once the water reaches a rolling boil.
Do not use
- NeverWater that may contain fuel, toxic chemicals or radioactive material. Boiling and bleach cannot make it safe. Use bottled water.
- NeverScented or colour-safe bleach, bleach with added cleaners, or bleach whose strength you cannot read on the label.
- NeverBleach mixed with any other cleaner or chemical.
- AvoidDirty or open containers. Store treated water in clean, sanitised containers with tight lids.
A planning aid based on Drinking Water Inspectorate, US CDC and US EPA guidance. Follow official notices. If someone is ill after drinking water, call NHS 111, or 999 in an emergency. See also the water purification guide and your local risks.
The method, in detail
When to use it
If there is a problem with your tap water, your water company will send a notice to homes in the area. The Drinking Water Inspectorate describes two situations:
- A boil water notice. Boil tap water before you drink it, brush your teeth with it, make ice or prepare food with it. You can still shower and bathe, but keep the water out of your mouth.
- An instruction to stop using your water. This is rare. Drink bottled water or the alternative supply your water company provides. Boiling does not help.
This tool is for the times you have no safe supply and no instructions: for example stored water you are unsure of, or water from a source you would not normally drink.
How the bleach dose is worked out
The tool multiplies your litres by the published dose per litre, then doubles it for cloudy water.
| Bleach strength | Clear water, per litre | Cloudy water, per litre |
|---|---|---|
| 5% to 9% | 0.1 ml (2 drops) | 0.2 ml (4 drops) |
| 1% | 0.5 ml (10 drops) | 1 ml (20 drops) |
Drops are millilitres divided by 0.05, which is the size of a typical drop. This matches the US CDC’s own pairs (2 drops = 0.1 ml). Droppers vary, so trust the millilitres, especially for larger amounts.
After adding bleach, stir and wait at least 30 minutes. The water should smell slightly of chlorine. If it does not, the US EPA says to repeat the dose and wait another 15 minutes. If it still does not smell of chlorine, use another method.
What we left out, and why
- Other bleach strengths. The official doses cover 5% to 9% and 1% bleach only. For anything else, or a bottle you cannot read, the tool will not give a dose.
- How many tablets per litre. Tablets differ. The official advice is to follow the packet, so the tool gives you a timer you can set to the packet’s waiting time.
- A cooling time for boiled water. The guidance says only to let it cool naturally.
Where the figures come from
Every figure on this page was checked against these official pages on 25 October 2026. No UK source gives a household bleach dose, so the dose comes from US public health agencies.
- Rolling boil, cool naturally, store in the fridge for up to 24 hours, what a notice covers, scalds: Drinking Water Inspectorate, Receiving a Boil Water Notice.
- 1 minute at a rolling boil (3 minutes above 6,500 feet), bleach doses for 5% to 9% and 1%, double for cloudy water, wait 30 minutes, tablets, filters, contaminated water, storage: US CDC, How to Make Water Safe in an Emergency.
- Bleach to avoid, the smell check, repeat and wait 15 minutes, bleach less than a year old: US EPA, Emergency Disinfection of Drinking Water.
- Never mix bleach with other cleaners: US CDC, Cleaning and Sanitizing with Bleach After an Emergency.
Sources
- Drinking Water Inspectorate, Receiving a Boil Water Notice.
- US CDC, How to Make Water Safe in an Emergency.
- US EPA, Emergency Disinfection of Drinking Water.
- US CDC, Cleaning and Sanitizing with Bleach After an Emergency.
Frequently asked questions
How long should I boil water to make it safe?
The Drinking Water Inspectorate says to boil water until it reaches a rolling boil, then let it cool naturally. The US CDC says to keep it at a rolling boil for 1 minute, or 3 minutes above 6,500 feet (about 1,981 metres).
How much bleach do I add to a litre of water?
For plain, unscented bleach of 5% to 9% strength, the US CDC gives 2 drops (0.1 ml) per litre of clear water. For 1% bleach, it is 10 drops (0.5 ml) per litre. Double the dose for cloudy water and wait at least 30 minutes.
What if I don't know the strength of my bleach?
Do not guess. There is no published dose for an unknown strength. Boil the water, use purification tablets as the packet says, or use bottled water.
Do water filters make water safe?
Not on their own. The US CDC says portable filters will not remove viruses. After filtering, disinfect the water too, by boiling it or using bleach or tablets.
How long can I keep boiled water?
The Drinking Water Inspectorate says to store boiled water in a clean container in the fridge and throw it away if you have not used it within 24 hours.
Can I make water with fuel or chemicals in it safe?
No. The US CDC says you cannot make water with fuel, toxic chemicals or radioactive material safe by boiling or disinfecting it. Use bottled water.
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