Prep

How to make water safe to drink

Choose where the water comes from and what you've got. You'll get the steps from UK official advice, in the order it gives them. Bottled water is the safest option, and boiling comes first.

Bottled water first, then boiling. Bleach is a last resort, for when you can't boil and have no purification tablets. None of these makes water safe if it has fuel or chemicals in it.

What have you got?

Where's the water from?
Can you boil water safely?

1. Boil it

  1. If it isn't tap water, filter it first through a coffee filter, pillowcase or bedsheet, until it runs clear.
  2. Bring it to a rolling boil. The US EPA says to keep it there for at least 1 minute.
  3. Let it cool naturally. Boiling water can scald.
  4. After a boil notice, keep it in a clean container in the fridge, and throw it away if it isn't used within 24 hours. With no fridge, store it in a cool, dark place.

In a power cut, never use a camping stove or barbecue indoors, because of carbon monoxide (US CDC).

2. Tablets

If you can't boil it, Be Ready and the Chief Medical Officers both list purification tablets. Use them only if they're in date, and follow the label, because strengths vary. Most sources need filtering first.

The US CDC says iodine and chlorine tablets don't kill Cryptosporidium, and that chlorine dioxide does if used correctly. Boiling works against it, says the Drinking Water Inspectorate.

3. Bleach, as a last resort

This is the UK Chief Medical Officers' advice for a national power outage, when you've run out of safe drinks, can't boil water and have no tablets. Filter the water first.

  1. Use plain, unscented household bleach. Don't use scented or colour-safe bleach, or bleach with added cleaners.
  2. Add about a quarter of a teaspoon for every 10 litres of water, about one bucket. It's important not to use too much.
  3. Leave it to stand for 15 minutes.
  4. It should have a slight chlorine smell. If the smell is very strong, leave it uncovered for 2 hours before drinking.
  5. Use this water for drinking rather than washing your hands or cooking.

Which bleach

  • ✓ USE Plain, unscented household bleach. Fresh is best: the US EPA says less than a year old, kept at room temperature.
  • × NOT Scented bleach.
  • × NOT Colour-safe bleach.
  • × NOT Bleach with added cleaners. Check the label.

What the dose assumes: the script doesn't name a strength, so we can't tell you how your bottle compares. It gives one figure, for about 10 litres, in teaspoons, and no smaller dose, so we haven't worked one out. It gives no extra dose for cloudy water, and says to keep filtering until the water runs clear. If there's no chlorine smell at all, the script doesn't say what to do, so don't assume it has worked.

When not to use the water

  • × NOT It has an oily film on top.
  • × NOT It's flood water, or might have fuel or chemicals in it. Boiling kills germs but doesn't remove harmful chemicals. Bleach and tablets don't either.
  • × NOT It came in a container that has held chemicals such as fuel or paint.
  • × NOT You've been told to stop using it. Very occasionally a notice says this. Use bottled water or the alternative supply from your water company.
  • × NOT A private well or spring has been covered by flood water. Assume it's unsafe and call your local authority.

Babies

Ready-to-feed formula is best, as you may not be able to boil water for powdered formula. Make powdered formula only with freshly boiled water. If water is short, the Chief Medical Officers say to rinse bottles and make up formula with your cleanest water.

From preppantry.app. Checked 2 November 2026. Independent tool, not official advice.

How it works

This page follows three UK sources. The Drinking Water Inspectorate covers boil notices. Be Ready covers water supply interruptions. The UK Chief Medical Officers' scripts for a national power outage cover what to do once safe drinks have run out, and they're the only UK official source we found that gives a bleach dose.

What the bleach advice is: the Chief Medical Officers published it in December 2025 as a radio script for a total power outage across Great Britain, which they call very unlikely. It's for when you can't boil water and have no tablets. They say the scripts aren't government policy, and Be Ready's water page doesn't mention bleach.

What it doesn't tell you: the script gives one dose, in teaspoons, for about 10 litres. It names no bleach strength and gives no smaller dose. It says to filter until the water runs clear, and gives no bigger dose for cloudy water. We've kept to what it says. We haven't converted it to drops or mL, scaled it to other amounts, or added a strength.

Scented bleach: the script says "ideally unscented" in one line and "unscented" in the dose. The US EPA is firmer and says not to use scented or colour-safe bleach, or bleach with added cleaners. We've used the firmer wording, so read the label before you use it.

Boiling time: UK sources say to bring water to a rolling boil and give no time. The US EPA says at least 1 minute. Nowhere in the UK is above 5,000 feet, so the US advice on boiling for longer at altitude doesn't apply.

When the sources disagree: Be Ready says water from a water butt shouldn't be drunk. The Chief Medical Officers' script, written for when there's nothing else, says it can be made safer. We show both and treat it as a last resort.

Common questions

How do I make water safe to drink in the UK?

Use bottled water if you have it. Otherwise filter the water until it runs clear, then bring it to a rolling boil and let it cool. If you can't boil it, use in-date purification tablets and follow the label.

Can you drink water with bleach in it?

Only as a last resort, and only with the small dose in the Chief Medical Officers' script: about a quarter of a teaspoon of unscented household bleach for every 10 litres, left to stand for 15 minutes. Don't use more. Never drink bleach itself, and don't use scented or colour-safe bleach.

How long does boiled water keep?

After a boil notice, the Drinking Water Inspectorate says to keep cooled boiled water in a clean container in the fridge and throw it away if it isn't used within 24 hours.

Do I need to boil water for brushing my teeth?

Under a boil notice, yes. Boil it for drinking, brushing teeth, making ice cubes, preparing food, cleaning feeding equipment and giving to pets. You can still shower and bathe, keeping it out of your mouth.

What about water for a baby's formula?

Be Ready says ready-to-feed formula is best, as you may not be able to boil water for powdered formula. Powdered formula should only be made with freshly boiled water. Our sources don't cover bleach-treated water for formula, so don't assume it's suitable.

Will a water filter make it safe?

Filtering through cloth only takes out solid bits, so it comes before boiling, not instead of it. The US CDC says portable filters don't remove viruses, and that you should add a disinfectant after filtering.

Sources

Contains public sector information licensed under the Open Government Licence v3.0.

Checked on 2 November 2026. This is an independent tool and not official advice. See sources and method.

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