Water Purification Calculator
How many drops of plain household bleach make water safe to drink when the tap water can’t be trusted, and how long to boil it instead.
Last updated
Bleach
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- Let the water settle, then pour it through a clean cloth, paper towel or coffee filter into a clean container.
- Use plain, unscented chlorine bleach: not scented, colour-safe or “with added cleaners”. Check that the label says it contains sodium hypochlorite.
- Add the bleach with a clean dropper or measuring spoon, stir well and leave it for at least 30 minutes.
- It should smell faintly of chlorine. If it doesn’t, add the same amount again and wait another 15 minutes. If it still doesn’t, throw the water away and find another source.
- Keep it in clean, covered containers.
At this strength, for other amounts
| Water | Clear water | Cloudy water |
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Boiling
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Bring the water to a rolling boil (big bubbles that don’t stop when you stir) and keep it there for that long, then let it cool and keep it covered. If the water is cloudy, let it settle and filter it first. Boiling is the surest way to kill germs if you have the fuel.
How to make water safe to drink
- If you can, boil it: a rolling boil for 1 minute, or 3 minutes above 2,000 metres (6,500 feet).
- If you can’t boil it, enter how much water you have and the strength printed on your bleach.
- Add that many drops, stir, and wait at least 30 minutes before drinking.
- If the water is cloudy, let it settle and filter it first, then use double the bleach.
Where the amounts come from
The doses are the US Environmental Protection Agency’s (EPA) emergency disinfection table: 8 drops (⅛ teaspoon) of 6% bleach, or 6 drops of 8.25% bleach, for each US gallon of clear water, and 2 drops for a litre. Cloudy water gets double. The calculator scales that to the amount you enter, and to another strength by the amount of chlorine, so 3% bleach needs twice as much as 6%. A drop is taken as about 0.08 ml, the size the EPA’s “8 drops = ⅛ teaspoon” implies; droppers vary, so the measured amount is given too for larger volumes.
The boiling times are the US Centers for Disease Control and Prevention’s (CDC): a rolling boil for 1 minute, or 3 minutes above 2,000 metres (6,500 feet), where water boils at a lower temperature.
Bleach slowly loses strength once opened, so use a bottle that’s in date and has been stored somewhere cool. Water-purification tablets work too: follow the instructions on the packet, as different kinds need different waiting times.
Sources
Checked on 18 October 2026.
- EPA: Emergency disinfection of drinking water bleach drops per gallon and litre, double for cloudy water, settling and filtering, the chlorine smell check.
- CDC: How to make water safe in an emergency boiling or disinfecting, waiting 30 minutes, storing treated water.
- CDC: Boil water advisory a rolling boil for 1 minute; 3 minutes above 6,500 feet.
Questions about making water safe
How many drops of bleach per litre of water?
Two drops of plain 5–8.25% household bleach per litre of clear water, then stir and wait 30 minutes. Use four drops if the water is cloudy, after letting it settle and filtering it.
How much bleach for a gallon of water?
For a US gallon (3.8 litres) of clear water, 8 drops of 6% bleach or 6 drops of 8.25% bleach. Double it for cloudy water.
How long do you boil water to make it safe?
Bring it to a rolling boil and keep it there for 1 minute, or 3 minutes if you’re more than 2,000 metres (6,500 feet) above sea level.
Can I use scented or colour-safe bleach?
No. Use only plain, unscented chlorine bleach. Scented, colour-safe and “with cleaner” bleaches contain other chemicals and may not have enough chlorine.
Does boiling or bleach remove chemicals?
No. Both kill germs but leave chemicals, fuel, salt and heavy metals in the water. If the water may be polluted with those, use bottled water instead.