How to make water safe to drink in an emergency
The CDC says boiling is the best way to kill germs in water: bring clear water to a rolling boil for 1 minute (3 minutes above 6,500 feet). If you cannot boil it, add 8 drops of unscented household bleach per gallon, stir, and wait at least 30 minutes before drinking.
- Which method should I use?
- How to boil water
- How much bleach per gallon
- When no method will make it safe
- Gear
Which method should I use?
Use bottled water first if you have it. If you do not, pick the first method on this list that you can actually do. The table is based on the CDC's emergency water guidance.
| Method | What it does | Watch out for |
|---|---|---|
| Boil | CDC calls it the best way to kill germs. | Needs fuel and a stove used outdoors or properly vented. |
| Bleach | Kills germs if you wait at least 30 minutes. | Must be unscented. Weaker than boiling against Giardia and Cryptosporidium. |
| Filter | Removes bacteria and parasites if the pore size is small enough. | Portable filters will not remove viruses. Treat with a disinfectant afterwards. |
| Tablets or drops | Chlorine dioxide kills Cryptosporidium if used correctly. | Iodine and chlorine tablets do not kill Cryptosporidium. |
| UV light | Kills germs in clear water. | Useless in cloudy water. Filter first. |
| Sunlight (SODIS) | Clear plastic bottles in the sun for 6 hours, or 2 days if cloudy. | Last resort. Clear water only. |
How to boil water
- Clear it. If the water is cloudy, pour it through a clean cloth, paper towel or coffee filter, or let it settle and draw off the clear water.
- Boil it. Bring it to a rolling boil and hold it there for 1 minute. Above 6,500 feet, boil for 3 minutes.
- Cool and store. Let it cool, then pour it into clean containers with tight lids.
- Fix the taste. Boiled water tastes flat. Pour it back and forth between two clean containers, or add a pinch of salt per quart.
How much bleach per gallon
For regular unscented household bleach (5% to 9% sodium hypochlorite, which is most bleach sold in the US). Check the label first. If it gives its own disinfecting instructions, follow those. Amounts from the CDC.
| Water | Clear water | Cloudy or very cold water |
|---|---|---|
| 1 quart or liter | 2 drops | 4 drops |
| 1 gallon | 8 drops (a little under ⅛ tsp) | 16 drops |
| 5 gallons | 40 drops (½ tsp) | 80 drops (1 tsp) |
Stir it well. Wait at least 30 minutes before you drink it or use it.
When no method will make it safe
The CDC is clear that boiling and bleach cannot make water safe if it has fuel, toxic chemicals or radioactive material in it. Floodwater that has run through garages, roads and drains can carry all three. Use bottled water or another source and call your local health department.
Never drink water from the radiators or boiler of your home heating system.
Common mistakes
- Using scented, splash-less or color-safe bleach. Use plain unscented bleach only.
- Trusting a hiking filter alone. Portable filters do not remove viruses, so treat the water afterwards.
- Skipping the 30 minute wait after adding bleach.
- Bleaching cloudy water without clearing it first, or without doubling the dose.
- Treating water that smells of fuel or chemicals. Boiling will not fix it.
- Giving iodine-treated water to someone who is pregnant or has thyroid problems. The CDC says they should avoid it.
Gear for this
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Unscented bleach and a glass eye dropper
The cheapest way to treat water. The dropper lets you count drops properly.
A water filter rated 0.3 micron absolute or smaller
The CDC says that pore size is what it takes to remove bacteria. Check the box for the absolute rating.
Chlorine dioxide tablets
The only tablet type the CDC says can kill Cryptosporidium when used correctly. Light enough for a go-bag.
Backpacking stove and spare fuel
So you can boil water with the power out. Use it outdoors only.
Questions people ask
How long do you boil water to make it safe?
The CDC says 1 minute at a rolling boil, or 3 minutes if you are above 6,500 feet.
How many drops of bleach purify a gallon of water?
The CDC says 8 drops of unscented 5% to 9% bleach per gallon of clear water, or 16 drops if it is cloudy, then wait 30 minutes.
Do water filters remove viruses?
Not portable ones. The CDC says they will not remove viruses, so add a chemical disinfectant after filtering.
Sources
Related entries
- W-02How much water to store for an emergency1 gallon per person per day. Work out your household total in 10 seconds.
- D-01Hurricane emergency kit checklistThe FEMA kit list as a tick-off checklist, plus the hurricane rules that keep people alive.
General safety information only, not medical or professional advice. Rules can change, so check the linked sources and follow your local officials. As an Amazon Associate I earn from qualifying purchases.