How do I purify water in an emergency? Every method ranked

By Owen Hartley, The Steady Prepper · October 29, 2026 · 13 min read · Water

Short answer (checked October 29, 2026): boil it. A rolling boil for 1 minute kills bacteria, viruses and parasites, and the CDC calls boiling the best way to kill germs in water. If you can't boil, let cloudy water settle, filter it, then disinfect it with chlorine dioxide tablets or unscented household bleach (8 drops of 6% bleach per gallon, per the EPA) and wait at least 30 minutes.

The catch most guides skip: no home method removes fuel, toxic chemicals or radioactive material. The CDC says plainly that boiling and disinfecting can't make that water safe. Treatment handles germs. Only water you stored before the emergency handles everything else, which is why storage comes first and purification is the backup.

Below is every common method ranked, what each one does not do, the exact doses, and a worked example of what a family of four actually needs.

This is general information. In a real emergency, follow any boil water notice or instructions from your local health department.

Every method ranked

Rank is for an ordinary household with a stove or camp stove, based on what the CDC and EPA say each method kills and what it leaves behind.

How to read it: "Less reliable" and "No" for bleach and iodine come from the CDC, which says chemical disinfectants "do not work as well as boiling" against Cryptosporidium and Giardia, and that iodine and chlorine tablets will not kill Cryptosporidium. Chlorine dioxide is the exception when you follow the label exactly. The filter rows come from the CDC's pore size rules (0.3 micron absolute or smaller for bacteria, 1 micron or smaller for parasites, and no portable filter for viruses) and Sawyer's published removal rates for its 0.1 micron filter.

Before any method: settle and strain

Every official guide starts the same way. If the water is cloudy, let it stand so the dirt settles, pour off the clear part, and strain it through a clean cloth, paper towel or coffee filter. Cloudiness matters for two reasons: particles use up chlorine before it reaches the germs, and they shade germs from UV light and sunlight. The CDC says UV and solar disinfection don't work well in cloudy water.

Prefer flowing water to still, stagnant water, and skip any source with floating material, a dark color or a strange smell, the EPA advises.

1. Boiling: the best germ killer

How to do it (CDC):

1. Settle and strain cloudy water first.
2. Bring it to a rolling boil for 1 minute.
3. At high altitude, boil for 3 minutes. The CDC sets the line at 6,500 feet; the EPA uses 5,000 feet. If you live anywhere between the two, just boil for 3 minutes.
4. Let it cool naturally and store it in clean, covered containers.

Boiled water tastes flat. Pour it back and forth between two clean containers, or add a pinch of salt per quart or liter.

What boiling does not do: it doesn't remove heavy metals, salts or most other chemicals, the EPA notes, and some water evaporates, so anything dissolved ends up slightly more concentrated. It also costs fuel. Never run a camp stove, grill or generator indoors, because of the carbon monoxide risk.

2. Filter, then disinfect: the best no-fuel combo

A good hollow fiber filter plus a chemical disinfectant covers all four germ groups without any fuel, and it handles cloudy water better than chemicals alone.

• The filter takes out the parasites and grit. Sawyer rates its Squeeze at 0.1 micron absolute and publishes removal of 99.99999% of bacteria and 99.9999% of protozoa such as Giardia and Cryptosporidium.
• The disinfectant takes out the viruses. The CDC says portable filters will not remove viruses and tells you to add iodine, chlorine or chlorine dioxide after filtering.

This order also fixes chlorine's weak spot: the filter removes the Cryptosporidium that bleach can't kill, and the bleach handles the viruses the filter lets through.

What it does not do: most portable filters pass dissolved things. Sawyer's own FAQ says tastes, colors, odors and anything in solution, such as salts, go straight through.

3. Chlorine dioxide tablets: the best single chemical

These are the only common tablets that the CDC says will kill Cryptosporidium, "as long as you follow the instructions correctly". Using Katadyn Micropur MP1 as the example:

• Dose: 1 tablet per liter (about a quart).
• Wait: the label requires 4 hours in an opaque container or in the dark. Katadyn's brochure explains why: in cold, dirty test water, cysts such as Cryptosporidium take 4 hours, while bacteria and viruses take 15 minutes. In clear 20°C (68°F) water, cysts took 30 minutes. Follow the label, because in an emergency you rarely know how cold and dirty your water really is.
• Shelf life: Katadyn lists 5 years from manufacture, sealed.

The limit nobody mentions: quantity. A gallon is 3.79 liters, so you need 4 tablets per gallon. At the 1 gallon per person per day that agencies recommend, a family of four goes through about 16 tablets a day, so a 30-tablet pack lasts under two days. Tablets are a backup and a travel tool, not a plan for weeks.

4. Household bleach: cheap and stockable

Bleach is the cheapest way to disinfect a lot of water, with one big weakness: it doesn't kill Cryptosporidium.

What to buy: regular, unscented liquid chlorine bleach whose label says it's suitable for disinfection, with 6% or 8.25% sodium hypochlorite (the CDC accepts 5% to 9%). No scented, color safe or "with cleaners" versions.

EPA doses for clear water:

For a standard 5-gallon jug, the CDC gives 40 drops, 2.5 mL or 1/2 teaspoon of 5% to 9% bleach.

Steps (EPA):

1. Settle and strain if cloudy. Double the dose if the water is cloudy, colored or very cold.
2. Add the bleach with a clean dropper, stir and wait 30 minutes.
3. Sniff. It should smell slightly of chlorine. If not, repeat the dose and wait another 15 minutes. Ready.gov adds: if it still doesn't smell of chlorine, throw it out and find another source.
4. Too strong? Pour it between clean containers and let it stand a few hours.

The detail that catches people out: the EPA says to use bleach stored at room temperature for less than a year. That bottle under the sink from three moves ago may be too weak. Date your emergency bottle and replace it yearly when you rotate your water.

5. Iodine: works, with health limits

The EPA's dose for 2% tincture of iodine is 5 drops per quart or liter, or 10 if cloudy or colored, then wait 30 minutes. Tablets: follow the label. Like chlorine, iodine won't kill Cryptosporidium.

The CDC says people who are pregnant, have thyroid problems or are sensitive to iodine should avoid iodine-treated water, and no one should drink it for more than a few weeks at a time. That makes iodine a short-term fallback, not a long-emergency plan.

6. Portable filter on its own

Good for clear backcountry streams, risky for flood water. A 0.1 micron hollow fiber filter stops bacteria and parasites, but the CDC is blunt: portable filters "will not remove viruses". Flood water and anything that could contain sewage may carry viruses, so treat filtered water from those sources with a disinfectant too.

When you buy, look for the word absolute next to the pore size. The CDC's thresholds are absolute pore sizes: 1 micron for parasites, 0.3 micron for bacteria.

7. UV pens and lights

A portable UV unit kills germs in small amounts of clear water, per the CDC. It needs batteries or charging, and particles shield germs, so filter first. It's a good compact tool, but not a way to treat a 5-gallon jug.

8. Solar disinfection (SODIS): the last resort

With nothing else available, sunlight reduces germs. The CDC method:

1. Use clean, clear plastic bottles and clear water (settle and strain first).
2. Lay the bottles on their side in the sun, ideally on a dark surface.
3. Leave them 6 hours if sunny, 2 days if cloudy.

The CDC describes this as reducing germs, not as reliably killing all of them. Use it when you have no fuel, no bleach and no tablets.

Special case: distillation removes what others can't

Ready.gov notes that distillation kills germs and removes heavy metals, salts and most other chemicals, because you collect only the condensed steam. Its kitchen method: fill a pot halfway, hang a cup from the inside of the upturned lid so it doesn't touch the water, and boil for 20 minutes. The water that drips into the cup is distilled.

It's slow and uses a lot of fuel for a small amount of water, so treat it as the tool for salty or chemically doubtful water when nothing else is available, not your daily method. It is still not a fix for known fuel or chemical spills; the CDC's advice there is a different water source and a call to the health department.

What none of these methods fix

This is the strongest argument for storage. In the scenarios people worry about most, from floods that mix fuel into water supplies to a nuclear emergency, the safest water is the water you sealed in clean containers beforehand.

How much water to store, and where else to find it

Ready.gov and the CDC say to store at least 1 gallon per person per day, for at least 3 days, and the CDC adds to aim for 2 weeks if you can. Store more for hot climates, pregnancy, illness and pets.

Example scenario (an illustration with stated assumptions, not a real household): a family of four planning for 14 days at 1 gallon each per day needs 4 x 14 = 56 gallons. That's about eleven or twelve 5-gallon containers. Treating all 56 gallons with 6% bleach at the EPA's 2/3 teaspoon per 8 gallons takes 7 x 2/3 = about 4.7 teaspoons, or roughly 9.3 teaspoons if all of it were cloudy. A single bottle of fresh bleach covers that many times over, which is why the bleach matters less than its age.

Storage rules (CDC and Ready.gov):

• Use food-grade containers with tight lids; never containers that held bleach, pesticides or other chemicals. Avoid glass.
• Wash with soap, then sanitize with 1 teaspoon of unscented bleach in 1 quart of water, shake so it touches every surface, wait at least 30 seconds, pour out and air dry.
• Label each one "drinking water" with the date, keep it cool (50°F to 70°F) and out of the sun, away from fuel and chemicals.
• Replace home-filled water every 6 months. Keep store-bought water sealed and watch its date.
• Use a clean scoop each time. Never dip hands into stored water.

Hidden water at home: the EPA lists melted ice cubes and the water in your hot water tank and pipes. Treat it if you're unsure of it.

Don't ration. Ready.gov says to drink what you need today and look for more tomorrow, and to avoid caffeine and alcohol in place of water.

If you live in the UK, Canada or Australia

The science is the same everywhere, but the official numbers differ, and in a real emergency your own authorities' instructions are the ones to follow. Here's what each government's own guidance says, checked October 29, 2026.

Three things worth knowing from that table:

• The UK and Canadian drinking amounts are smaller than the US gallon because they count drinking water separately. Canada's 2 plus 2 litres (4 litres in total) is close to the US gallon of about 3.8 litres. If you're building a two-week store, the US figure is the simpler one to plan around.
• Match the dose to your bottle. The Canadian dose is for 5.25% bleach and the NSW dose for 4% to 5%. The EPA table earlier is for 6% and 8.25%. Read the strength on your label and use the table written for it; stronger bleach needs fewer drops, not the same number.
• Cold water needs longer. NSW Health is the only one of these sources that spells out the wait by water temperature. If your water is near freezing, give bleach the full 2 hours.

Emergency numbers: 999 or 112 in the UK, 911 in Canada, 000 in Australia.

The 10-minute water kit checklist

• At least 3 days of water per person, working up to 2 weeks
• A bottle of unscented 6% or 8.25% bleach, dated, replaced yearly
• A clean medicine dropper and a 1/8 teaspoon measure, taped to the bleach
• A 0.1 micron absolute filter for bulk clear water
• One pack of chlorine dioxide tablets for when you can't boil
• Coffee filters or a clean cloth for straining
• A pot and a safe outdoor way to boil, with fuel
• A printed copy of the bleach table above

More water guides are on our water topic page. For keeping a fridge and well pump running in a shorter outage, our sister site The Outage Planner covers what size power station runs a fridge.

Sources

Checked on October 29, 2026.

• CDC: How to make water safe in an emergency (boiling times, bleach table for 5% to 9% bleach, Cryptosporidium and iodine cautions, filter pore sizes, UV, solar disinfection, fuel and chemical warning, radiators and boilers)
• CDC: How to create an emergency water supply (1 gallon per person per day, 3 days to 2 weeks, containers, sanitizing, 6 month rotation, storage temperature)
• EPA: Emergency disinfection of drinking water (bleach table for 6% and 8.25%, 5,000 foot altitude rule, bleach age, iodine dose, hidden water sources)
• Ready.gov: Water (storage amount, rationing, chlorine smell test, distillation method and comparison table)
• UK Government Be Ready: Be ready for emergencies (3 litres per person per day for 3 days) and Drinking Water Inspectorate: Receiving a boil water notice
• Health Canada: Food and drinking water safety in an emergency (72 hours, 2 plus 2 litres per person per day, bleach dose)
• NSW Health: What to do if a boil water alert is in place (boiling, bleach dose and wait times by temperature) and Queensland Government: Pack an emergency kit (10 litres per person for 3 days)
• Katadyn: Micropur MP1 product page (1 tablet per liter, 4 hour exposure, 5 year shelf life) and Micropur brochure (contact times in clear and cold, dirty water)
• Sawyer: Squeeze water filter (0.1 micron absolute, published removal rates) and FAQs (dissolved substances pass through)

Spotted something out of date? Tell us with the source and we'll fix it.

Owen Hartley is the pen name of The Steady Prepper's editorial team. Guidance comes from the official and manufacturer sources linked above, checked on the date shown. Follow instructions from your local authorities in a real emergency. Example scenarios are illustrations with stated assumptions, not real households. Some links may be affiliate links that pay us a commission at no extra cost to you.