Water Purification Tablets, Bleach and Boiling Compared

When the tap is unsafe and bottled water has run out, CDC lists three household treatments: boiling, chemical disinfectants such as water purification tablets, and plain unscented bleach. This page prints CDC's amounts as CDC states them, dated 2026-11-02, and says what each method does not remove.

What this page is and is not

This is a summary of federal guidance, not advice of our own. Every number below is attributed to the agency that publishes it, and each was read on 2026-11-02. The CDC bleach table was re-read in a browser on that date and matched the earlier read. Agencies revise these pages, so check the CDC page linked under Sources before you rely on a figure.

We do not give home-chemistry advice beyond what CDC and EPA print. We have not tested any method, and nothing here promises that treated water will be safe to drink. If the water may be contaminated with fuel, toxic chemicals or radioactive material, CDC says none of these methods can make it safe, and it says to use bottled water or another source instead.

If you are choosing equipment before an emergency, the hub page on a survival water filter covers filters. This page covers the three methods that need no filter at all.

Start with the water you have

Both CDC and EPA say cloudy water should be cleared first. CDC says to let it settle, or pour it through a clean cloth, paper towel or coffee filter, then treat the clearer water. EPA's page gives the same step before boiling or adding chemicals.

If your supply is a stored one, emergency water storage covers amounts, containers and rotation. If the supply is still running but under an advisory, CDC says to use bottled water or to boil tap water for drinking, cooking, brushing teeth and making ice, and to boil it even if it has been filtered.

Boiling

CDC says to bring clear water to a rolling boil for 1 minute, and for 3 minutes at elevations above 6,500 feet. EPA's page gives the same 1 minute and the same 3 minutes, but starts the higher-altitude time at 5,000 feet (1,000 meters). The two agencies disagree on where "high elevation" begins.

If you live or camp between 5,000 and 6,500 feet, the two pages give different answers. The longer boil costs a few minutes of fuel, so following the longer time there is the cautious reading. That is our reading of two differing pages, not a rule from either agency.

Other points CDC and Ready.gov make about boiling:

• Ready.gov says boiling is the safest method of treating water, with a full minute at a rolling boil, then letting the water cool.
• CDC says boiling kills viruses, bacteria and parasites including Cryptosporidium and Giardia.
• CDC says to store boiled water in clean, sanitized containers with tight covers.
• CDC says boiled water that tastes flat can be poured back and forth between containers and left to stand a few hours, or given a pinch of salt per quart or liter.
• CDC does not give a cooling time.

Boiling has limits. CDC says it cannot remove fuel, toxic chemicals or radioactive materials. EPA adds that it does not remove heavy metals, salts or most other chemicals.

Household bleach

CDC says to use unscented household bleach and to follow the instructions on its label. If the label gives none, CDC says to look for the active ingredient, sodium hypochlorite, which it says is typically 5% to 9% in the United States.

CDC's table for bleach with 5% to 9% sodium hypochlorite, read 2026-11-02:

CDC adds these directions:

• If the water is cloudy, murky, colored or very cold, add double the amount in the table.
• Stir well and let the water stand at least 30 minutes before drinking.
• Store the treated water in clean, sanitized containers with tight covers.

CDC prints a second table for a weaker 1% bleach: 10 drops per quart or liter, 40 drops per gallon, 200 drops per 5 gallons. Do not mix columns from the two tables. Check the percentage on your bottle first and follow the label of your bleach.

Other agencies word this differently. Ready.gov gives 1/8 teaspoon per gallon for regular liquid bleach with 5.25 to 6.0 percent sodium hypochlorite, and does not recommend products with other active ingredients. EPA's page, for 6% or 8.25% bleach, gives 8 drops per gallon at 6% but 6 drops per gallon at 8.25%, and says to use fresh bleach or bleach stored at room temperature for under a year. The CDC figure for 8 drops per gallon is higher than EPA's 8.25% figure because CDC uses one dose across its whole 5% to 9% range. This page prints only CDC's table so that readers do not combine numbers from different tables.

EPA also says treated water should have a slight chlorine smell, and that if it does not, the dose should be repeated and the water left another 15 minutes. CDC's page does not say this. For an overly strong chlorine taste, EPA says to pour the water between clean containers and let it stand for a few hours.

Water purification tablets

Tablets are a packaged version of chemical disinfection. CDC says chemical disinfectants kill viruses and bacteria but may not kill parasites, and it says to follow the package label because tablets are made for specific volumes of water. EPA likewise says tablets of chlorine, iodine, chlorine dioxide and other chemicals exist and that strengths vary, so the label governs.

The detail that matters most is the parasite Cryptosporidium. CDC says:

CDC's hiking and camping page words chlorine dioxide more loosely: it says the chemical can kill Giardia and is somewhat effective against Cryptosporidium. The two CDC pages do not match, so read the label and the CDC page for your situation.

CDC does not print contact times for tablets, and neither does this page. They are on the package. CDC also says to filter water before disinfecting it where the water is cloudy. For iodine, CDC says to avoid it if you are pregnant, have thyroid problems or are sensitive to iodine, and says no one should drink iodine-treated water for more than a few weeks at a time. Ready.gov's water page does not mention tablets.

The three methods side by side

What none of these does

CDC says that boiling, disinfecting or filtering with these methods cannot make water contaminated with fuel, toxic chemicals or radioactive material safe. CDC's page on finding water in an emergency lists in-home sources such as the water heater tank and melted ice cubes, and says water from streams, rivers, ponds and lakes may carry livestock waste, sewage and chemicals and should only be used after treatment.

Water treatment is one layer of a kit. A filter handles particles and parasites, and CDC says filters alone do not remove viruses, which is why the page on the difference between a water filter and a purifier pairs the two ideas. For gear that works without power, see gravity water filters and the Sawyer Squeeze review. Where these items sit in a larger plan is on the emergency kit checklist and the bug out bag page, and the food side of an outage is covered under survival food kits and freeze-dried food shelf life. If you run pumps or a kettle from a battery, the Jackery review and what a solar generator can run show how much energy that uses.

FAQ

How many drops of bleach go in a gallon of water?

CDC says 8 drops (1/2 mL) per gallon for bleach with 5% to 9% sodium hypochlorite, doubled if the water is cloudy, murky, colored or very cold, then stirred and left at least 30 minutes. CDC also says to follow the label of your bleach. Other agencies print different numbers for different strengths.

Which bleach does CDC say to use?

CDC says unscented household bleach. EPA says regular, unscented chlorine bleach and says to avoid scented, color-safe or added-cleaner products. Ready.gov says products with other active ingredients are not recommended.

How long should I boil water?

CDC says a rolling boil for 1 minute, or 3 minutes above 6,500 feet. EPA says 1 minute, or 3 minutes above 5,000 feet. At high elevation, the longer time is the cautious reading of the two pages.

Do water purification tablets kill Cryptosporidium?

CDC says chlorine dioxide tablets do when the instructions are followed, and that iodine and chlorine tablets do not. Its hiking page describes chlorine dioxide as somewhat effective against Cryptosporidium, so the two CDC pages differ in wording.

How long do tablets need to work?

The CDC pages read for this guide do not give contact times. The time depends on the product, so use the figure printed on the package.