How long to boil water: the 1-minute and 3-minute rule explained

Bring clear water to a rolling boil and hold it there for 1 minute. Above 6,500 ft (1,981 m), CDC advises 3 minutes. Boiling kills viruses, bacteria and parasites, and CDC rates it as the best way to kill germs in water. It does not remove chemicals. Water with germs in it sometimes looks clean, so appearance cannot tell you whether the boil was needed.

What the CDC rule covers

The rule has two parts: a time at a rolling boil, and an elevation threshold. A rolling boil is one where bubbles keep breaking the surface while the water is stirred. A simmer, with small bubbles rising from the bottom while the surface stays mostly still, does not count; start the timer only once the boil keeps rolling.

CDC says boiling kills all types of germs, including viruses, bacteria and parasites such as Cryptosporidium and Giardia. CDC lists boiling first among its options, and filtering followed by disinfection second. Emergency guidance from both CDC and EPA says to use bottled water where you have it, and to boil when you do not.

Why altitude changes the time

Water boils at a lower temperature when the air pressure is lower, so a pot at altitude reaches a boil at a cooler temperature than one at sea level. The boiling point guide works through the physics. For scale, the standard-atmosphere figures are about 100.0 °C at sea level, 96.7 °C at 1,000 m, 93.4 °C at 2,000 m and 90.0 °C at 3,000 m. CDC's answer is a longer boil rather than a higher temperature: 1 minute up to 6,500 ft (1,981 m), and 3 minutes above it.

At 2,000 m (6,562 ft) the standard-atmosphere pressure is 795 hPa, about 78% of the 1,013 hPa at sea level, and water boils near 93.4 °C. Weather moves real pressure by a few hectopascals, so treat these as reference values. The Altitude tool shows the boiling point and the CDC boil time for any elevation.

threshold   6,500 ft = 1,981 m
2,400 m     = 7,874 ft, above the threshold
boil time   3 min per 1 L batch
4 batches   4 × 3 = 12 min at a rolling boil
sea level   4 × 1 = 4 min for the same batches

Where the EPA rule differs

EPA's emergency guidance asks for a rolling boil of at least one minute, and three minutes "at altitudes above 5,000 feet (1,000 meters)". The two units on that page disagree: 5,000 ft is about 1,524 m, and 1,000 m is about 3,281 ft. Either reading puts EPA's threshold below CDC's 6,500 ft (1,981 m).

Above 1,000 m, a 3-minute boil meets both agencies' wording whichever unit is read. The extra time costs fuel, which the planning section below covers.

Cool, store and taste the boiled water

CDC says to let boiled water cool before use, to avoid burns. It also says to store the water in clean containers with tight covers, and EPA gives the same storage advice. EPA says to let the water cool naturally before storing it. Neither page gives a storage time.

Boiled water can taste flat. CDC suggests pouring it between clean containers and letting it stand for a few hours, or adding a pinch of salt for each litre. EPA also suggests pouring it between clean containers several times.

Planning fuel for the boil

Fuel for boiling depends on the number of batches, the minutes at a rolling boil, and the time to bring each pot to the boil, which depends on the stove and the pot. Count the batches you need for drinking, cooking and other uses that need treated water, then multiply by the boil time for your elevation. Stove, pot, wind and water temperature all change how much fuel each boil uses, so measure your own setup at home. For the wider packing list, see the ten essentials guide.

Clear the water before the timer starts

If the water is cloudy, CDC says to filter it through a clean cloth, paper towel or coffee filter, or to let it settle and then draw off the clear water. EPA gives the same two options. Both pages place this step before the boil, and neither gives a settling time. EPA also advises against water with floating material in it, or with a dark colour or questionable odour, whatever the treatment. Where you have a choice, EPA says flowing water is generally better than still, stagnant water.

Clearing the water is a separate step from the boil, and the boil is still needed afterwards. The cloudy water guide covers the settling and straining steps in more detail.

What boiling does not remove

Boiling kills germs. It does not remove dissolved chemicals, salts or fuel. EPA says boiling and disinfection will not destroy heavy metals, salts or most other chemicals, and CDC says the same for water containing harmful chemicals, toxins or radioactive material. Boiling also does not show whether the water carried germs before you heated it. When the water is cloudy, settle or filter it first, as covered in the cloudy water guide. The filter guide sets out what a filter adds to a boil, and the chemical treatment guide covers disinfectants used instead of heat.