Water when the tap stops

How much, and why that number

Ready.gov's figure is the one to plan on: "Store at least one gallon of water per person per day for several days, for drinking and sanitation." A gallon covers drinking plus the cooking, teeth, and hand-washing that make a bad week livable. The sanitation half is the part people quietly leave out of the math and then miss.

Ready.gov also states the adjustments, and they matter more than the base number: children, nursing mothers, and sick people may need more; a medical emergency may require additional water; and in very hot temperatures water needs can double. Plan for the hot version, because the bad week is not guaranteed to happen in March.

• One gallon per person per day, several days minimum (Ready.gov). Two weeks is the version worth working toward.
• Count pets. A dog is a household member with a water requirement.
• Water weighs about 8.3 pounds per gallon. Three days for two people is roughly 50 pounds — which is why storing it is a house problem and carrying it is a completely different problem.
• Commercially bottled water stays sealed, cool, and dark. Water you bottled yourself gets replaced every six months (Ready.gov).

The water already in your house

Before treating anything, inventory what is already clean and captive. A conventional water heater holds 30 to 50 gallons of drinkable water behind a drain valve — cut its power or gas first, and open a hot tap somewhere to let air in. Ice in the freezer is water. The tank behind the toilet, the tank and not the bowl, is clean water unless you use in-tank chemical pucks.

And the first minutes of an outage or an advisory are the window to fill the bathtub, every pot, and every container you own while pressure still exists. Nothing on this page is as effective as that ten minutes.

Boiling, quoted exactly (EPA)

When you have heat, boiling is the method with the fewest ways to get it wrong. The EPA's emergency disinfection guidance says it plainly: "Bring water to a rolling boil for at least one minute. At altitudes above 5,000 feet (1,000 meters), boil water for three minutes."

Longer is not better; it is just fuel. Let it cool on its own, and pour it back and forth between two clean containers — that puts air back in and makes it taste like water again instead of like a pot.

Bleach ratios, quoted exactly (EPA)

If you cannot boil, unscented household bleach is the fallback, and the doses are small enough that guessing is genuinely dangerous in both directions. EPA's numbers, verbatim:

• One gallon: "8 drops of 6% bleach, or 6 drops of 8.25% bleach."
• Two gallons: "16 drops (1/4 tsp)" of 6% bleach, or "12 drops (1/8 teaspoon)" of 8.25%.
• Cloudy water: "Double the amount of bleach if the water is cloudy, colored, or very cold."
• Then: "Stir and let stand for 30 minutes. The water should have a slight chlorine odor." No chlorine smell after 30 minutes means repeat the dose and wait another 15.

The limits of boiling and bleach

Read the bleach label for the sodium hypochlorite percentage — 6% and 8.25% are both common on shelves and the doses are different. Use plain unscented bleach with no added cleaners, thickeners, or scent. Filter cloudy water through a clean cloth or coffee filter and let it settle first; sediment shelters germs from the chemistry.

And the limit EPA states directly, which is the reason none of this is a universal answer: "Boiling or disinfection will not destroy other contaminants, such as heavy metals, salts, and most other chemicals." If the concern is floodwater, industrial runoff, or a fuel spill, no amount of boiling or bleach fixes it. That is a bottled-water problem, not a treatment problem.

Filters and purifiers are not the same word

A filter is a physical sieve. Water is pushed through a membrane with a pore size, and anything larger than the pore does not get through. CDC's emergency guidance is that a filter needs an absolute pore size of 1 micron or smaller to remove protozoan parasites like Cryptosporidium and Giardia. Most decent backpacking filters are 0.1 to 0.2 micron hollow fiber, comfortably past that bar for protozoa and bacteria.

Viruses are the gap. They are far smaller than the pores of a typical hollow-fiber filter, so a filter alone is not the answer where viral contamination is plausible — sewage, floodwater, a lot of international travel. A "purifier" is a device that adds a step for viruses: a chemical stage, ultrafiltration or reverse osmosis, or UV. CDC's emergency advice is the two-step version — filter first, then add a chemical disinfectant to the filtered water, which handles what the filter may have let through.

The asymmetry worth memorizing runs the other way too. CDC states that chlorine and iodine tablets kill most germs but do not kill Cryptosporidium, which is poorly inactivated by either — while a 1-micron-absolute filter removes it mechanically. Chlorine dioxide is the chemical that does get Crypto, at the cost of a long contact time. Filters and chemicals cover each other's blind spots, which is precisely why the recommendation is both.

What actually fails in the field

• Freezing. Water inside a hollow-fiber membrane expands as it freezes and cracks the fibers, and a cracked filter looks and flows exactly like a working one. Anything that froze in the truck is unverifiable — sleep with it in the bag in winter, and retire the one you are not sure about.
• Silt. Turbid water clogs a filter fast. Pre-filter through a bandana or let it settle overnight, or you will spend the event backflushing.
• Flow rate as a lie detector. The slow-down IS the maintenance signal. Backflush when it slows, and replace the cartridge on the manufacturer's schedule rather than on optimism.
• Contact time. Chemical treatment is not instant, and it is slower in cold water. The wait printed on the package is doing real work; starting the timer and then drinking early is the most common way people treat water and get sick anyway.
• The dirty-to-clean handoff. Wet hands, the outside of the dirty bag, and the cap you set down in the mud all recontaminate treated water. Keep a dirty side and a clean side and never let them touch.
• Certification claims. A percentage printed on a box is a manufacturer statement. Look for testing against a named standard — NSF/ANSI 53 and 58 are the ones CDC points at for Cryptosporidium — and be skeptical of any claim with no standard behind it.

Turn this into a list you own

Water is the one slot in a kit with no substitute, so it is the slot worth over-building: a stored supply at home, a container to move it in, a filter, and a chemical treatment that covers what the filter misses. Our kit builder will put a version of that together from the catalog inside a budget you pick, and the 72-hour kit guide covers where water sits among everything else.