How Much Water to Store for 90 Days
Ready.gov and the American Red Cross use a baseline of one gallon of water per person per day for drinking and sanitation. Multiply people by days. Ready.gov says needs can double in very hot weather. For four people, two weeks is 56 gallons and 90 days is 360 gallons, more than most homes should stack as raw weight, so store what you safely can and plan how to treat more.
The question how much water to store for 90 days has a simple formula and a hard logistics answer. The formula is official. The logistics are why 90 Days Without Power by Robert Smythe spends a full chapter on treating water, not only on buying barrels.
Water storage calculator
This calculator uses Ready.gov’s baseline of one gallon of water per person per day. Checking extra heat doubles the rate, matching Ready.gov’s note that needs can double in very hot weather. Counting each pet as one extra gallon per day is a conservative household choice, not a separate official pet formula.
Sources: Ready.gov water guidance and Ready.gov notes on heat. Water weighs about 8.34 pounds per gallon.
The official baseline, not a slogan
Use one gallon per person per day as your starting number, then add for heat, illness, pregnancy, and pets.
Ready.gov and the Red Cross both use one gallon per person per day for drinking and sanitation. That gallon is not only drinking water. It also covers brushing teeth, washing hands, cleaning wounds, and preparing food. Ready.gov says never to ration drinking water unless authorities tell you to.
CDC water-storage language tells households to store at least one gallon per person per day for several days and to try for a two-week supply if possible. That two-week figure is the most important official target for most families. Ninety days is a planning ceiling you reach only after that layer is real.
Ready.gov notes that very hot weather can double water needs. Desert summers are not a one-gallon problem. The calculator on this page lets you toggle a heat factor that uses two gallons per person per day as an official-based planning factor, not a medical prescription.
What the numbers look like for real households
Four people need 12 gallons for three days, 56 gallons for 14 days, 120 gallons for 30 days, and 360 gallons for 90 days at the one-gallon baseline.
A gallon of water weighs about 8.34 pounds. Fifty-six gallons (four people, two weeks) weigh about 467 pounds. Three hundred sixty gallons weigh about 3,000 pounds. A 55-gallon food-grade barrel holds about 55 gallons and weighs about 460 pounds when full. Do not put that load on a weak apartment floor or an upstairs landing.
A 7-gallon jug is a useful renter-sized unit. Four of them cover two people for about two weeks at the baseline. Commercially bottled water in original sealed containers, kept cool and dark, is Ready.gov's best default. If you fill your own containers, use food-grade jugs, sanitize them (Ready.gov: one teaspoon of unscented household bleach per quart of water, then rinse), label the fill date, and replace self-bottled water about every six months.
Pets need a planning figure too. The worksheets in the book use about one gallon per pet per day, more for large dogs in heat. Add that in the calculator.
Why you should not try to warehouse 90 days if the floor cannot take it
Store a realistic first layer (several days to two weeks) and keep a way to treat more water from the heater tank, pipes, rain, or an outdoor source that is not chemically contaminated.
Chapter 4 of the book, Making Any Water Safe to Drink, walks through hidden water already in a home, rain catchment, and why pool water is for flushing and cleaning, not a drinking plan. It also explains what boiling, bleach, tablets, filters, SODIS, and distillation each can and cannot do.
Boiling and bleach do not remove chemical pollution, fuel, or salt. If officials say water is chemically contaminated, find another source. See how to make water safe to drink and the water filters and storage items in the Top 50 list.
How to store water so it stays drinkable
Commercially bottled water in sealed bottles is the simplest store. If you fill your own food-grade containers, use clean vessels, label the fill date, keep them cool and dark, and replace the water about every six months (Ready.gov).
Ready.gov’s water page tells people to store commercially bottled water if they can, in its original sealed container. If you bottle your own, use food-grade containers, never containers that held milk or juice if you cannot sanitize them thoroughly (sugars support bacteria), and do not use containers that held toxic chemicals. Leave headspace if the storage area might freeze.
A 7-gallon rigid jug is a renter-sized unit: two people at the baseline use about 14 gallons in a week, so two jugs cover that week if you can lift them. A full 7-gallon jug weighs about 58 pounds (water is about 8.34 pounds per gallon), which is why a stack of jugs on a weak closet shelf is a bad plan. A 55-gallon barrel holds a two-person two-week layer by itself but weighs about 460 pounds when full and usually needs a food-safe siphon. A bathtub bladder such as a WaterBOB is a fill-now tool when a storm is coming, not a standing 90-day tank.
Keep stored water away from gasoline, pesticides, and household chemicals. Do not store jugs directly on a dirty garage floor if you can put them on a pallet or shelf. If a container looks damaged, smells like plastic solvents or fuel, or has been opened in a dirty place, treat that water as needing disinfection or discard it and use a better container.
Pets share the household water plan. Ready.gov’s published gallon figure is written per person; counting each pet as an extra gallon per day is a conservative household choice, not a separate official pet formula published on that page. Livestock and large dogs will need more. Write the number down so a guest who comes to help does not guess.
If you cannot store two weeks, store what you can and invest earlier in treatment. That is the honest answer for apartments. The book’s water chapter shows where water already lives in a typical home (the water heater, if you can isolate it and the tank itself is sound) and why that is a backup, not your only plan. See making any water safe to drink.
Ninety days is a treatment plan more than a warehouse plan
Four people for 90 days at one gallon each is 360 gallons, about 3,000 pounds, before extra heat. Most families should store a real two-week layer and then know how they would treat more.
The calculator above is honest about volume because search results that say “just store 90 days of water” skip the weight. A two-week layer for four people is 56 gallons at the baseline, or 112 gallons with the heat factor. That is already a serious closet. Going further without a treatment method usually means buying containers you cannot rotate or lift.
CDC’s emergency water page describes boiling and, when boiling is not possible, household bleach disinfection for microbial risk. EPA publishes a dose table for 6% and 8.25% unscented bleach. Filters that are rated for bacteria and protozoa help you use surface water that you would not drink raw; they do not automatically make chemical-contaminated water safe. Chlorine dioxide tablets follow their own label. SODIS is a last-resort sun method described by WHO and Eawag for clear PET bottles.
Rotation is how stored water stays a comfort instead of a science experiment. Drink the oldest sealed bottles in ordinary life and replace them. Every six months, refill containers you bottled yourself (Ready.gov). Put a reminder on a calendar you will actually see.
City kit pages, including Phoenix emergency management’s household supply list, still start from the same one-gallon, several-day idea. A desert household that stops there in July is under-prepared for heat even if the outage lasts only a few days. Use the heat checkbox. Name a cooling place that will also have water. Details are in staying cool without power and in the Arizona edition at /states/arizona/.
Questions people ask
How much water do I need for two weeks?
At Ready.gov’s one gallon per person per day, four people need 56 gallons for 14 days, or 112 gallons if you apply the extra-heat factor Ready.gov notes can double needs.
Can I store water in milk jugs?
Ready.gov advises commercially bottled water when you can. If you fill your own containers, use clean food-grade vessels and avoid containers that held milk, juice, or chemicals unless they can be thoroughly sanitized.