Why one gallon per person per day is the number
Federal preparedness guidance settles on one gallon per person per day and splits it roughly in half: about two quarts for drinking and the rest for food preparation and basic hygiene. The drinking half is a physiological figure and rises in heat, at altitude, with exertion, and for anyone who is pregnant, nursing or unwell. The other half is the one that gets squeezed in a real emergency, because washing hands and rinsing a pot both cost water you might rather drink.
The guidance sets two horizons. Three days is the evacuation figure: what you can carry to a car and what you need to reach somewhere with supplies. Two weeks is the shelter-at-home figure, and it is the one that produces surprising numbers, because two weeks for a family of four is 56 gallons of drinking and hygiene water before you have accounted for a single pot of pasta.
Everything beyond that baseline is a household decision this calculator asks you to make explicitly. Pets drink about a fluid ounce per pound of body weight a day, so a 60 lb dog needs half a gallon. Cooking rice or freeze-dried food adds water the baseline does not include. And sanitation is the wildcard: if the mains are down but the sewer works, a conventional toilet consumes 1.6 gallons per flush and can outrun your drinking supply in a day.
From gallons to containers, weight and floor space
The gallons are the easy part: add the daily demands, multiply by days. The three conversions that follow are what turn a plan into something you can actually execute.
Containers is a ceiling function, so it always rounds up — 56 gallons in 5 gallon jerrycans is 11.2, which is 12 cans. The container size you choose is a trade-off between refill effort and portability. A 5 gallon can holds 41.7 lb of water, which most adults can carry; a 7 gallon can is 58.4 lb, which many cannot carry far; a 55 gallon drum is 458.7 lb, which nobody moves and which therefore has to be filled where it will stand and emptied with a siphon or a pump.
Weight is gallons times 8.34 lb, the weight of a US gallon of fresh water. It is worth writing out in full because the number is so easy to underestimate. Two weeks for a family of four with pets and cooking is 84 gallons, which is 700 lb — a quarter of a tonne of water sitting on one part of your house.
Floor loading is where that matters. The International Residential Code sets a design live load of 40 lb per square foot for residential living areas and 30 for sleeping rooms. Water in 5 gallon cans on a 1 sq ft footprint is 41.7 lb per square foot per layer, so a single layer is already at the limit and a two-high stack is at double it. That does not mean the floor collapses — design loads carry safety factors and are specified for uniform loading across a span — but it does mean you should spread the containers out, keep them near a bearing wall rather than mid-span, and prefer a basement slab or a garage floor if you have one.
Rotation divides the total by the interval you choose. Water does not spoil, but containers and the water in them pick up contamination and taste over time, and public-health guidance is to replace self-bottled water about every six months. Commercially sealed bottles carry their own dates and can go longer. Rotating a fixed fraction each month is easier to keep up with than replacing everything twice a year.
Worked example: a family of four with a dog, two weeks at home
Four people, one 60 lb dog, a fourteen-day target. You add half a gallon a day for cooking and a gallon a day for washing, and you plan to store the water in 5 gallon stackable cans that each take about a square foot of floor. You already have 30 gallons of bottled water in the garage.
- People. 4 × 1.0 = 4.0 gal/day.
- Pet. 1 × 0.5 = 0.5 gal/day.
- Cooking and sanitation. 0.5 + 1.0 = 1.5 gal/day.
- Daily demand. 4.0 + 0.5 + 1.5 = 6.0 gallons a day.
- Fourteen days. 6.0 × 14 = 84 gallons.
- Containers. 84 ÷ 5 = 16.8, rounded up to 17 cans.
- Weight. 84 × 8.34 = 700.6 lb.
- Floor area. 17 × 1 sq ft = 17 sq ft on a single layer, which is 41.2 lb per square foot.
- Current coverage. 30 ÷ 6.0 = 5.0 days, so you are 54 gallons short.
- Rotation. 84 ÷ 6 months = 14 gallons a month, which is about three cans.
Two things stand out. The pet and household allowances added 50% to the baseline — 84 gallons against the 56 the one-gallon rule alone would suggest — and that is before any flushing. And the 5.0 days of existing coverage is the honest answer to "are we ready": a third of the way there, with a specific 54 gallon gap to close.
How to read the numbers and where to put the water
The days-covered figure is the one to check first, because it converts a shelf full of bottles into a number you can act on. Anything under three days does not meet the evacuation baseline. Between three and fourteen you are covered for a short outage but not for the two-week scenario that hurricane, wildfire and earthquake planning assumes. Once you are past your target, the useful question shifts from quantity to rotation and access.
Read the weight and floor-loading figures before you decide where the water goes. Concentrated load is the real constraint in a house: an upstairs closet is the worst place for 700 lb, a slab-on-grade garage the best, and a basement floor almost as good. If you must store on a framed floor, distribute the containers along a wall rather than stacking them in one corner mid-span, and keep the stack to a single layer where you can.
Split the storage physically as well. A three-day evacuation portion in containers one person can lift, near the door, is a different asset from a two-week supply in the garage. If you have to leave in ten minutes, only the first one matters.
Finally, remember the water you already own but have not counted. A conventional water heater holds 40 to 50 gallons that can be drained from the bottom valve once the power or gas is off and the supply is isolated. Toilet tanks (not bowls) hold a few gallons each. Neither is a substitute for planned storage, but both extend the coverage figure materially, and both are worth locating before you need them.
Water needed at the one-gallon baseline, by household size
| People | 3 days | 14 days | 30 days | Weight at 14 days |
|---|---|---|---|---|
| 1 | 3 gal | 14 gal | 30 gal | 117 lb |
| 2 | 6 gal | 28 gal | 60 gal | 234 lb |
| 3 | 9 gal | 42 gal | 90 gal | 350 lb |
| 4 | 12 gal | 56 gal | 120 gal | 467 lb |
| 5 | 15 gal | 70 gal | 150 gal | 584 lb |
| 6 | 18 gal | 84 gal | 180 gal | 701 lb |
These are the baseline only. A household that also flushes, cooks from dry goods or keeps pets will land 30 to 60% above the corresponding row, which is what the calculator adds.
Storage mistakes worth avoiding
- Reusing milk or juice jugs. Residual sugars and proteins support bacterial growth and the plastic is not made for long storage. Use food-grade containers intended for water, or commercially sealed bottles.
- Storing on bare concrete. Plastic can pick up flavour and, over long periods, permeate. Put a sheet of plywood or a pallet between the containers and the slab.
- Storing next to fuel, pesticides or solvents. Plastic containers are permeable to vapours. Keep water away from the gas can and the lawn chemicals.
- Leaving it in sunlight. Light drives algal growth in translucent containers and degrades the plastic. Cool, dark and dry is the storage condition.
- Forgetting a way to treat more. Storage runs out; a filter, unscented household bleach, or the ability to boil extends the plan indefinitely. Note that boiling does not remove chemical contamination.
- Buying containers you cannot lift when full. Five gallons is 41.7 lb. Anything above about seven gallons becomes a two-person job or a siphon job, which changes how you can use it.
Where water sits in a household preparedness plan
Water is the first item on every emergency supply list because the tolerance is shortest: food can wait days, water cannot. It is also the heaviest and bulkiest item on the list, which is why it dominates the storage plan and why the weight and floor-area outputs on this page matter as much as the gallon count.
The rest of the plan follows the same pattern of a daily rate multiplied by a horizon. If you are on well water, the pump is the single point of failure and the water softener sizing calculator covers the everyday side of the same supply. Households whose emergency plan includes a failing appliance decision — a freezer, a water heater — can use the appliance repair vs replace calculator to decide whether to fix it now or before the season.
For evacuation rather than sheltering, the constraint shifts from storage to transport, and drive time and fuel become the planning quantities. The road trip drive time calculator handles the first, and the tire tread life remaining calculator is worth a look before hurricane season for the vehicle you would leave in. If your plan involves being outdoors for extended periods, the UV index sunburn time calculator covers exposure, which also raises the drinking-water figure at the top of this page.
Stored water is not treated water
Water you bottled yourself from a safe municipal supply keeps for months in a clean, sealed, food-grade container kept cool and dark, but it is not sterile and the container is not the barrier a commercial seal is. If stored water looks cloudy, smells odd or has been in an open or damaged container, treat it before drinking. Follow current public-health guidance for the treatment method and dose rather than a remembered ratio, and remember that boiling handles microbes but not chemical contamination.