Water Storage Calculator

Approximate water-storage planning estimates based on a baseline of about one gallon (≈3.8 litres) per person per day. This is common emergency-preparedness guidance, not a universal engineering rule: actual needs vary with local climate, activity, health, drinking versus non-drinking use, household size, emergency duration, and local guidance.

How much water should you store?

A widely used preparedness guideline is to store at least one gallon (about 3.8 litres) of water per person per day. Roughly half of that is for drinking, and the rest covers food preparation and basic hygiene. A common recommendation is to keep at least a three-day supply for evacuation and, where storage space allows, a two-week supply to stay at home. These figures are general emergency-preparedness guidance rather than a universal rule, and the right amount depends on your local climate, intended use, household size, emergency duration, and the guidance of your local authorities.

For a household of four planning a two-week supply at one gallon per person per day, that works out to 4 × 14 = 56 gallons. The calculator lets you adjust the per-person rate, the number of people, the number of days, and an optional allowance for pets.

Factors that increase water needs

Several situations raise the amount you should store above the baseline. Hot climates and physical exertion can roughly double drinking-water needs. Children, nursing mothers, and people who are ill or pregnant generally need more. Pets add to the total — a rough planning figure is about one ounce of water per pound of body weight per day for dogs and cats, which you can convert and add as extra gallons per day. Medical needs and sanitation beyond the basics also add up.

Storing water safely

Use food-grade containers intended for water, keep them sealed and out of direct sunlight, and label them with the date. Commercially bottled water is the most reliable option; if you store tap water, replace it periodically. This calculator estimates quantity only — it does not address water treatment, purification, or rotation schedules, which are important parts of a complete plan.

Water tank volume and capacity in gallons and litres

Storage planning tells you how much water you want; this section shows how to find how much a physical water tank actually holds. The volume depends on the tank's shape:

• Rectangular tank: volume = length × width × height.
• Upright round (vertical cylindrical) tank: volume = π × radius² × height. This applies to a full vertical cylinder; a horizontal cylinder, or any cylinder that is only partly full, is not a simple π r² h calculation.

Once you have the volume, convert it with these standard factors:

• 1 cubic foot ≈ 7.48052 US gallons
• 1 cubic metre = 1,000 litres
• 1 US gallon = 3.785411784 litres

Worked examples. A 1 cubic metre tank holds 1,000 litres, which is about 264.2 US gallons. A rectangular tank measuring 4 ft × 3 ft × 2 ft is 24 cubic feet, which is about 179.5 US gallons (24 × 7.48052) or roughly 680 litres.

To compute capacity from your own measurements — including partial fills and shapes beyond a simple box — use the tank volume calculator. For specific shapes, see the rectangular tank volume calculator, the vertical cylindrical tank volume calculator, or, for tanks lying on their side or partly full, the horizontal cylindrical tank volume calculator. To read how much water is in a tank at any dipstick level, the tank dip chart generator builds a level-to-volume table for your dimensions.

Frequently asked questions

Assumptions and limitations

The figures here are approximate planning estimates, not guaranteed requirements. They assume a steady per-person daily rate and do not account for leaks, evaporation, spillage, or contamination losses. The default of one gallon per person per day is a general baseline; your actual needs may be higher, especially in hot weather or for vulnerable household members. Use this as a starting point and adjust for your own situation.

Water tank capacity figures are geometric estimates of internal volume. The internal volume may differ from the usable volume: wall thickness, fittings, inlet and outlet levels, baffles, and sediment can all reduce how much water a tank effectively holds. The emergency-storage examples are planning examples only.

Disclaimer: This calculator is for general planning only and does not constitute emergency, medical, or safety advice. Tank capacity results are approximate geometric estimates, not custody-transfer measurements. For authoritative guidance on emergency water storage and treatment, follow official preparedness and public-health resources from your local emergency-management authority.

References and cross-checks

• Ready.gov — Water (one gallon per person per day for drinking and sanitation).
• CDC — How to Create and Store an Emergency Water Supply (at least 1 gallon per person per day for 3 days; try for 2 weeks if possible).
• American Red Cross — Survival Kit Supplies (3-day supply for evacuation, 2-week supply for home).

These official preparedness sources are provided as references for the one-gallon-per-person-per-day baseline and the 3-day / 2-week supply guidance. Actual requirements depend on climate, activity, health, and local conditions.

Need to know the volume or capacity of a specific water tank or cistern? Use the tank volume calculator to compute full and partial volume for cylinders, rectangles, and other shapes, or see the methodology page for the formulas and unit conversions used across this site.