Water Tank Size Calculator
Sizing a water tank — for storage, rainwater, an RV or emergencies? Enter the dimensions to get the capacity in gallons, and compare it to the reserve you need from the number of people, their daily use and how many days you want to cover.
Your numbers
Result
- Reserve neededgallons
- 12
- Covers the reserve?
- Yes
- Weight when fulllb
- 2,994.6
| Item | Value |
|---|---|
| Tank capacity | 359 gallons |
| Reserve for 4 people, 3 days | 12 gallons |
| Weight when full | 2995 lb |
How to calculate tank gallons
Rectangular tank: length × width × height (in feet) gives cubic feet; multiply by 7.48 for gallons. Cylindrical: π × radius² × height × 7.48. A 4 × 3 × 4 ft tank holds 48 cu ft ≈ 359 gallons.
How much reserve you need
Multiply people × gallons per day × days. For emergency storage, FEMA suggests 1 gallon per person per day (drinking and basic hygiene). For full household use — showers, laundry, dishes — plan 50 to 100 gallons per person per day. A family of four wanting 3 days of drinking water needs about 12 gallons; three days of full use is far more.
Mind the weight and support
Water is heavy: about 8.34 pounds per gallon, so a full 359-gallon tank weighs roughly 3,000 lb (1.5 tons). Before placing a tank on a stand, floor, deck or roof, make sure the structure can carry it full — this is a real safety issue, not just a number. Keep stored water covered and rotate it to keep it fresh.
Frequently asked questions
Sources
- FEMA / Ready.gov — emergency water storage (1 gal/person/day)
- Volume geometry; 1 cu ft = 7.48 US gallons; water ~8.34 lb/gal
Embed this calculator
Copy the code and paste it into your site or blog. Credit with a link back to Contania is included, and the height adjusts itself.
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<script>window.addEventListener("message",function(e){if(e.data&&e.data.type==="contania:embed-height"){var f=document.querySelector('iframe[src="https://contania.app/embed/home-improvement/water-tank-size"]');if(f){f.style.height=e.data.height+"px"}}});</script>Related calculators
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