How to Use the Water Tank Capacity Calculator
For a rectangular water tank, you find the volume by multiplying length × width × height, then convert that volume into gallons. One cubic foot holds about 7.48052 gallons, so multiplying the volume in cubic feet by that factor gives you the tank's storage capacity. This same formula works for rooftop water tanks, rainwater cisterns, aquariums, and rectangular fish tanks — any rectangular storage container.
Keep in mind the calculated value is the theoretical maximum if the tank were filled all the way to the brim. In practice, it's safer to plan around about 90% of that capacity to leave room for overflow prevention, plumbing pressure, and thermal expansion. This calculator shows both the maximum capacity and the recommended 90% usable capacity, which is handy when sizing a tank purchase or planning water storage.
Frequently Asked Questions
For safety, plan on using about 90% of the calculated capacity. Filling all the way to the brim can cause overflow or pressure issues.
No, this calculator is for rectangular tanks. Cylindrical tanks use the formula pi times radius squared times height, which requires a separate calculation.