Calculators/ Landscaping/Water Tank Calculator
Water Tank Calculator
Capacity in gallons for tanks and cisterns.
Water Tank Calculator
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Water is heavy: 8.34 pounds per gallon, so a full 500 gallon tank weighs over two tons. The capacity is simple geometry, but the weight is what determines whether the stand or slab under it is adequate.
How this is calculated
- Cylindrical: π × radius² × height. Lay the same tank on its side and the diameter becomes the height.
- Multiply cubic feet by 7.48 for US gallons.
- Rainwater harvesting: one inch of rain on one square foot of roof yields about 0.623 gallons, before losses.
Frequently asked questions
How much does a full water tank weigh?
8.34 lb per gallon plus the tank. A 500 gallon tank holds over two tons of water, which needs a proper base, not bare soil.
How much roof do I need to fill a tank?
Divide the tank capacity by 0.623 to get the square feet of roof needed per inch of rainfall. Real collection runs 75 to 90% of that after first flush and evaporation.