Operator math, worked
How to calculate the volume of a round tank in gallons
A round (cylindrical) tank holds water in a circle shape. You find the gallons by getting the area of the circle, multiplying by the water depth for cubic feet, then turning cubic feet into gallons.
Last reviewed 2026-08-04
The formula
gallons = 0.785 x diameter x diameter x depth x 7.48
The steps
Find the area of the circle
Multiply 0.785 by the diameter, then by the diameter again. That is the top area in square feet.
Get the cubic feet
Multiply that area by the water depth in feet.
Turn cubic feet into gallons
Multiply the cubic feet by 7.48.
Worked example
A round tank is 30 feet across (diameter) and 20 feet of water deep. How many gallons?
- Area: 0.785 x 30 x 30 = 706.5 square feet
- Cubic feet: 706.5 x 20 = 14,130 cubic feet
- Gallons: 14,130 x 7.48 = 105,692 gallons
Answer: about 105,692 gallons
Common questions
- What is 0.785?
- It is pi divided by 4, the shortcut for the area of a circle from its diameter.
- Do I use radius or diameter?
- Diameter, twice. The 0.785 formula is built for diameter.
- What if I only know the radius?
- Double it to get the diameter, or use area = 3.14 x radius x radius.