The internal volume of a cylinder is very important to industrial applications such as chemical plant design. The most common mistake made when calculating internal volume is the inclusion of vessel wall thickness in the calculations. Internal volume is typically provided in cubical units such as cubic feet or cubic meters, but can also be converted to liquid volumes such as gallons or liters.
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Write down the assumed dimensions of a cylindrical vessel. These dimensions are height of 25 feet, outside diameter of 10 feet and a wall thickness of 1/4 inch (0.25 inches).

Calculate the actual internal diameter by converting the wall thickness to feet and subtracting two wall thickness values from the outside diameter. The equation becomes 0.25 inches divided by 12 (12 inches equals one foot) to give a wall thickness of 0.021 feet. The internal diameter is now 10 feet minus (2 x 0.021 feet), or 9.958 feet.

Determine the internal area of the vessel by multiplying the internal diameter squared by pi (3.14) and divided by 4. This equation is 9.958 squared x 3.14 divided by 4 for an answer of 77.88 square feet.

Calculate the internal volume of the cylinder by multiplying the internal area by the vessel height. The equation is 77.88 x 25 for an answer of 1,947 cubic feet. This can be converted to gallons using the conversion factor of 1 cubic foot equals 7.48 gallons for an answer of 14,563.6 gallons.
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