To find the gallons of fluid that a tank can hold, first find the volume of the tank. Then, convert the volume of the tank into gallons.

The volume of a cylindrical tank is pi times r^2 times h, where pi is the mathematical constant, r^2 is the radius of the base of the cylinder squared and h is the height of the cylinder. If the tank is rectangular or a cube, then the volume is l times w times h, where l is the length, w is the width and h is the height of the cylinder.

To determine the number of gallons in the tank, first convert the dimensions of the tank into feet so that the result of the volume formula is in cubic feet. There are 7.48 gallons in a cubic foot; therefore, to get the number of gallons, divide the volume by 7.48. There are other conversation factors if the volume is expressed in other units, such as cubic meters or cubic yards.

For example, a rectangular tank with dimensions of 5 feet by 8 feet by 2 feet has a volume of 80 cubic feet; dividing 80 by 7.48 gives 10.7. Thus, this rectangular tank can hold 10.7 gallons of fluid.