First measure the diameter of the base usually easier than measuring the radius then measure the height of the cylinder. Visual in the figure below. The difference of the external and internal radius forms wall thickness of a cylinder ie.
The height is the distance between the top and the bottom bases.
Volume pr 2 h where r is radius and h is height of the tank. In other words the space covered by a cylinder is called the volume of a cylinder. Volume of a cylinder ph r12 r22 Where r 1 external radius and r 2 internal radius of a cylinder. The result of the cos -1 function in the formula is in radians.