This is a test of commonsense. Complete the next sentence:

How to work out water tank capacity
Measure the radius of the circle at the bottom of the cylinder.
The region enclosed by the circle at the bottom of the cylinder is your bottom base surface (b). A radius is any line segment that runs from a circle's center to its perimeter.
To find the radius, simply measure from the mid-point of the cylinder's bottom to the outside of the circle. A diameter is any straight line segment that passes through the center of the circle and has endpoints on the circle's perimeter.