[header] How to work out water tank capacity [title] Measure the radius of the circle at the bottom of the cylinder. [step] 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. [substeps] A diameter is any straight line segment that passes through the center of the circle and has endpoints on the circle's perimeter.