Identify the axis of symmetry for the object. For example, the axis of symmetry for a cylinder is the imaginary line directed down the middle and extending the length of the cylinder.
Imagine a plane oriented perpendicular to the symmetry axis slicing through the object. The intersection of the plane with the object forms a two-dimensional projection on the plane.
Identify the geometric shape formed by the projection of the object on the cross-sectional plane. For example, the cross section of a sphere or a cylinder is a circle, while the cross section of a cube will be a square.
Measure the dimensions of the geometric shape using the ruler. For example, for a circular shape, measure the radius. If the shape is rectangular, measure the width and length of the rectangle.
Calculate the cross-sectional area by determining the area of the geometric shape. For a rectangle, multiply adjacent side lengths using this formula: area = length x width. For a circle, the area = (3.14) x radius x radius. For example, the cross-sectional area of a wire with a radius of 0.1 inch is: area = (3.14) x (0.1) x (0.1) = 0.0314 inches squared.