You have probably learned how to calculate the perimeter of a shape. For most regular shapes, you just measure the length one of the sides and multiply it by the number of sides in the shape. For an irregular shape, you must measure all of the sides and add up their lengths. Calculating the perimeter of a 3D object follows similar logic.
Measure one of the edges.
Count the number of edges that the object has. For example, a cube would have 12 edges.
Multiply the length of each edge by the number of edges. The result will be the perimeter of the 3-dimensional object.
Measure each of the edges.
Write down the length of each edge.
Add the values of the lengths together. The result will be the perimeter of the object.
TL;DR (Too Long; Didn't Read)
An alternative method of finding the perimeter of a regular object is to multiple the length of a side by half of the number of faces that the object has. This should give you the same result as the method above.
Do not simply multiple the perimeter of each face by the number of faces. Because each edge is shared between two faces, this will give you twice the perimeter of the object.