What is the length of a diagonal of a cube with a side length of 6 inches? Length of a diagonal of the face: 6 squared + 6 squared = c squared. 36 + 36 = c squared. 72 = c squared. StartRoot 72 EndRoot = c. Use the diagonal of the face of the cube and the side length of the cube to find the length of the diagonal of the cube. Round to the nearest tenth of an inch. The diagonal through the interior of the cube is about inches.