If I have an octagon that is 12 feet across (side to opposite side) how can I find the length of the sides? THank you Hi,I assume you have a regular octagon so you can think of it as a 12 ft by 12 ft square with the corners cut off. Suppose that each side has length S. In the diagram the corner triangle, ABC, has CA = BC and thus using the theorem of Pythagoras 2 CA^{2} = S^{2} CA = S/ Since the octagon is 12 feet across, S = 4.97 feet Harley
