13m greenhouse arch is the most reasonable. Take a greenhouse 8 meters wide and 4 meters high as an example. At this time, the radian of the greenhouse is just a semicircle, that is, 4x2x3. 14/2= 12.56 (m), and its calculation formula is: diameter x3. 14/2. At this time, the radian of the greenhouse is just a semicircle, that is, 4x2x3. 14/2 =12.56 (m), and its calculation formula is: diameter x3.14/2.