The area of a rectangular elbow is the bending radius of the elbow multiplied by the circumference of the rectangular elbow. In general, the bending radius of rectangular elbow is given. The inner arc and outer arc of rectangular elbow are both rectangular, and there are two sectors on both sides. This calculation method is relatively simple. You can ask questions if you want to learn the basics of other elbow joints.
Ventilation hose
elbow joint
reducing elbow