All online tools use javascript to calculate the dimensions. Use at your own risk. If the answers look wrong then verify the answers with a calculator.

This circular tower rafter script - calculator will return the circular rafter geometry angles and trigonometry rafter measurements for circular tower roof framing. The script returns the correct number of common rafters based on the rafter spacing for the circumference of the tower plate line. It also returns the correct number of 4' x 8' plywood roof sheathing for the circular tower. The circumference of the fascia is also displayed in the results.

  In a circle of radius r, the length of the chord that subtends a central angle x is r * sin( x/2).

Most structural engineers will call out 24" O.C. rafter spacing for the circular towers. However, the script default is 8" O.C. rafter spacing at the outside edge of the circular tower radius plates so you can actully attach the fascia correctly.