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 polygon rafter script - calculator will return the polygon rafter geometry angles and trigonometry rafter measurements for equal sided polygon roof framing.

  In a circle of radius r, the length of the chord that subtends a central angle x is r * sin( x/2).
This script uses the theory of an polygon inscribed inside of a circle. So the radius is the run of the polygon hips and the chord is the length of the sides of the polygon.

polygon-rafter.dxf -- auto generated DXF CAD file of polygon rafters drawing each time script is run
polygon-rafter.pl source code for perl script that generates these octagon rafter calculations
dxf_header.txt DXF header text file used by this script