An octagon roof is commonly used for gazebos or turrets. Easy Rafters will calculate the overall dimensions of an octagon roof given either the common rafter run (inscribed radius) or the hip rafter run (circumscribed radius) as well as calculate the rafter dimensions and the layout of the jack rafters.
Easy Rafters is designed to calculate octagons where eight equal length hip rafters with 67.5º double top bevels meet at the center. The jack rafters are calculated to start one half of the spacing distance to either side of center allowing for easier nailing of the longest jack rafters. In practice it can be very difficult to cut the steep bevels required for the hip rafters to meet at the center. You may need to modify the results obtained with Easy Rafters if you plan to use a different method of octagon framing.