Good rafter length calculators can be found at many sites, including Lowe's, Blocklayer.com and Wendricks Truss. The calculators found at these locations are free to use.
The Lowe's rafter length calculator requires the rise and run dimensions of the roof. The calculator can provide estimates on the required rafter length. Helpful tips can be located in the how-to and ideas section on the home page.
The rafter calculator on Blocklayer.com can provide measurements in feet or inches. Rafter cut templates and birds-mouth templates are available on the site. Ridge thickness should be factored in order to determine the rafter run to the outer wall.