I building a 12 ft x 20 ft building I want to put a 4 12 pitch roof on what angle do I cut my rafters and how long should they be thanks Dale

I thought their might be a Carpenter on this site, since no answer:
Pitch to Angle - Angle to Pich Converter (http://www.blocklayer.com/PitchAngle.aspx)

Which run are the rafters? The 12' or the 20'?