The angle on a 4/12 pitch is 18.43 degrees. The pitch is the comparison of the roof rise and run.
The roof angle of incline is what primarily affects pitch. Homes are constructed using one of a common set of roof angles that are described in terms of the pitch. The 4/12 pitch is the standard roof found on most homes. It is not too steep and relatively easy for roofers to walk on.
To visualize pitch, imagine a ruler held exactly horizontally to the ground, placed over a roof. The length of the ruler is 12-inches, which is the run. The vertical distance between the open end of the ruler and the roof surface is the rise.