There are 2 methods that I like to use:

1) Create a poly torus with 4 sides. Set the twist to 45. Adjust the subdivisions and radius to my liking. Delete half of the torus. Extrude the bottom edges to create the height of the arch.

or

2) Draw a curve that represents the path of the arch. Draw another curve that represents the profile of the arch (in this case a square or rectangle will work). Select the profile then shift select the path. Perform a Surfaces > Extrude with the output type set to polygons.

In both cases, you can simply adjust elements in the history the original components to tweak your arch.