You can create a arch from a circle, or cylinder.

1. Start by deleting the bottom half of the vertices.
2. model the circle in to the arch shape you want.
3. add a mirror modifier for a symmetrical arch, and delete half of the vertices.

You can create a arch from a circle, or cylinder.

1. Start by deleting the bottom half of the vertices.
2. If you want a symmetrical arch add a mirror modifier and delete half of the vertices.
3. Model the circle in to the arch shape you want.