Several arc tools are available for sketching curves or rounded corners.
There are several options for sketching arcs in Revit:
You specify a start-end-radius arc by drawing the chord that connects the 2 ends of the arc, and then you specify the angle or radius with the third point.
To create an arc from the center and end points, first specify the radius for the arc, and then specify the angle. You cannot specify an arc of more than 180 degrees using this method, although you can modify the arc after you draw it.
You can create tangent arcs from the end points of existing walls or lines.
Use fillet arcs when you need to create rounded corners.
For information on constraining arc sketches, see "Using Constraints When Sketching" in About Sketching Elements.