The Concentric tool 
enables you to make selected circles, arcs or ellipses concentric
to another circle, arc or ellipse.
First, select one or more circles (or arcs/ellipses).
Then, select a circle, arc or ellipse that will define the position of the concentric circles/arcs/ellipses.
Finally, select the Concentric tool.
If none of the entities are constrained, the entities that are selected first move to be concentric with the entity that is selected last.
If entities are constrained, whenever possible, the independent entities will move to be concentric with the constrained entity.
Note: If the original circle / arc / ellipse is not in the current sketch, you must add it to the sketch as a reference entity.
To make circles/arcs/ellipses concentric to a selected circle/arc/ellipse:
Select the Add
Constraints icon
from
the Sketcher toolbar.
The cursor will be a pointer-and-grid 
symbol.
Click one or more circles/arcs/ellipses that you wish to make concentric to another circle/arc/ellipse.
In the example above, two circles are selected. Both these circles will be concentric to the circle/arc selected last (in step 3).
Click the circle/arc/ellipse to which the entities selected in step 2 will be concentric.
Click the Concentric
tool .
The entities selected first (in step 2) move to be concentric with the entity selected last (in step 3).
In the example above, none of the entities were previously constrained. If entities are already constrained, whenever possible, the independent entities will move to be concentric with the constrained entity.
Notes:
You can also first pick the entities to constrain (while in Select mode), then select the Add Constraints option, then choose the desired constraint.