Diameter Dimension
The Diameter Dimension 
tool
measures the diameter of a circle and places a diameter dimension.
Creating a diameter dimension
-
Select the Diameter Dimension tool.
-
Pick a circle.
-
Move the mouse to position the dimension, then click to place it. Hold Shift for a full-diameter dimension.
The Text and Tolerance input boxes let you override the dimension text and add a tolerance. See the section overview for the options shared by all dimension tools.