Measure Radius And Diameter
Measure radii and diameters.
Click the 
Measure/Markup tool in the Tools group. Click the 
Radius button to measure radii or click the little black arrow in the button to select another measure function.
Measure Radius
Measure the radius of a circle.
Move the cursor across the model, and the automatic selection highlights possible circles in light blue.
Click to select a circle.
If the automatic selection does not find the wanted circle, activate the option By 3 points and select three points on the circle to be measured.
To make the selection easier, the points are displayed.
Measure Diameter
Measure the diameter of a circle.
Move the cursor across the model, and the automatic selection highlights possible circles in light blue.
Click to select a circle.
If the automatic selection does not find the wanted circle, activate the option By 3 points and select three points on the circle to be measured.
To make selection easier the points are displayed.