Choose Inspect tab→3D Measure group→Normal Distance 
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Using the Element Types box on the Measure Normal Distance command bar, define the type of elements you want to measure from.
Select an element to measure from.
Select a keypoint to measure to.
Tip:
In an assembly, you can activate inactive parts that you want to measure to or from by selecting the Activate Part button on the Measure Normal Distance command bar and then selecting the part.
You can use the Keypoint option on the Measure Normal Distance command bar to specify the type of keypoint you want to identify when measuring the distance.
You can select a user-defined coordinate system to define one of the points. If you use a coordinate system, the returned values will be relative to the specified coordinate system.
You can copy the measurement value to the Clipboard by highlighting the value and pressing Ctrl+C.