How to calculate %overlap of 2 segmentation objects

Hi! I am new to Slicer, and am trying to do segmentation of objects in MRI head. I would like to compare the overlap of 2 manual segmentation objects, how should I go about in doing it in the segmentation statistics?

Also would you recommend working with dicom or nii for best result?

You can use the Logical operators tool in the Segment Editor module then get the volume in Segment Statistics.

You can also find some segment comparison metrics in Segment Comparison module (provided by SlicerRT extension).