Voxler 3 and later can calculate volume from an Isosurface module or a FaceRender module. 

1. Select the Isosurface or FaceRender module in the Network Manager.
2. Click the General tab in the Property Manager.
3. Expand the Isosurface or Facerender section. 
4. Check the box next to Compute volume to display the volume values. The Isosurface will display the value above and below the specific isovalue.

The FaceRender module shows approximate volume which is controlled by the blocks that are visible.

Updated November 27, 2018