All, 

Upon looking at online forums and the FreeSurfer wiki, I have come to the understanding that "BrainSeg" volume in the aseg.stats file includes all cortical and subcortical gray matter, cerebellar gray matter; cortical, subcortical and cerebellar white matter and white matter hypointensities; and all ventricle volumes, but excludes the brain stem.

When using the aseg.stats file for included subject "bert" to check my understanding of which volumes are used, I cannot seem to produce identical "BrainSegVol-to-TIV" ratios. 

I am adding the following volumes, all in mm^3, to obtain the brain volume: 

TotalGrayVol: 658087.056198

CorticalWhiteMatterVol: 485627.161537

Left-Lateral-Ventricle: 6562.6

Left-Inf-Lat-Vent: 228.3

Left-Cerebellum-White-Matter: 15708.2

3rd-Ventricle: 997.5

4th Ventricle: 1714.2

Right-Lateral-Ventricle: 6039.9

Right-Inf-Lat-Vent: 160.3

Right-Cerebellum-White-Matter: 15694.5

5th-Ventricle: 14.1

WM-hypointensities: 1002.6

Sum of individual measures: 1191836.418

Output of BrainSegVol: 1193318

And when normalizing the subject's brain volume by the eTIV measure (1594882.911364), I obtain the following:

Individual measures: 0.7473

BrainSegVol-to-eTIV: 0.7482

Is there a reason that these two measurements are slightly off?  I'm wondering whether there's certain volumes used for BrainSegVol that I missed, or ones that I included that would not be present in the output.  Moreover, I'm wondering whether this could potentially be a result of using surface volumes (BrainSegVol) versus using both surface and voxel-space calculations (my addition method). 

Any help is appreciated.  Thank you in advance. 

Best,