Hi Charlie, There are two tool in Chimera that can calculate volume enclosed in a surface, menu entries Tools / Volume Data / Measure Volume Tools / Surface Binding & Analysis / Surface Area & Volume The first one allows you to compute the volume enclosed in the solvent excluded surface. Just open your PDB model. Use menu entry Actions / Surface / show. That shows the solvent excluded surface. Then use the above Measure Volume tool to get the enclosed volume. I don't know of a way to get the volume within the solvent accessible surface (which will be larger) because Chimera does not have a way to compute that surface. The second tool above can compute the Voronoi volume by sending the PDB model to a web server http://molbio.info.nih.gov/structbio/basic.html I don't know the definition of that volume, but I assume it is neither the volume for solvent accessible nor solvent excluded surfaces. The paper that defines it is referenced in the Chimera manual if you are interested: http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/surfvol/surfvol.htm... Tom