Hi Marky, there isn't currently a built in way but there is a plug-in you can install to get that.

To get it go to Max Smirnov's file archive here: http://moi.maxsm.net/media/files/ and on there find the one named CVolume.v.2.0.2015.09.27.zip . There are instructions for how to install a plug-in on the FAQ here.

There are also various STL viewing programs that should be able to calculate that for you as well.

It is something that I want to add as a built in function in MoI in the future.

- Michael