  The mass evolution of a coronal mass ejection (CME) is an important parameter
characterizing the drag force acting on a CME as it propagates through
interplanetary space. Spacecraft measure in-situ plasma densities of CMEs
during crossing events, but for investigating the mass evolution, we also need
to know the CME geometry, more specific, its volume. Having derived the CME
volume and mass from remote sensing data using 3D reconstructed CME geometry,
we can calculate the CME density and compare it with in-situ proton density
measurements near Earth. From that we may draw important conclusions on a
possible mass increase as the CME interacts with the ambient solar wind in the
heliosphere. In this paper we will describe in detail the method for deriving
the CME volume using the graduated cylindrical shell (GCS) model (Thernisien et
al., 2006,2009, see Figure 1). We show that, assuming self-similar expansion,
one can derive the volume of the CME from two GCS parameters and that it
furthermore can be expressed as a function of distance.
