The use of packing algorithms to fill build volumes in 3D printing promotes both more efficient processes and better utilisation of the available build space. There are various packing techniques, and the choice of an appropriate one is often highly dependent on the characteristics of the parts to be printed, among other factors. Part complexity, and particularly convexity, is an important factor. This paper presents metrics for quantitatively evaluating part complexity, extending a 2D metric to a 3D situation. These metrics are then available for classifying problems and identifying appropriate packing algorithms.