This study compared digital measurement of aneurysm volume by 3D rotational angiography (3D-RA) with an approximation technique using three diameters of an aneurysm to re-interpret previously reported optimal packing densities (volume embolization ratio, VER) in coil embolization of intracranial aneurysms. Estimation of the volume of saccular aneurysms is important for calculation of the VER, which is in turn reported to be useful for prediction of coil compaction. The conventional formula for the volume estimation is V=4/3 π(A/2) (B/2) (C/2), where A, B, and C are the aneurysmal height, length, and width measured on 3D-RA image respectively. Using 3D rotational angiography data from 74 aneurysms, the approximated volume generated using the conventional formula was directly compared with the digitally measured volume. The digitally measured volume varied from 0.003 ml to 7.935 ml, and the dome-to-neck ratio (D/N) from 0.79 up to 4.62. We found that the conventional formula almost systematically underestimated the volume by up to 50 %, particularly when the neck was large relative to the dome (D/N<2). On average, digitally measured volume was 1.26~1.29 times larger than the approximated volume obtained using the conventional formula.Conventional 2D angiography based aneurysm volume calculation tends to underestimate an aneurysm volume, so the so-called VER (volume embolization ratio) could be overestimated.