An apparatus for calibrating an x-ray computed tomography device has a plurality of objects formed from a material that is visible to x-rays. The plurality of objects are configured to receive x-rays without changing shape, and have substantially the same shape. The objects each have an object attenuation value to x-rays and a center point such that they are symmetrically shaped relative to their respective center points. The apparatus also has a base at least in part fixedly supporting the plurality of objects so that each of the plurality of objects contacts at least one of the other objects. Like other physical objects, such as the objects, the base has a base attenuation value to x-rays, and that value is greater than the base attenuation value. The center points of the plurality of objects together form a three-dimensional volume.