A computer-implemented method for automatically determining estimated item dimensions may facilitate operations of a materials handling facility when exact dimensions of items are not known. Such a method may involve determining dimensions of a container, portal, or dimensionally constrained path in which an instance of a given item is handled and determining estimated dimensions of the item dependent on dimensions of the container, portal, or path. In some embodiments, the method may include determining dimensions of a second container, portal, or path in which an instance of the item is handled and updating the estimated item dimensions dependent on the dimensions of the second container, portal, or path. In some embodiments, the method may include recommending a container, portal, or path suitable for storing or transporting one or more items dependent on the estimated dimensions. In some embodiments, the estimated dimensions may facilitate identifying items in an inventory area.