A method and system for computing disk space used by a directory and its descendants that may proceed while input and output are occurring that involve the directory and any of its descendants. A calculation of the total amount of disk space consumed by a set of objects is commenced. While performing the calculation, changes to the objects are monitored. If a change to one or more of the objects affects the disk space consumed by the objects, the calculation is changed so that the total reflects the change to the one or more objects.