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How to explain differences in reported sizes

This section explains why GrandPerspective may sometimes report different sizes than other tools, for example Finder.

Differences in the reported size for a given folder or volume

Sometimes the total size of a folder or total used size of a volume as reported by GrandPerspective may differ from that reported by other means, e.g. via Finder's Info Panel. There can be various reasons for these differences:

Differences in the reported amount of free space

The free space of a volume as shown by GrandPerspective in a view window when the entire volume is shown, can be less than the (initial) free size shown in the Info Panel. The free space shown in the Info Panel is the free space as reported by the file system. Let's call it X. GrandPerspective also calculates the size of the volume minus the total size of scanned files. Let's call that Y. As the total size of scanned files can under-estimate but never over-estimate the actual used storage size, it should be the case that Y ≥ X. However, it can occur that Y < X, apparently because either the volume size or the free space as reported by the file system is inaccurate. In this case, Y is used as the size of free space in the view window. This way the mismatch between the volume size in the view and the volume size as reported by the file system is minimised.