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:

• Usage of different file size measurements. There are two different ways to measure a file's size. The logical size corresponds to the size of the file's contents. The physical file size is the space needed to store it, which is typically slightly larger. Finder uses logical file sizes, whereas GrandPerspective can use either depending on your Preferences setting. So when you scan using the physical size measure, the size reported by GrandPerspective will typically be larger.
• Usage of different file size unit systems. There are two common unit systems for reporting file sizes. When the binary unit system is used, 1 KB corresponds to 1024 bytes. When the decimal unit system is used, 1 KB corresponds to 1000 bytes. Finder uses the decimal unit system, whereas GrandPerspective can use either depending on your Preferences setting. So when you scan using the binary unit system in GrandPerspective, the size reported by GrandPerspective will be smaller.
• The difference caused by hard-linked files. Hard links allow multiple copies of the same file to exist, without the content being replicated in the physical storage. The file is only stored once. So for example, if you have a file of 1 MB and two hard-linked copies of it, this requires only 1 MB of storage instead of 3 MB. GrandPerspective accounts for this and reports the actual storage space that is required. Finder, on the other hand, counts every copy of the file thus over-estimating the actual storage space. So when GrandPerspective scans a folder with multiple hard-linked copies of the same file, the size reported by GrandPerspective will be smaller.
• The difference caused by ignored files and folders. When you scan using a filter, GrandPerspective may exclude certain files and folders. Their size will then not be accounted for in the total folder size. Similarly, when GrandPerspective does not have the required permissions to read certain files and folders, they are also not accounted for. When this happens the size reported by GrandPerspective will therefore be smaller than the actual storage space that is used.

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 tab. The free space shown in the Info tab 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.