The area of each rectangle in a view window is proportional to the size of the file it represents. There are different ways to measure a file's size. The measure that GrandPerspective uses when scanning a folder is determined by its Preferences.

File size measures

GrandPerspective supports the following file size measures:

• Logical size corresponds to the size of the information in the file. For example, the logical size of a simple ASCII text file is the number of characters (including spaces and linebreaks) it contains.
• Physical size represents the space that a file takes up on disk. A file's physical size is typically slightly larger than its logical size due to the way files are stored. For example, a file with a logical size of ten bytes can have a physical size of 4096 bytes.
• Tally size simply counts the number of files. Each file has size "one" irrespective of its actual size on disk. The size of a folder therefore equals the number of files it contains.

Which file size measure to use is partially a matter of taste but also depends on the situation. Physical size best shows how much space is freed up when you delete a file but exposes more details of the underlying file system. Logical size best represents the intrinsic size of the file content but can occasionally severely over-estimate the disk usage. In particular, the logical size can be larger than the actual required storage for:

• Files stored using HFS+ compression. In fact, very small files can even have a physical size of zero bytes. This happens when their content can be stored entirely as part of the file's metadata. Such a file is then invisible in the view window.
• Sparse files in the APFS file system. Sparse files are used to efficiently store data that may contain many empty blocks. A typical use case is a data volume used by a Virtual Machine or Docker container. The logical size of such files can be very large, yet they require relatively little storage space when these volumes are largely empty.
• Cloud files that are not (yet) downloaded. These have an icon in Finder. The size on disk for these files is minimal. The reported logical space is typically much larger, as it corresponds to the content. The logical size does not help you free space on your hard-drive but can help you to reduce your cloud storage.
• Online-only Dropbox files. These files are part of Dropbox Smart Synch feature. They appear in the Dropbox folder on your computer, but their content is stored remotely. These files therefore similarly take up less space on your drive than the actual content.

Tally size should only be used for exceptional circumstances. For example when you want to find folders that contain many (possibly small) files.

File size unit systems

When reporting file sizes, GrandPerspective uses one of the following unit systems:

• Binary, in this case one kilobyte contains 2¹⁰=1024 bytes. Similarly, a megabyte contains 2²⁰ bytes, and so on.
• Decimal, in this case one kilobyte contains 10³=1000 bytes. Similarly, a megabyte contains a million bytes, and so on.

See also

• How to explain differences in sizes reported by GrandPerspective compared to that reported by other tools