Size on Disk

Windows allocates diskspece in units called clusters. A cluster is made up of one or more sectors (a sector is almost universally 512 bytes). The size of a cluster depends on the total size of the disk but is a power of two multiple of 512 bytes (512, 1024, 2048, etc.).

Say you have a file that is exactly 5,000 bytes. If your cluster size is 512 bytes, it will use 10 clusters, or 5120 bytes. If your cluster size is 2048 bytes the same 5,000 byte file would use 3 clusters, or 6144 bytes.

If you are using NTFS and have compression turned on, the 'size on disk' number may be somewhat smaller than the size of the file.

In certain filesystem types like the File Allocation Table (FAT) filesystem of MS-DOS or the NTFS filesystem of Windows NT, a cluster is the unit of disk space allocation for files and directories.

If you are not using NTFS compression for any files or folders contained on the volume, the difference between SIZE and SIZE ON DISK may represent some wasted space because the cluster size is larger than necessary.

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This sort of a question should be in Tech Q & A.

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