Why is the difference between File size and File size on disk ?

What is the difference between file size and file size on disk in Windows ? Why there are two types of file size ? Which one is relevant ?

Hi,

The difference between file size and file size on disk is:

File size - is the actual size of the file.

File size on disk - is the size that file occupies on disk. The file size on disk is usually bigger, because the disk is divided into a units (clusters) where data are stored. Depending on device, the size of a cluster may vary. When you copy the file onto your disk, it is usually stored in a multiple clusters. This increases the amount of used space on the disk.

File size on disk is usually greater, because there is some "extra space" used to store the file on disk. So the file occupies a bit more space on disk, than it really has.