Windows – Whats the actual file size

Size is the actual size of the file in bytes.

Size on disk is the actual amount of space being taken up on the disk. They differ because the disk is divided into tracks and sectors, and can allocate blocks of discrete size.

Editing

For a more detailed explanation, see this text which I copied from another site:

We know that a disk is made up of Tracks and Sectors. In Windows that means the OS allocates space for files in "clusters" or "allocation units".

The size of a cluster can vary, but typical ranges are from 512 bytes to 32K or more. For example, on my C:\ drive, the allocation unit is 4096 bytes. This means that Windows will allocate 4096 bytes for any file or portion of a file that is from 1 to 4096 bytes in length.

If I have a file that is 17KB (kilo bytes), then the Size on disk would be 20.48 KB (or 20480 bytes). The calculation would be 4096 (1 allocation unit) x 5 = 20480 bytes. It takes 5 allocation units to hold a 17KB file.

Another example would be if I have a file that is 2000 bytes in size. The file size on disk would be 4096 bytes. The reason is, because even though the entire file can fit inside one allocation unit, it still takes up 4096 of space (one allocation unit) on disk (only one file can use an allocation unit and cannot be shared with other files).

So the size on disk is the space of all those sectors in which the file is saved. That means,usually, the size on disk is always greater than the actual size.

So the actual size of a file(s) or folder(s) should always be taken from the Size value when viewing the properties window.

Source: What's The Difference Between Size And Size On Disk In Windows Folder Properties.

File size on FAT is stored in a 32-bit field.

On FAT32 the field is treated as unsigned allowing for files up to 4294967294 bytes (apparently 4294967295 is not allowed for some reason, possiblly "-1" was used as a flag somewhere).

On FAT16 the field is limited to the range of a signed 32-bit number. I expect the reason for this is legacy code that was written back when 2GB would have been considered an insane size for a file. FAT16 filesystems beyond 2GB were an oddity anyway*.

AIUI on FAT12 the file size is limited to 32MB (not 32KB!) not because of any limitation specific to file size, but simply because that is the maximum size of the volume and you can't have a file bigger than the volume. The same would apply to early versions of FAT16.

* The version of FAT16 used by DOS 4 through windows 95 was limited to 64 sectors per cluster which on typical 512 byte sectors meant a roughly 2GiB volume size. NT doubled the maximum sectors per cluster allowing ~4GiB volumes with 512 byte sectors and 98 added support for reading/writing but not creating or repairing such volulems. Bigger volumes were theoretically possible if the underlying device had bigger sectors but I have seen no evidence that this actually happend in practice.