What is the difference between ''Size" and "Size on Disk?

When you view properties of folders/files in Windows, why do you see Size and Size on disk. What's the difference?

• Answer:

  As nullpointer pointed out the below answer has to do with hard drive size not file size so I've added the file size difference. FILE SIZE DIFFERENCE The difference between the "file size" versus the "file size on disk" is that the hard drive is divided into clusters and each cluster has a minimum size in terms of KB (each cluster is fixed in terms of its size regardless of the space the file actually uses). The minimum size is determined by your file system. Under NTFS, the minimum cluster size is 2KB to 4KB (depending how your drive is partitioned). Keep in mind that the difference between the 10base and 2base still applies on top of the affects of the cluster size so the minimum size of 4KB is 4KB in base10 but 4096 in base2. Since a file might not use the entire cluster space a difference between the size of the cluster used (file size on disk) and its actual file size creates the difference. If you have a 43KB file and each cluster is 4KB, then the difference would be 1KB (11X4KB clusters = 44KB - 43KB = 1KB). This occurs since only 1 file can use each cluster. You can see in the attached file that my file size is 994bytes where as the file size is the minimum 4KB cluster size for my NTFS file system. HARD DRIVE DIFFERENCE The difference is related to the practice of defining a KB which can be 1000 (base 10) or 1024 (base 2) depending on who you ask. The computer industry when selling you a drive will use based 10 or 1000 bytes to equal 1KB in their advertising while your computer only sees the drive in terms of 1024 bytes per KB. So that means that 1GB in terms of base 10 is 1,000,000 bytes (10^9) while in base 2 it is 1,073,000 bytes (2^30). To get the actual size of your drive you need to multiply the advertised drive size by approx 93.13 (1,000,000/1,073,000) which for a 40GB drive would be 37.2GB. Another factor to subtract is that when the hard drive is formatted part of the drive is unavailable for use so the size of the drive is also decreased by that amount and does show under properties so my 40GB drive would actually shows 39.925GB versus the 40GB. So my actual base 2 drive size is 37.1GB but was advertised as 40GB. Hope this helps.