Difference in File Size and File Size on Disk

When copying files from place to place in Windows you may have noticed that there is a difference between the size of the files in a folder and what Windows calls the 'Size on disk'.

The reason for this is that data is allocated in Windows in clusters, and each cluster must take up a certain number of bytes. Each file no matter how many individual bytes it has, must take up a fixed number of clusters. This leads to wasted space. For example a file that is 13.4 KB (13,753 bytes) will take up 16.0 KB (16,384 bytes) when the cluster size is 4096 bytes. You need four 4096 clusters to contain all of the data. You can determine the number of clusters taken up by a data set on your PC by using the Cluster Size Calc utility, available for download here. In this screen grab we can see how the utility calculates possible cluster sizes for the C:\FooFolder directory for which the Windows properties are shown above. We can see that since the size of the files in the folder is 81,550,109 bytes and the size on the disk is 81,625,088 bytes, with a difference of 74979 bytes, the cluster size must be 4096 bytes.