All file systems that Windows XP uses to organize the hard disk are based on cluster (allocation unit) size, which represents the smallest amount of disk space that can be allocated to hold a file. The smaller the cluster size, the more efficiently your disk stores information. If you do not specify a cluster size for formatting, Windows XP Disk Management bases the cluster size on the size of the volume. Windows XP uses default values if you format a volume as NTFS by either of the following methods:
- By using the format command from the command line without specifying a cluster size.
- By formatting a volume in Disk Management without changing the Allocation Unit Size from Default in the Format dialog box.
The following table shows the default cluster size for NTFS when Windows XP uses formatting. 
Default cluster size for FAT file system. These sizes apply to any operating system that supports FAT: 
To support FAT partitions that are greater than 4 GB using 128- or 256-KB clusters, the drives must use sectors that are greater than 512 bytes. Note: On very small FAT partitions, a 12-bit FAT is used instead of a 16-bit FAT. The FAT file system supports only 512-byte sectors, so both the sectors per cluster and the cluster size are fixed. On Volumes, having cluster size more than 4 KB compression is not supported
Why change cluster size?
Cluster is an allocation unit. If you create file lets say 1 byte in size, at least one cluster should be allocated on FAT file system. On NTFS if file is small enough, it can be stored in MFT record itself without using additional clusters. When file grows beyond the cluster boundary, another cluster is allocated. It means that the bigger the cluster size, the more disk space is wasted, however, the performance is better. However, when you format the partition manually, you can specify cluster size 512 bytes, 1 KB, 2 KB, 4 KB, 8 KB, 16 KB, 32 KB, 64 KB in the format dialog box or as a parameter to the command line FORMAT utility. What it gives us? Determine average file size and format the partition accordingly.
How to determine proper cluster size?
The simplest (but rough) way is to divide number of files on the drive by total disk usage in kilobytes. Another idea is to estimate the approximate data size in advance before formatting the hard drive. If you are going to store multimedia stuff that is usually huge in size, make cluster bigger to increase a performance. If you plan to store small web pages or text documents, make cluster size smaller not to lose a lot of disk space. Think!
How to change cluster size without data losing?
If you want to change the cluster size of an existing partition, what would you do, by formatting again? No, files would lost after formatting the partition, then is there a way to change cluster size without data losing or reformatting? The answer is yes. By partitioning software, you can easily complete this job.
Change cluster size with Acronis Disk Director
For detailed operations, please refer how to change cluster size with Acronis Disk Director.
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3 Responses to How to change cluster size without reformatting?
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[...] This is just my own idea. As we know, Windows use cluster to organize files, that is to say, files was built up by several clusters. After you format a partition, the cluster size was fixed, for example, 4KB, so this file will use 8 sectors. If you create a file that only use 2 sectors, Windows still allocate 8 sectors (4KB) to this file, so the other 6 sectors are wasted. By compressing files, Windows can use the wasted sectors. This is the reason we write another article Change cluster size to better manage disk space. [...]
well written blog. Im glad that I could find more info on this. thanks
FYI Acronis Disk Director 11 Home does not have the cluster resize feature. Supposedly there will be an 11 “advanced” version released at some point.