Set The Right Allocation Unit Size When Formatting a Drive

Have you ever formatted a disk on your own?


Attempting to format a disk for the very first time?

Then, you should definitely read this post as it is going to tell you why it is important to set the right allocation unit size while formatting a drive.

Now, what do you mean by ‘Allocation unit size’?

It is also known as or can be addressed as ‘Cluster size’. The cluster size determines the can be defined as the tiniest chunk of disk space that holds a file.

When you begin to format a partition into one of the file systems such as NTFS, FAT, exFAT etc., Windows makes use of the default values if formatting is done by one of the following methods and they are:

  • By using the ‘FORMAT’ command from the command line without mentioning the cluster size;
  • When the Allocation Unit box in the Format dialog box lists Default Allocation Size while formatting a drive from Windows Explorer.

However, it is advisable to keep the allocation size small if lots of small files are present. This will help in saving disk space. But, if the size of the files is large, it is a good idea to keep the allocation size big. This will eventually enhance the system’s performance.

Our Hard drives are divided into clusters and the size of the allocation unit determine the size of a single cluster. The file system in which the hard drive is formatted keeps track or record of the state of the clusters present. When a file or a portion of the file is written on any cluster, it is marked as or considered to be occupied. If the size of the clusters is small, the speed with which the hard drive performs becomes slower. It happens because each file is broken into small pieces and it takes a significant amount of time gather all the broken pieces of the file in one place and access them as one.

And, if the size of the cluster is large, the space on the disk will be wasted.


Hence, there is no absolute benchmark for setting the size of the allocation unit. If the size of the files you wish to store on the drive is large then a large cluster size is recommended. This will also increase the speed with which the drive will perform. If you are planning to store smaller sized files than a small cluster size will do the deed. It will preserve the space on the disk.

Note: Formatting a drive into any file system comes with a catch. If the formatting is done in an improper way or if the process faces some kind of interruption, you may lose the data on your drive.

Read: Here’s how you can retrieve an exFAT partition.

Default image
John Harris

Senior Editor, Content Analyst and a fan of exceptional customer service. John develops and publishes instructional and informational content regarding partition management, Windows hot-fixes, data management and computer troubleshooting.

As a tenured data recovery specialist, John shares exceptional insights and blog posts about data loss and data recovery across any storage device. With 8+ years’ experience in writing for Data Recovery for both Mac OS and Windows OS computers, he is an avid learner who always wants to polish and simplify the data recovery process. John passes his free time playing Chess and reading Science Fiction novels.

Articles: 544

Leave a Reply