[2024] exFAT Allocation Unit Size: Which is the Best and How to Set it?

What is exFAT allocation unite size? Which size is the best for exFAT? In this passage, you can get all you want to know with a powerful tool for easily changing the cluster size.


By Irene / Updated on April 12, 2024

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What is the allocation unit size?

When you use an SD card, USB flash drive, or storage devices, you may be confused in choosing the suitable allocation unit size for exFAT when creating or formatting the disk.

What is the allocation unit size? The allocation unit size, also known as the cluster size and block size, refers to the smallest logical amount of disk space that be used for storing or managing disk files. Most hard drives are split up into many clusters (allocation units ) for storing data. When the partition is created or formatted, this size is picked and tells the computer how much space a file can take up on the hard drive.

That is to say, each cluster size can only hold a single file. If your file needs 4097 bytes, the allocation unit size should be 4096 bytes. On the contrary, if the cluster size is 512 bytes, 9 clusters are needed for storage, and the actual space occupied is 4608 bytes.


As you might think, the smaller clusters are, the more space is saved. However, in fact, saving small files on a file system with a large allocation unit size wastes space on the disk, and saving big files on a file system with a small cluster size slows down the system. Besides, smaller clusters also need more time to search for files when reading them. Because of this, choosing the right allocation unit size is necessary for your system to work well.

What is the best allocation unit size for exFAT when formatting an SD card or USB? That's what we'll talk about next.

What is the best exFAT allocation unit size?

Microsoft made the exFAT file system, and this is what they say is the standard exFAT allocation unit size:

  • 7MB - 256MB: 4KB
  • 256MB – 32GB: 32KB
  • 32GB – 256TB: 128KB
  • >256TB: Not supported

So, if you want to know what the allocation unit size is for 64GB exFAT, based on the information we give you, you can figure out that. If you don't know much about the close relationship between the cluster size and partition performance, setting the allocation unit size at its default setting for exFAT can also be a good choice. Based on the partition's size, this setting chooses the best cluster size automatically.

How to change the allocation size for exFAT?

To change a suitable exFAT allocation unit size, it is better to use a professional tool like AOMEI Partition Assistant Standard. This is a free and multifunctional disk manager that is compatible with Windows 11/10/8/7. You can use it to format a USB drive or SD card to the exFAT file system, and simply configure and change a cluster size between 0.5KB and 32MB based on your needs, which is more than enough for most uses. 

For extra requirements, you can also convert the exFAT to NTFS, FAT32, and Ext2/3/4 with ease. Even if the disk partition is damaged or write-protected, it can also successfully format the hard drive. It is strongly to recommended to its Professional edition as it can provide more advanced features such as cloning partition to another drive, merging partitions, recovering lost partitions, and so on. You can free download and use it to change the allocation size for exFAT.

AOMEI Partition Assistant

The Best Windows Disk Partition Manager and PC Optimizer

Step 1. Connect USB drive to your PC. Install and run AOMEI Partition Assistant and you’ll see your drive at the main interface.

Step 2. Then right-click the partition on your hard drive and select Format Partition.

Format FAT32 to exFAT 30GB

Step 3. In the pop-up small page, choose exFAT from the drop-down menu next to File System.

Select exFAT File System 30GB

Step 4. In this stpe, you can change or set the allocation unit size for exFAT by selecting the Cluster Size.

Specify Cluster Size 30GB

Step 5. Preview the virtual result. Click Apply and later Proceed after your confirmation.

Apply Proceed 30GB

1. If your USB drive shows up as Unallocated, you can right-click the unallocated space and select "Create Partition" to create a partition with exFAT formatting.
2. If you have not made backup before changing the allocation unit size for exFAT by formatting and suffer data loss, you can follow the guide to recover data from the formatted hard drive.

Final words

Now, you know what is the exFAT allocation unit size and how to set it with AOMEI Partition Assistant. If you still worry about whether the allocation unit size you set will affect your disk performance or cause space waste, you can directly run the downloaded software for troubleshooting. For example, you can move installed programs to another drive, allocate free space from partitions have sufficient space, or use it to monitor disk health.  If you are a Windows Server user, you can choose the AOMEI Partition Assistant Server edition.

FAQs about allocation unit size for exFAT

1. What is the size limit for exFAT drive?
exFAT has a maximum capacity limit of 128 PiB (approximately 134.2 billion TB), which is considered virtually unlimited for practical purposes.

2. What is the allocation unit size for exFAT 128GB SD card?
32KB is the recommended and default allocation unit size for an exFAT-formatted 128GB SD card, offering a balance between performance and space efficiency.

3. Is it best to reformat the hard drive to exFAT using 512kb chunk, or smaller or bigger chunks?
The choice depends on your usage. For a mix of file sizes, the default (typically 32KB) is recommended. Smaller sizes improve space efficiency for tiny files, while larger sizes enhance performance for large files.

4.Why is exFAT's default allocation size in Windows so high?
Larger allocation unit sizes improve read/write performance, especially for larger files. This default setting in Windows aims to balance performance and compatibility with various systems.

5. How can I check the cluster size (allocation unit size) of an exFat partition on Windows?
You can use the Command Prompt with administrator privileges: fsutil fsinfo ntfsinfo :. The "Bytes Per Cluster" value represents the allocation unit size in bytes.

Irene · Staff Editor
Irene is an Editor of AOMEI Technology. She devotes herself in giving insightful thoughts on common computer problems with simple and clear guidance. Irene loves to help people solve problems and explore more solutions on relevant issues. She loves reading, singing and travelling.