What's the Best Allocation Unit Size for exFAT (Change & Check)

"What allocation unit size should I use for exFAT?"

This is a question frequently asked by users who want to choose the best allocation unit size for exFAT when partitioning or formatting an external drive for PC or Mac. After all, an improper exFAT allocation unit size can lead to increased fragmentation, slower file access, and inefficient use of storage space.

If that's also you, this article will give you the answer. I'll explain the basics about the allocation unit size for exFAT and tell you how to choose the best allocation unit size for exFAT when erasing an SD card, flash drive, or external hard drive.

What is exFAT allocation unit size?

The allocation unit size, also referred to as the cluster size or block size, is the smallest disk space used for storing a single file. It determines the minimum size of a file or directory on a storage device and affects the efficiency of disk space utilization and disk performance.

Suppose you're saving a 26KB file to a 128GB SD card.

・If the allocation unit size for exFAT is 128KB:

The file will be allocated to 1 cluster, with 26KB of used space and 102KB of wasted space.

・If the allocation unit size for exFAT is 16KB:

The file will be allocated to 2 clusters, resulting in 6KB of wasted space.

So despite your file being only 26KB, it will take more disk space, depending on the allocation unit size. That's why you see two categories of disk sizes when right-clicking on a file and choosing "Properties" on Windows or "Get Info" on macOS.

Size: The actual size of your file.

Size on disk:  The size of the clusters the file is allocated = the actual amount of space being taken on the disk.

If you don't want to waste storage space, choose the optimal allocation unit size for exFAT with the principles I'll discuss in the following paragraphs.

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What's the best allocation unit size for exFAT?

There's no single "best" allocation unit size for exFAT that applies to all cases.

To choose the optimal exFAT allocation unit (cluster) size, it's important to strike a balance between space utilization and data read/write speed.

Since the allocation of storage must occur in whole clusters, a file that's not a multiple of the cluster size will always render wasted space, also called slack space.

Usually, the larger the cluster size, the more space will be wasted, especially for small files. But that doesn't mean the smaller the cluster size, the better. When saving a file to a drive with smaller cluster size, it will be spread across several clusters, causing increased fragmentation. 

As a general rule of thumb, you can choose a larger allocation unit size for exFAT (e.g., 32KB, 64KB) if primarily working with large files like movies, or a smaller allocation unit size (e.g., 4KB, 8KB, 16KB) if most of your files are small documents.

Besides, you should also consider your storage device type.

For mechanical hard disk drives (HDDs), larger allocation unit sizes are recommended to enhance performance and reduce fragmentation. However, smaller allocation unit sizes are usually more suitable for flash drives like SSDs, USB flash drives, or SD cards because they have faster random access times and are less affected by fragmentation.

All in all, you should select the allocation unit size for exFAT based on space efficiency, disk performance, and storage types. If you're uncertain about which one to choose, you can stick to the default allocation unit size suggested when you format the drive. 

Here's a table of the recommended allocation unit sizes for exFAT-formatted drives of different capacities:

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How to change exFAT allocation unit size?

If you're dissatisfied with the current allocation unit size of exFAT, you can change it by reformatting the drive. This can be easily done in Disk Management on Windows or Terminal on Mac without third-party tools. Just remember to back up the drive before proceeding.

Change exFAT allocation unit size on Windows:

Change exFAT allocation unit size on Mac:

How to check allocation unit size of exFAT?

If you want to check the allocation unit size of exFAT on a Windows PC or Mac, take the following steps:

On a Windows PC:

On a Mac:

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