What Is Allocation Unit Size For SD Card? Everything You Should Know

Cici updated on Feb 18, 2024 | Home >Computer Instruction

In this article, you'll learn:

  • What Is Allocation Unit Size for SD Card
  • How To Check Your Allocation Unit Size For SD Card
  • How to Change Allocation Unit Size for SD Card
  • Conclusion
  • FAQs about What Is Allocation Unit Size for SD Card

The smallest possible data block on your storage disc is called "cluster size." To be more precise, when you save a file to your storage drive, its size will be determined by the allocation unit size you specify, and as you increase the allocation unit size, the file size on the disc will also rise.

There is no question that allocation unit size affects the speed of file access, the size of files, and how much space is used on your SD card and USB. When your PC writes and reads a file, it stores it in the allocation units. Therefore, the file will occupy fewer allocation units the greater the allocation unit size you specify, which will increase the file access speed by your OS. 

What is allocation unit size for SD card? This article will tell you everything you should know. Continue to read!

What Is Allocation Unit Size for SD Card

You can see that there are many options for the allocation unit size when formatting an SD card, USB flash drive, or external hard drive partition in Windows 11, 10, 8, and 7. These options range from 4K to 64 KB.

Which one is the best for your SD card, then? The allocation unit size has little effect on the SD card's performance. When formatting, you can frequently just select the default allocation unit size. Usually:

  • For SD cards that are 32GB and smaller, select the file system FAT32 and allocation unit size 32 KB.
  • For 64GB cards and larger, select the file system exFAT and allocation unit size 128KB.

Employing various allocation unit sizes is not equivalent. Small allocation unit sizes can cause you to spend more time accessing data, while large allocation unit sizes can result in more disc space waste and wear and tear. If you just plan to keep a few small files on the SD card, you can use a small size, but if you need to store many huge files, such as film files, a large allocation size may improve reading and writing speeds.

For other knowledge about allocation unit size, such as the best SSD cluster size, please check the following article:

related articles

What's The Best SSD Allocation Unit Size in Windows 11/10

On this page, we will introduce what is the best SSD allocation unit size in Windows 11/10. And you can also get the best solution to change it.

SSD allocation unit size

How To Check Your Allocation Unit Size For SD Card

By now, you have an in-depth understanding of the allocation unit size in an SD card. However, to answer your query about how to check your allocation unit size for an SD card. You can use Diskpart to finish the task. The steps are listed below: 

Step 1. Ensure the SD card or USB device is connected to the computer.

Step 2. In the Search box, type diskpart, right-click the best match, and choose Run as administrator. Then, in the UAC window, click yes to confirm it.

run as administrator

Step 3. Type the following instructions into the elevated diskpart.exe window, hitting Enter after each one to complete it:

List disk
Select disk (identifies the disc number on your USB or SD card)
List partition
Select partition (indicates the partition number of the partition whose cluster size on the drive you want to check)
file systems

Step 4. Now, as seen in the screenshot below, you can view the current allocation unit size for a USB or SD card and the default sizes for various file systems.

view allocation size

How to Change Allocation Unit Size for SD Card

In the above two parts, we know the suitable allocation unit size for SD card. And we also know how to check the clusters of SD card. If your cluster size does not meet your needs, you may want to change it. Setting exFAT or FAT32 allocation unit size is necessary to optimize partition efficiency. EaseUS Partition Master is a powerful tool to assign the unit size easily.

Free Download 

Windows 11/10/8/7100% Secure

As a professional disk partition tool, you can change the cluster size easily and quickly. It can adjust the allocation unit size of SD card without any data loss issues.

Step 1. Right-click the partition that you need to change its cluster size, select "Advanced" and click "Change Cluster Size".

Right-click to change cluster size

Step 2. Click the size list below the New size section, and select a desired cluster size you want to adjust for the disk partition, click "OK".

Adjust cluster size

Step 3. Click "Execute 1 Task(s)" and click "Apply" to confirm the operation.

confirm the opreation

EaseUS Partition Master is a comprehensive tool with many features. You can also change SSD cluster size easily.


To summarize what has been stated so far, we have answered the query "what is allocation unit size for SD Card" in detail. We have mentioned the methods to allocate unit size for; allocation. You can choose the one that helps you allocate the unit size.

Besides, we recommend you try EaseUS Partition Master to allocate unit size for the SD card. It helps easily allocate the unit size for SD cards, clone system partitions, merge partitions, and support Windows 10,11,7 and 8. Download EaseUS Partition Master and try it yourself!

Free Download 

Windows 11/10/8/7100% Secure

FAQs about What Is Allocation Unit Size for SD Card

You already know the answers after reading this post. If you have any suggestions for this topic or have any questions, please leave them in the comments section.

1. What is allocation unit size in formatting SD cards?

The size of the cluster of hard drive partitions is the allocation unit size. Clusters form the foundation of the computer's hard disc storage arrangement. Using a 4097-byte file as an example, if the cluster size is 4096 bytes, the file must be saved in two clusters, taking up 8192 bytes in total; if the cluster size is 512 bytes, nine clusters are needed for storage, taking up 4608 bytes in total.

2. What allocation unit size should I use?

The ideal allocation unit size for your drive can frequently depend on your disc size and operating system. A single movie file typically weighs in at several hundred MB or even a few GB, so using a big allocation unit size makes sense if, for instance, you have a partition or drive that you use only for movies. You may choose the 2MB maximum allocation unit size, but remember that this will force smaller files (such as subtitle files) to occupy the bare minimum of that space.

3. What is the allocation unit size for a 128 GB SD card?

USB allocation unit size for 128GB: 8096 (8kb) USB 512GB allocation unit size is 32k; USB beyond 512GB allocation unit size is 64k or higher; USB 256GB allocation unit size is 16k; When the USB flash disc is formatted as exFAT and has a capacity more than 64GB, the system automatically adjusts the allocation unit size to 8096 (128G), 16K (256G), and 32K (512G).

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Updated by Cici

Cici is the junior editor of the writing team of EaseUS. She accepted the systematic training on computers at EaseUS for over one year. Now, she wrote a lot of professional articles to help people resolve the issues of hard drive corruption, computer boot errors, and disk partition problems.

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