By Emily / Last Updated September 26, 2021

What is allocation unit size on SSD?

If you have ever formatted an SSD, HDD, USB thumb drive, SD card or other hard drives, you might have seen the “Allocation Unit Size” setting. So, what is the allocation unit size? The allocation unit size is also called as cluster size. All the file systems organize your hard disk based on the allocation unit size/cluster size. It represents the smallest amount of disk space that can be used to save a file.

Allocation Unit Size

Each allocation unit size can only hold one file. This file will be divided into several parts by the allocation unit size and stored on the hard disk. If the file sizes do not come out to an even multiple of the cluster size, additional space will be used to hold the file up to the next multiple of the cluster size.

For example, when you store a 512-byte file into a partition with 512-byte allocation unit size, this file will take up 512 byte storage space. However, if you save a 513-byte file on the partition of 512 byte allocation unit size, it will occupy 1024 (512*2) bytes storage space, which will waste some space (you can do a rough match with the equation: (cluster size)/2 * number of files.

What allocation unit size should you use for SSD?

When you are formatting an SSD drive, the allocation unit size is set by default based on the partition size. But you can change it if you want. So, you might wonder what’s the best SSD allocation unit size.  

Generally speaking, the smaller allocation unit size on the hard drive can help save storage space better. Although the larger allocation unit size can help save much time in reading data, it will waste some space. It seems that the smaller the SSD cluster size is, the more drive space you can save. In fact, that is not the case. When a large file is divided into several chunks based on the given allocation unit size, it will cost more time when reading the data. Besides, larger unit sizes can lead to more writes over time and then cause more wear and tear on your SSD.

So, the recommended allocation unit size for SSD should be the defaults: 4KB, which are set to reduce the space that is lost. To get default allocation unit size for SSD with NTFS, FAT32 and exFAT, you can go through this link.

How to change SSD allocation unit size in Windows 11, 10, 8, 7?

If your SSD has been formatted to NTFS or FAT32 with other allocation unit size, but now you want to change the cluster size, you need to reformat it, which will delete everything on the drive. To format the SSD for the best allocation unit size, you can make use of Windows Disk Management.

Step 1. Open Disk Management, right-click the SSD partition and select “Format ...”.

Formart Drive

Step 2. In the next window, change the SSD allocation unit size (4096). Then, click on “OK” icon to execute quick format.

Allocate Unit 4k

This can help set the SSD with a proper cluster size so as to reduce space waste. If you want to enjoy the better SSD performance, you can also perform 4K alignment. The 4K alignment is not allocation units but allows SSD to adopt the smallest 4K sector in the file system to store data. Then the speed of reading and writing data will be boosted significantly. For this task, you can the AOMEI Partition Assistant Professional. Its “Partition Alignment” enables you to make 4K SSD alignment without data loss.

Step 1. Install and launch the software. Right-click the partition on SSD and select “Advanced” > “Partition Alignment”.

Partition Alignment

Step 2. In the pop-out window, select 4096 sectors from the given list and click ”OK”.

4K Alignment

Step 3. Click “Apply” > “Proceed” to commit the pending operation.

Apply

Summary

Now, you should know what’s the best SSD allocation unit size and how to change the SSD allocation size to the best value without losing data. The software mentioned earlier can also do other disk management tasks: clone SSD drive, convert MBR to GPT for Windows 11, secure erase SSD drive of all brands, recover lost partition on SSD, HDD, USB flash drive, SD card, migrate OS from HDD to SSD, and more.