What Is Allocate Unit Size When Formatting for Flash Drive, SD Card, Hdd, SSD


This page tells about allocating unit size when formatting storage media SSD HDD external hard drive USB SD card and the best solution for allocating unit size fat32 exfat or ntfs, and what's the best solution for allocating unit size for gaming.

Andrew Wright


by Andrew Wright

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What is allocate unit size and is default cluster size better?

Allocating unit size is the option to choose which block size you would like your hdd/ssd/usb/sd to have (from 512bytes to 1024k) when you format a storage media.

Tech terms (What is allocate unit size) : Allocation unit size is the size of the hard disk partition cluster. The hard disk storage organization of the computer is based on clusters. Assuming that the file size is 4097 bytes, if the cluster size is 4096 bytes, the file needs to be stored in two clusters, that is, the actual space occupied by the file is 8192 bytes; If the cluster size is 512 bytes, 9 clusters are required for storage, and the actual occupied space is 4608 bytes.

This shows that the smaller the cluster, the more space is saved, but when reading files, the smaller the cluster will bring more search time, which is bound to lose performance...

Default Allocation Unit Size by MicroSoft Windows OS

NTFS - default value of allocation unit size

Volume Size

7 MB - 512 MB

NTFS Cluser Size

512 Bytes

513 MB - 1,024 MB
1 KB

1,025 MB - 2 GB
2 KB

2 GB - 2 TB
4 KB

2 TB - 4 TB
8 KB

4 TB - 8 TB
16 KB

8 TB - 16 TB
32 KB

16 TB - 32 TB
64 KB

Also read: allocation unit size for ntfs

FAT32 - Default Allocation Unit Size for FAT32

128-256MB: 2KB
256MB - 8GB: 4KB
8GB - 16GB: 8KB
16GB - 32GB: 16KB
32GB - 2TB: not supported (You may set with third party free tool)
> 2TB: not supported
(You may set with third party free tool)

Also read: allocation unit size for fat32

EXFAT - The default value of allocation unit size for exfat drive:

7MB-256MB: 4KB
256MB - 32GB: 32KB
32GB - 256TB: 128KB
> 256TB: not supported

Also read: allocation unit size for exfat

Resource: Default cluster size for ntfs fat exfat on Windows

[5 Ways] Change allocation unit size during formatting or initializing hdd/ssd/usb/sd

#1 - Change allocation unit size by formatting it by right click format in file explorer

Here are the steps.

  • 1, Insert your disk/ssd/usb/sd if it's an external drive at first
  • 2, Double clicking your THIS PC on desktop to see your drives in file explorer
  • 3, Right click the drive that you want to change its allocation unit size, and open the Format... option which will lead to the window that we can change the block sizeright click format
  • 4, The file system option are available on this window, together with the allocation unit size selection column (We recommend you leave this size to default allocation size if there is no specific need.)

change cluster size by formatting disk and choose unit size

#2 - Change the allocation size by Windows disk management format option

Right click your drive in disk management, and then go to the format option

choose block size

Note: how to open disk managment? This route: Right click 'This PC' ->Manage -> Disk Management, and every drive will load in this map.

#3 - Initialize drive in disk management and set the allocation unit size

select to initialize disk

There is another chance that you may also find the way to change the allocation unit size. Here are the methods.

  • Delete partition in disk management if you right click it if it's an old disk or usb
  • Right click the drive again -> Initialize the drive, and then it will also offer the window to set the allocation unit size

Details: how to initialize disk in Windows 10

#4 - Delete partition and create new volume in disk management

You may also do this in disk management if you right click the drive in disk management -> Delete Partition; and then right click the drive -> Create new volume, which will also lead to the allocation unit size window.

create new partition

choose block size

#5 - Set allocation unit size by third party free tool

IM-Magic Partition Resizer Free is able to help set allocation unit size and file system. You may right click the drive in this tool, and then format partition which will lead to the following window.

format partition to change cluster size

choose block size

FAT32, NTFS, EXFAT, which one?

There are fat32 exfat and ntfs file system while formatting in current Windows operating systems.

FAT32: Both Windows and Mac supported, but a single file cannot exceed 4G, however it can be fixed by using packet compression.

NTFS: Windows-specific format, which is not recognized by Mac in general, but can be cracked by command without the need to install other software.

exFat: supported by Windws and MAc, the disadvantage is that it is not very stable and should be used with caution, especially when the compressed package will be damaged after copying to another computer.

What is allocation unit size:

The so-called allocation unit size is the smallest unit that the system can read and write to the disk and removable storage devices. The file is divided into several pieces and stored on the disk according to the size of the allocation unit. Generally speaking, the smaller the allocation unit, the more space efficient it is, and the longer it takes to read; while the larger the allocation unit, the more wasteful it is, and the shorter it takes to read.

For example, for a 500-byte file, when you set the allocation unit to 512 bytes, it only occupies 512 bytes of storage space; when you set the allocation unit to 1024 bytes, it occupies 1024 bytes of storage space; when you set the allocation unit to 4096 bytes, it occupies 4096 bytes of storage space; so that the smaller the allocation unit is, the more space-saving space, it takes longer time in the process of reading. But the smaller we set the allocation unit, the more we divide a file into many pieces and store it on the USB drive, so it will be slower to read.

When formatting a USB drive or hdd/ssd, the system automatically adjusts the size of the allocation unit for the file system format. So generally we choose the one by the default system.


Hot Question: Does allocation unit size affect speed?

Answer: It depends on the file size and what kind of file it is. As to the system installation, the 4k is good at block cluster size. If it's for the drive that stores the large game files, movie files, music files, it's the large the better.

Also read: Does allocation unit size affect speed

Question: Is the larger the flash drive format allocation unit size, the faster usb speed?

Answer: No. The speed of a USB flash drive is determined by the memory chip, the type of master and the combination between the two.

Usually, you can format the USB flash drive to FAT32 or EXFAT format and format the allocation unit as the system default, no need to change it.

Question: Is 64k better for large hard drive formatting allocation unit size? ->YES for non system disk.

Warehouse disk.6*1TB RAID5 mainly play HD and lossless music. When formatting large files, is it recommended to choose a 64K family size for good performance and not wasting space? I saw someone suggested using 128K unit size whether it will waste space.

Answer: For the allocation unit size, it mainly depends on the type and size of the stored files, if it is a trivial application file and Windows 10 system, it is inevitable to choose 4K size, but if it is a warehouse disk, mainly storing large volume continuous, audio and video files, for this kind of data continuous data files are not easy to cause space waste, so it's better to choose 64K or 128K allocation unit size.

Again, for the waste of space often refers to the small size of the file, it is easy to cause incomplete writing of individual file clusters, resulting in a waste of disk space, so if the storage disk is also used as a download disk, it is natural not to choose a large allocation unit size.

See the following image that displays the option.

choose block size

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