What Should I Set the Allocation Unit Size to When Formatting? (2023)

What Should I Set the Allocation Unit Size to When Formatting? (1)

You should use the default allocation unit size that is recommended when you format the drive. For NTFS drives, which are common on Windows, that will probably be 4096 bytes. If you're only going to be storing large files on a drive, feel free to use a larger allocation unit size --- it'll improve your storage efficiency.

When you format a new storage device, be it aninternal solid-state driveor anexternal flash drive, you can pick the file system you want to use and the allocation unit size. But what is the allocation file unit size, and what does it do? Here’s everything you need to know.

Table of Contents

What Is Allocation Unit Size?
What Allocation Unit Size Should You Use?
Why Change the Allocation Unit Size?

What Is Allocation Unit Size?

Allocation Unit Size — otherwise known as “Cluster Size” or “Block Size” — refers to the size of the chunks that asolid state drive (SSD)orhard disk drive (HDD)is divided into. When you increase the size of the allocation unit, the “chunk,” you decrease the total number of allocation units on your drive. If you decrease the size of the allocation unit, the opposite happens — you increase the number of allocation units on the drive.

Note:The term “Allocation Unit,” “Block, ” and “Cluster” all refer to the same thing in this context, and we’ll use them interchangeably in this article. The term block is typically seen when talking aboutLinux, especially the ext4 file system, whereas cluster and allocation units are seen with Windows.

If a file is too big to fit in a single block, then it will be split and span multiple blocks. If a file is smaller than the block size, then it will be stored in that block, but the entire block volume will be used up. This can result in inefficient storage in some extreme cases, but we’ll go into that in more detail later.

Here’s a brief example of how allocation units, the size of allocation units, and the volume of your storage drive are related. To make the math easy, we’re going to talk about a tiny hypothetical hard drive with a total volume of 16,384 kilobytes (16 MB) — absurdly small by 21st-century standards, but convenient to illustrate the point.

So, you plug in your 16,384 kilobyte drive and choose to partition it as anNTFS file system. The default allocation unit size for an NTFS drive of that size is 4069 bytes, or 4 KB. Your drive will have 16,384/4 (4,096) units — or blocks— on it. If you increased your allocation unit size to 32 kilobytes, you’d instead have 16,384/32 (512) clusters. This holds for any HDD or SSD of any size, though the larger drives we’re used to today will have many millions of blocks rather than a few thousand.

What Allocation Unit Size Should You Use?

You should stick with the default allocation unit size that is suggested when you format your storage device unless you have an extremely specific reason to change it. For the average NTFS drive, that will be 4,096 bytes, or 4 KB.

What Should I Set the Allocation Unit Size to When Formatting? (2)

The default size does change depending on thesize of the partitionand thefile systemyou’re using. Generally speaking, as the partition gets larger, so does the allocation unit size. We’ve put together some tablescomparing NTFS, FAT32, and exFAT, since they’re the most common formats you’ll encounter, and are regularly used withUSB flash drives,storage drives, andSD cards.


Partition VolumeAllocation Unit Size
7 MB – 16 TB4 KB (4096 Bytes)
16 TB – 32 TB8 KB
32 TB – 64 TB16 KB
64 TB – 128 TB32 KB
128 TB – 256 TB64 KB
256 TB – 512 TB128 KB
512 TB – 1 PB265 KB


Partition VolumeAllocation Unit Size
32 MB – 64 MB0.5 KB (512 Bytes)
64 MB – 128 MB1 KB
128 MB – 256 MB2 KB
256 MB – 8 GB4 KB
8 GB – 16 GB8 KB
16 GB – 32 GB16 KB


Partition VolumeAllocation Unit Size
7 MB – 256 MB4 KB (4096 Bytes)
256 MB – 32 GB8 KB
32GB – 256TB16 KB

Why Change the Allocation Unit Size?

At least in theory, changing the allocation unit size can optimize drive speed or storage space. In practice, you probably won’t notice this or care very much if you stick with the default value, since modern drives are both large and extremely fast. There are really only two extreme cases where you might have to worry about this:

  1. When you have a few enormous files, and you choose a tiny allocation unit size
  2. The opposite scenario, where you have a ton of tiny files, and you choose a large allocation unit size.

If you have only a few enormous files and you use the 4096 byte option, you’ll wind up creating a ton of clusters (volume size in bytes/4096 bytes) that are filled with only a few files. Each allocation unit winds up holding only a fraction of the total file. For example, if you have a 4096-byte allocation unit size and you have a 12 KB file, you’d need to use three allocation units (or blocks) to store the complete file. The more blocks you use to store a file, the greater the possibility for fragmentation, and the more blocks your operating system needs to keep track of. This can slow down your drive’s performance.

Note:Fragmentation isn’t the problem it once was.Windows defrags itself regularly, and you’ll probably never notice it happening.

In the opposite situation — many small files with a large allocation size — you wind up wasting storage space. Each allocation unit can only hold one file at the most. If you have an allocation unit size of 64 kilobytes and you write a 3-kilobyte file to it, that entire block will be filled. That means you’ve used up 64 kilobytes of storage to store only 3 kilobytes. If you had enough 3 KB files to fill an entire drive formatted that way, you’d wind up wasting more than 95% of the drive’s total volume. If you shrink your allocation unit size to 4KB, you’d only waste 25% of the drive’s total volume. That is an extreme example, however, and you won’t typically see losses like that with average use.

Theaverageamount of wasted spaceis half of the allocation unit (block) size per file on the volume. So, if you choose a 4096 byte block size, and you’ve got 100,000 files, you’ll waste only about 200 megabytes (4KB/2*100,000) of storage space. On the other hand, if you pick a 64 KB block size with 100,000 files, you’ll waste an average of 3.2gigabytes. It’s not really something to lose sleep over considering the sizes of modern SSDs and HDDs.

If you know in advance that you’re exclusively going to be storing larger files on a drive, like if you’rehosting your own Jellyfin Media Server, then it definitely won’t hurt to use a larger allocation unit size. Otherwise, you should stick to the default value.

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What Should I Set the Allocation Unit Size to When Formatting? ›

9 Answers. If you are a "Standard User" by Microsoft's definition, you should keep the default 4096 bytes. Basically, the allocation unit size is the block size on your hard drive when it formats NTFS.

How do you choose allocation unit size when formatting? ›

How to Set NTFS Allocation Unit Size
  1. Right-click the partition that you need to change its cluster size, select "Advanced" and click "Change Cluster Size".
  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".
May 11, 2023

What should be the allocation unit size while formatting pen drive? ›

If your USB flash drive is smaller than 8GB, the most proper allocation unit size is 4096 bytes. If your flash drive is larger than 8GB but smaller than 16GB, the most proper allocation unit size is 8192 bytes. If your flash drive is larger than 16GB, the most proper allocation unit size is 16 kilobytes.

What is the allocation unit size when formatting SD card? ›

For Allocation unit size, select '4096 bytes'

What is allocation unit size when formatting SSD? ›

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.

What is the allocation unit size for FAT32 format? ›

FAT32 Allocation Unit Size FAQ

In general, you can use 512 bytes, 1 KB, 2 KB, 4 KB, 8 KB, 16 KB, 32 KB, etc., as the allocation unit size for FAT32 partition. But when it comes to how to choose from them, it depends on the size of the FAT32 partition and the size of files you store in the partition.

What should be allocation unit size for exFAT? ›

The best allocation unit size for exFAT 64GB is 128KB.

What is the default allocation unit size? ›

Windows's default allocation unit size is 4096 bytes (4 kilobytes), which is pretty small, and on most computers, it's unlikely this will lead to a lot of wasted space.

Which is better NTFS or FAT32? ›

The FAT32 file system cannot store individual files larger than 4 GB, while the NTFS file system can. Compared to FAT32, the NTFS file system has higher disk utilization and can manage disk space more effectively. The read and write speeds of NTFS are also faster than that of FAT32.

What is the NTFS or exFAT allocation unit size? ›

The allocation unit sizes also vary from different file system types. The default FAT32 allocation unit sizes range from 512bytes to 64KB, while the default NTFS allocation unit sizes range from 4KB bytes to 64KB and the default exFAT allocation unit sizes range from 4KB and 128K.

What is the best allocation unit size for camera? ›

The best allocation unit size for video files is 64 KB (the default option is 4 KB). A larger allocation unit size of 2 MB may be better if your drive is solely used for storing large movies.

What is difference between NTFS and exFAT? ›

In a debate regarding exFAT vs NTFS vs FAT32, NTFS is the best format for storage devices with Windows operating system. However, exFAT is best for portable storage devices due to more efficient power and memory management. It also allows you to use a storage device on both Windows and macOS.

What is the best allocation unit size for gaming? ›

Conclusion. The 64K allocation unit size is the best for gaming as it improves system performance and offers the fastest lookup/loading times while playing games.

What should I set the allocation unit size to when formatting NTFS? ›

SQL Server uses extents to store data. Therefore, on a SQL Server machine, the NTFS allocation unit size for hosting SQL database files (including tempdb) should be 64 KB. To check the cluster (NTFS allocation) size of your drives, you can use PowerShell or the command line.

Why can't I format my SD card to FAT32? ›

The most common issue is that your SD card, probably is too large in volume. In Windows 10, it is difficult to format a flash drive into FAT32 if its memory size is more than 32 GB. This is due to its longevity as a file system.

Is ExFAT the same as FAT32? ›

ExFAT is a file system developed by Microsoft as an enhanced version of FAT32. It was created to provide better compatibility with external devices such as flash drives and digital cameras, while also increasing the capacity of storage available.

What is the allocation unit size for 512GB? ›

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 ...

Can I change allocation unit size? ›

Press Windows + E keys simultaneously, right-click on the partition you want to change cluster size, select Format... Here we take G: for example. Step 2. Click Allocation Unit Size (Cluster size), select one cluster size you need to change to, here is 64 kilobytes, select Quick Format > Start.

Is higher allocation unit size faster? ›

The data loading speed is similar. When creating a table space, the larger the AU, the faster the speed. That is to say, the larger the AU, the faster the data file resize. However, setting a too large AU will reduce the space for storing data on the disk.

What is allocation file size? ›

The file system allocation or cluster size is commonly referred to as the block size of the disk. If a logical disk is formatted at a 4 KB block size (default on Windows and Windows Server), then a file that is 44 bytes in size will use a 4 KB (4096 bytes) block on disk to store it making the “size on disk” 4 KB.

When should you use FAT or exFAT? ›

If you are sure that you won't be copying or moving files larger than 4GB, Fat 32 is the way to go. On the other hand, if you need to deal with a large file, then use exFAT. On similar lines, small storage like SD Card and USB go better with FAT32, while external hard drives can use exFAT.

Which format is faster NTFS or exFAT? ›

exFAT is a tradeoff between responsiveness for small files and write speed for large files (15mb/s). NTFS is very slow for numerous small files but the fastest for very large files (25mb/s).

Which formatting is better NTFS or FAT? ›

FAT is the more simple file system of the two, but NTFS offers different enhancements and offers increased security. Choosing the right operating system depends on your needs. Fault Tolerance: NTFS automatically repairs files/folders in the case of power failures or errors.

Should I quick format or full format? ›

In case you need to format a brand-new data storage device, you should perform a full format. But if your drive has been already formatted and you are absolutely sure that it doesn't have damaged nor logical bad sectors, a quick format would be enough.

Which is safer exFAT or NTFS? ›

The portable of external hard drives often means that they will be used across multiple operating systems. As NTFS has less compatibility outside of Windows, exFAT is the safer option.

Is exFAT slower than NTFS? ›

Yes, exFAT is slower than NTFS. exFAT as a responsive speed for small files and a 15MB/s write speed for large files. NTFS is very slow for small files but the fastest with 25MB/s speed for large files.

What is exFAT format used for? ›

The exFAT format allows individual files larger than 4 GB, facilitating long continuous recording of HD video, which can exceed the 4 GB limit in less than an hour. Current digital cameras using FAT32 will break the video files into multiple segments of approximately 2 or 4 GB.

Does formatting a drive wipe it? ›

Does formatting a hard drive erase everything on it? The answer is no. Formatting does not erase data on the disk, which only makes file recovery much more difficult.

Should I format my SSD? ›

If you plan on reusing an existing SSD, you should remember to format the drive before reinstalling the operating system. Note that clearing the drive partitions will erase the data, so be sure to back up your drive contents before proceeding.

What format should SSD be for Windows 10? ›

NTFS is best suited for using a drive with Windows devices. Choose this format if you're going to use the drive with a Windows computer. b. exFAT is best suited for using a drive with both Windows and Mac OS devices.

Which partition style for SSD? ›

Most PCs use the GUID Partition Table (GPT) disk type for hard drives and SSDs. GPT is more robust and allows for volumes bigger than 2 TB. The older Master Boot Record (MBR) disk type is used by 32-bit PCs, older PCs, and removable drives such as memory cards.

What is allocation unit size 128gb USB? ›

usb 64GB - exfat file system with default, 4K - 8k is good. usb 128gb allocation unit size - 8096(8kb)

What is FAT32 NTFS and exFAT? ›

FAT32, exFAT and NTFS are file systems used by the Windows Operating System. FAT32 is the oldest one, exFAT is a replacement of FAT32 and is compatible on multiple devices. NTFS is the latest file system. Read this article to learn more about FAT32, exFAT, and NTFS file systems and their unique features.

What is the maximum file size for FAT32? ›

The FAT32 file system uses smaller clusters than the FAT file system, has duplicate boot records, and features a root directory that can be of any size and can be located anywhere on the disk or partition. The maximum possible size for a file on a FAT32 volume is 4 GB.

What is the FAT32 format? ›

FAT32 is a disk format or filing system used to organise the files stored on a disk drive. The disk drive is marked up into addressable chunks called sectors and a “File Allocation Table” or FAT is created at the start of the drive so that each piece of information in the file can be found by the host computer.

Should I use FAT32 or exFAT? ›

Which format should I format my memory card to? FAT32 has better compatibility. If you are not sure whether your device supports exFAT, we recommend formatting the memory card to FAT32.

What is the best format for a USB drive in Windows 10? ›

Most USB flash drives will have a FAT32 file system out of the box. It is the most compatible file system for older/modern computers (PC and Mac), plus gaming consoles and other devices with a USB port. exFAT is the ideal file system for USB flash drives.

Why can't I format USB to FAT32? ›

There are a few reasons why your USB drive might not be formatting to FAT32. The first reason is that your USB drive might be larger than 32 GB. As a result, it can't be formatted to FAT32. The second reason is that your USB drive might have been formatted using a different file system, such as NTFS.

Can a 1TB drive be formatted to FAT32? ›

Can FAT32 Support a 1TB Hard Drive? The simple answer is no, it cannot. But you can still format it in this format, but we'll explain that later. For now, it's important to understand that even though FAT32 has a limit of 2TB hard drives, Windows only allows it to create partitions of 32GB.

How do I overcome FAT32 file size limitation? ›

How do I bypass 4GB limitation on FAT32? You can switch the file system on FAT32 device to NTFS or exFAT so to bypass the 4GB limitation and save big files on the device. For internal hard drives, set the file system as NTFS. For external hard drives or removable storage devices, set the format as exFAT.


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