Format disk allocation unit size​

When you format a disk be it a hard drive, USB flash drive, SD card, or SSD you’re presented with several options: file system type (NTFS, FAT32. exFAT), volume label, and a setting often overlooked yet crucial for performance and efficiency: Allocation Unit Size.

Also known as cluster size, the allocation unit size determines how data is organized and stored on the disk. While many users stick to the default value suggested by the operating system, understanding this option can improve your device’s speed, longevity, and storage efficiency.

What Is Allocation Unit Size?

Allocation unit size refers to the smallest amount of disk space that can be allocated to hold a file. Even if a file contains only 1 byte, it will still occupy at least one allocation unit on the disk.

For example, if your allocation unit size is set to 4096 bytes (4 KB), every file will take up space in chunks of 4 KB. A 2 KB file will still occupy 4 KB. A 5 KB file will take 8 KB (two allocation units).

This system is a foundational part of how file systems manage data and plays a significant role in how efficiently a drive performs.

Why Does Allocation Unit Size Matter?

Allocation unit size impacts four major aspects of storage:

Storage Efficiency

Smaller cluster sizes minimize wasted space (known as slack space) because less unused space is left in each cluster.

Performance

Larger allocation units mean fewer read/write operations for large files, resulting in better performance for large file transfers.

File System Overhead

Smaller cluster sizes require more metadata tracking, potentially reducing performance when handling many small files.

Compatibility

Some operating systems and devices may perform better or require specific cluster sizes, especially with FAT32 and exFAT file systems.

Available Allocation Unit Sizes

The available sizes typically range from:

512 bytes

1024 bytes (1 KB)

2048 bytes (2 KB)

4096 bytes (4 KB) – Most common default

8192 bytes (8 KB)

16 KB

32 KB

64 KB

128 KB

256 KB

512 KB

1024 KB (1 MB)

2048 KB (2 MB)

The upper limit depends on the file system. For instance, FAT32 limits cluster sizes to 32 KB for optimal compatibility, while NTFS and exFAT support much larger sizes.

How Allocation Unit Size Affects File Storage

Let’s say you have 100 files, each 3 KB in size, and your allocation unit size is set to 4 KB:

Each file uses one 4 KB cluster.

Total space used: 100 × 4 KB = 400 KB

Actual data stored: 100 × 3 KB = 300 KB

Slack space: 100 × 1 KB = 100 KB wasted

Now, if you choose 16 KB as the allocation unit size:

Each 3 KB file still occupies 16 KB

Slack space per file: 13 KB

Total slack space: 100 × 13 KB = 1300 KB

As you can see, the larger the cluster size, the more space is wasted when dealing with small files. Conversely, for a single large file (e.g., 1 GB), a larger cluster size results in fewer fragments, increasing read/write efficiency.

When to Use Specific Allocation Unit Sizes

512 Bytes or 1 KB

Best for: Drives storing tiny files like logs, config files, HTML pages

Pros: Minimum slack space, best space efficiency

Cons: Slower read/write performance, more file system overhead

2 KB or 4 KB (Default for NTFS)

Best for: General use, operating system installations, office documents

Pros: Balanced space efficiency and performance

Cons: None significant for average users

8 KB to 16 KB

Best for: Medium-sized files like images, PDFs, software installers

Pros: Good compromise between speed and space

Cons: Slightly more slack space

32 KB to 64 KB

Best for: Media files (videos, audio), gaming, virtual machines

Pros: Faster sequential read/write

Cons: Higher slack space for small files

128 KB and Above

Best for: Dedicated drives for large video files, backup images, databases

Pros: Maximized speed, reduced fragmentation

Cons: Massive slack space for anything small; not suitable for general use

Examples of Optimal Cluster Sizes

How to Choose the Right Size for Your Needs

Ask yourself the following:

What file types will you store?

Mostly small files → smaller allocation unit size

Mostly large files → larger unit size

What is your primary goal?

Maximize speed → use larger clusters

Minimize storage waste → use smaller clusters

What file system are you using?

NTFS → supports wide range

FAT32 → cluster size limits (especially for drives >32 GB)

exFAT → optimized for flash media; supports large sizes

Will this drive be used with specific hardware (game consoles, media players)?

Check compatibility and use recommended settings

How to Format with Custom Allocation Unit Size (Windows)

Using File Explorer

Insert or connect the disk.

Open File Explorer → Right-click the drive → Select Format.

Under “Allocation unit size”, choose from the dropdown.

Choose file system and volume label.

Click Start.

Note: The dropdown is limited to supported sizes based on the disk and file system.

Using Disk Management

Press Windows + X → Select Disk Management.

Right-click your disk → Delete Volume (if needed).

Right-click again → New Simple Volume.

Follow the wizard → Set file system and allocation unit size.

Using Command Prompt and format Command

Open Command Prompt as Administrator.

Use:

less

CopyEdit

format X: /FS:NTFS /A:64K /Q /V:MyDrive

Replace X: with your drive letter.

/FS sets the file system.

/A sets allocation unit size.

/Q for quick format.

/V for volume label.

Format Allocation Unit Size Options for Different File Systems

FAT32

Max volume size: 32 GB (Windows GUI), 2 TB (with CLI tools)

Max cluster size: 32 KB

Ideal for: USB sticks, SD cards, legacy systems

NTFS

Max volume size: 256 TB

Max cluster size: 2 MB

Ideal for: Windows OS drives, internal storage, external hard drives

exFAT

Max volume size: 128 PB

Max cluster size: 32 MB

Ideal for: Large external drives, flash storage, cross-platform compatibility

Format USB Flash Drive With Allocation Unit Size (Example)

To format a 64 GB USB drive to exFAT with 64 KB clusters:

Using File Explorer:

Right-click the drive → Format.

File System: exFAT

Allocation Unit Size: 64 kilobytes

Volume Label: MediaUSB

Click Start → Confirm

Using Command Prompt:

less

CopyEdit

format E: /FS:exFAT /A:64K /Q /V:MediaUSB

Tools for Advanced Formatting

1. Rufus

For creating bootable USB drives

Allows selecting cluster size

Ideal for Linux ISOs, Windows To Go setups

2. MiniTool Partition Wizard

GUI tool with advanced formatting, partitioning, and resizing

Easily choose cluster size and view space usage

3. AOMEI Partition Assistant

Helpful for cloning, converting, and formatting drives

Cluster size selection included

Performance Benchmarking

If you’re unsure what cluster size works best, test your drive using benchmarking tools:

CrystalDiskMark – Tests read/write speed for various file sizes

AS SSD Benchmark – Popular for SSDs and USB drives

ATTO Disk Benchmark – Offers file size and queue depth control

Format the disk with different cluster sizes and compare performance based on your intended workload (e.g., video editing, game loading, document storage).

File System Metadata and Overhead

Each allocation unit stores metadata about the file it contains. Smaller unit sizes increase the volume of metadata, which could slightly reduce performance and increase wear on SSDs.

For enterprise use or drives storing millions of files, especially tiny ones, consider using smaller clusters with a journaling file system like NTFS. For write-intensive uses (like surveillance video storage), larger clusters and exFAT might offer better durability and performance.

Real-World Scenarios

Photographer Storing RAW Images

RAW files are large (25 MB – 50 MB).

Use 64 KB or 128 KB allocation size with exFAT for performance.

Writer with Thousands of Text Files

Text files are tiny (1 KB – 20 KB).

Use 2 KB or 4 KB clusters with NTFS to reduce space waste.

External Backup Drive

Contains full-disk images or large archives.

Use NTFS or exFAT with 64 KB or higher.

Bootable Linux USB

Use FAT32 with 4 KB or 8 KB cluster size for compatibility with older BIOS.