iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
A file allocation table (FAT) is the on-disk data structure that FAT-family filesystems use to track which storage clusters belong to each file and directory, and how those clusters are linked together. A directory entry records a file’s first cluster, and the filesystem follows FAT entries from there to find the rest of the file. The FAT stores the links and allocation state. The file contents themselves sit in the data region of the volume.
How the FAT links clusters
A filesystem stores data in fixed-size units called clusters. A file rarely fits neatly into adjacent clusters, especially after files have been created, deleted, and resized over time. The FAT solves this by keeping one entry per cluster. Each entry either names the next cluster in the file’s chain or carries a special marker that says the chain ends there.
Microsoft’s FAT on-disk format overview (version 1.03, dated December 6, 2000) describes this arrangement. Microsoft’s exFAT File System Specification, in its current Microsoft Learn version, goes further and defines a cluster chain as a singly linked list of cluster indices. In its words:
The Tool Desk
Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →“A cluster chain is a series of clusters which provides space for recording the contents of files, directories, and other file system structures.” (Microsoft, exFAT File System Specification, section 4.1)
#1 Best Overall
Sale
Lexar D40E 128GB Dual USB 3.2 Gen 1 Type-C Jump Drive, Champagne Silver
- USB-C 2-in-1 storage OTG: The Lexar JumpDrive Dual Drive D40E features USB Type-A and Type-C connectors in a slim, portable form factor for easy device compatibility
- Transfer speeds up to 100MB/s: Based on internal testing, performance may vary depending upon the host device, interface, and usage conditions. 1MB=1,000,000 bytes
- Plug and Play: Widely compatible with USB Type-C smartphones, tablets, laptops, Macs, and traditional Type-A devices, no software installation required. The 360° swivel design allows for easy switching between connectors without the hassle of losing a cap
- Durable & Compact: The Lexar D40E USB memory stick features a metal enclosure, withstands temperatures from 0° to 50° C (32°F to 122°F), and is lightweight at 26g with dimensions of 70.4 x 16.9 x 11.7mm
- Security & Warranty: Securely protects files using an advanced security software solution with 256-bit AES encryption. Backed by a Lexar 3-year limited warranty
Following a chain from start to finish
Reading a file works in a fixed sequence:
- The filesystem reads the directory entry for the file, which holds the first cluster number.
- It reads that cluster’s FAT entry to find the next cluster in the chain.
- It repeats the lookup until it reaches an end-of-chain marker.
- It reads the data from each cluster in order, so the file reassembles correctly even when its clusters are scattered across the disk.
Reserved values are not all “next cluster” numbers
It is tempting to picture every FAT entry as a plain pointer. That is not accurate. Certain values carry meaning beyond a link: one marks a cluster as free, one marks a cluster as bad, and one marks the end of a chain. A filesystem driver that treats every entry as a simple pointer will misread the volume, which is why the format defines these reserved values explicitly.
Where the FAT sits on a volume
Microsoft’s format overview describes the regions of a FAT volume in this order:
Rank #2
- High-speed USB 3.0 performance of up to 150MB/s(1) [(1) Write to drive up to 15x faster than standard USB 2.0 drives (4MB/s); varies by drive capacity. Up to 150MB/s read speed. USB 3.0 port required. Based on internal testing; performance may be lower depending on host device, usage conditions, and other factors; 1MB=1,000,000 bytes]
- Transfer a full-length movie in less than 30 seconds(2) [(2) Based on 1.2GB MPEG-4 video transfer with USB 3.0 host device. Results may vary based on host device, file attributes and other factors]
- Transfer to drive up to 15 times faster than standard USB 2.0 drives(1)
- Sleek, durable metal casing
- Easy-to-use password protection for your private files(3) [(3)Password protection uses 128-bit AES encryption and is supported by Windows 7, Windows 8, Windows 10, and Mac OS X v10.9 plus; Software download required for Mac, visit the SanDisk SecureAccess support page]
- Reserved region, which holds boot-related structures at the start of the volume.
- FAT region, which holds the allocation table itself.
- Root-directory region, which exists on FAT12 and FAT16 volumes only.
- File and directory data region, which holds the actual contents of files and directories.
FAT32 has no separate fixed root-directory region. Its root directory is stored in the data area like any other directory, which is one of the structural differences between FAT32 and the older variants.
FAT12, FAT16, and FAT32
The three classic FAT types are named for the width, in bits, of their nominal FAT entries: 12, 16, and 32. Microsoft’s overview gives these as the nominal widths for each type.
Rank #3
- What You Get - 2 pack 64GB genuine USB 2.0 flash drives, 12-month warranty and lifetime friendly customer service
- Great for All Ages and Purposes – the thumb drives are suitable for storing digital data for school, business or daily usage. Apply to data storage of music, photos, movies and other files
- Easy to Use - Plug and play USB memory stick, no need to install any software. Support Windows 7 / 8 / 10 / Vista / XP / Unix / 2000 / ME / NT Linux and Mac OS, compatible with USB 2.0 and 1.1 ports
- Convenient Design - 360°metal swivel cap with matt surface and ring designed zip drive can protect USB connector, avoid to leave your fingerprint and easily attach to your key chain to avoid from losing and for easy carrying
- Brand Yourself - Brand the flash drive with your company's name and provide company's overview, policies, etc. to the newly joined employees or your customers
What the name does and does not tell you
The label describes the entry width, not the capacity or file count of a particular volume. The type of an actual volume is determined from its cluster count under the format rules. A volume labeled by its type name therefore does not automatically have that many addressable files or that many bits of usable capacity. Cluster count is the deciding factor.
exFAT and how it differs
exFAT belongs to the FAT family, and Microsoft identifies it as the successor to FAT32. Its most important structural change is how it records free and used space. FAT12, FAT16, and FAT32 use the FAT itself to record allocation state. exFAT uses an allocation bitmap for that purpose. exFAT still uses FAT entries to represent cluster chains in relevant cases, so the FAT concept does not disappear entirely.
Rank #4
- GOOD VALUE PACKAGE - 1 Pack 32GB Memory Stick USB 2.0 Flash Drives with great cost performance and high quality.
- BIG CAPACITY - The available capacity: 29.10GB-29.8GB, You can save the data of movies, music, photos, designs, programs, manuals, handouts in a high speed.Good performance in digital data storing, transferring and sharing with families, friends, workmates, clients and machines.
- EASY TO USE & PLUG AND WORK - Support windows 7 / 8 / 10 / Vista / XP / 2000 / ME / NT Linux and Mac OS, Compatible with USB2.0 and below.
- TWISTTURN DESIGN & EASY CARRY - The metal clip rotates 360° round the ABS plastic body which with rubber oil skin feeling finish. The capless design can avoid lossing of cap, and providing efficient protection to the USB port.
- WARRANTY & SUPPORT - SIMMAX logo is laser printed on the USB connector surface, our products are of good quality and we promise that any problem about the product within one year since you buy.
Microsoft’s Tech Licensing page on exFAT (accessed October 7, 2026) states that FAT32 has a 4 GB single-file size limit, in contrast to exFAT’s support for single files larger than 4 GB. That is a format comparison, not a general statement about which devices can read which format.
Outdated Drivers Are Slowing You Down
One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWindows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallComparing the FAT variants
| Format | Nominal FAT entry width | How allocation state is recorded | Single-file size limit (as stated in cited Microsoft sources) |
|---|---|---|---|
| FAT12 | 12 bits | FAT entries | Not stated in the cited sources |
| FAT16 | 16 bits | FAT entries | Not stated in the cited sources |
| FAT32 | 32 bits | FAT entries | 4 GB |
| exFAT | Not stated in the cited sources | Allocation bitmap; FAT entries still used for cluster chains in relevant cases | Supports single files larger than 4 GB |
Choosing between formats for a real device
The definition alone does not settle which format a drive should use. When the choice matters, check these four points before formatting:
Best Value
- 【16GB Flash Drive】USB flash drives with 16GB capacity, meet your needs of daily use on work, school, home and travelling for photos, music, videos, files storage and transfer. IMEASON thumb drives can be used to store different files, easy to data backup.
- 【Metal Swivel Cap Design】USB thumb drive is metal swivel cover provides extra protection for the usb thumbdrive connector, no usb drive cap to lose; keychain design makes it easier to carry without worrying lose it.
- 【Wide Compatibility】USB drive supports Windows 7/8/10/11 / Vista / XP / Unix / 2000 / ME / NT Linux and Mac OS, also Supports USB 2.0 and 1.1 ports. USB Stick support TV, desktop, notebook computer, car, audio and other device. The USB Memory Stick is your great data storage and transfer companion with traveling and working.
- 【Easy to use】usb memory stick is plug and play without any software installation. Just simply plug the Flashdrive into the port of your USB-compatible devices such as computer, laptop to start data storage or transmission.
- 【What You Get】16 GB USB Flash Drive Thumb Drive, The default format of the usb storage flash drive is FAT32.
- The largest single file you expect to store. FAT32’s 4 GB single-file limit rules it out for larger files.
- The volume size and cluster count the format will produce, since the cluster count determines the FAT type.
- Whether the drive is a fixed internal disk or removable media. Microsoft describes exFAT’s use with removable storage and interchange between devices.
- Whether every device and operating system that will mount the drive supports the format you choose. Verify this for the specific hardware and software involved, because Microsoft’s overviews do not establish compatibility for every device.
In short, FAT12, FAT16, and FAT32 are the classic FAT-based formats that track allocation with the FAT itself, and exFAT is the FAT-family successor that uses an allocation bitmap and removes FAT32’s 4 GB single-file ceiling. The FAT is the filesystem’s map of cluster chains, not the file data it describes.
Quick Recap
The Bottom Line
“”
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
