What is the UDF file format?
.udf (Universal Disk Format) is a file format used to store disc image files. A .udf file contains a collection of files using the directory structure defined in the UDF standard. The format was developed by the Optical Storage Technology Association (OSTA) and standardized as ECMA-167.
The UDF standard is commonly used for burning CDs, DVDs, and Blu-ray discs, as well as storing file collections on other storage devices such as flash drives. A .udf disc image identifies itself through NSR02 or NSR03 volume descriptors at sector 16 (byte offset 0x8000) - distinct from the CD001 signature used by ISO 9660 images. Some images carry both signatures in a UDF Bridge layout, making them mountable as either format.
Although the UDF standard itself is widely adopted - it is mandatory for DVD-Video and Blu-ray Disc - .udf image files are not commonly used as a standalone file type. Many disc-authoring and recording applications use UDF internally but save the resulting image in .iso or .img format instead.
By using the UDF standard to create media, it is possible to delete or modify files on the target device even after they have been written. This packet-writing capability makes .udf-formatted media well suited for rewritable discs such as DVD-RW and BD-RE.
Security & safety
RISK: LOWA UDF disc image is passive data, so opening or mounting one is generally safe. The usual optical-image caution applies: a mounted image presents a virtual drive that can contain autorun installers or executables, so treat software/OS images from untrusted sources with care and verify checksums where the source provides them. The unrelated NT-era .udf text file is plain ASCII and harmless on its own.
Format details
in a nutshellFULL NAMEUniversal Disk Format disc image
DEVELOPEROSTA (Optical Storage Technology Association) / ECMA-167since UDF spec from OSTA, 1995; based on ECMA-167. Common for DVD/Blu-ray images.
CATEGORYDisk Image Files
MIME TYPEapplication/x-udf-image
TYPEOptical-disc image using the UDF file system (binary)
STANDARDOpen · royalty-free
This extension is also used by…
- Windows NT/2000/XP Uniqueness Database File - An obsolete plain-text answer-file supplement for unattended Windows NT-era installs (per-machine settings); unrelated to optical-disc images and the format the DB name referred to.
- Generic user-defined function / data file - Some applications (e.g. firmware tools, niche databases) save '.udf' files with their own private meaning unrelated to disc images.
MAGIC BYTES · FILE SIGNATURE
OFFSET
8000800180028003
HEX
4E535230
ASCII
NSR0
A UDF volume has no fixed signature in the first bytes. From sector 16 (offset 0x8000 = 32768) the Volume Recognition Sequence holds standard identifiers: "BEA01", "NSR02" or "NSR03", and "TEA01". The bytes "NSR0" mark the UDF descriptor. Many UDF discs/images also carry an ISO 9660 "CD001" descriptor at the same area for compatibility ("UDF Bridge"), so an image can match both. A plain .udf disc image is otherwise structurally similar to an .iso.
Programs that open UDF files
Windows3 apps
7-Zip
Open-source Right-click > 7-Zip > Open archive to browse and extract files from the UDF image without mounting or burning. 
ImgBurn
Free Use 'Write image file to disc' to burn the .udf image to a DVD/Blu-ray. (Download only from imgburn.com to avoid bundled adware.) 
Windows File Explorer (built-in mount)
Built-in Rename the file to .iso, then right-click > Mount to open it as a virtual drive (Windows 8+ mounts UDF images natively). 
macOS1 app
Finder / DiskImageMounter (built-in)
Built-in Double-click the image (rename to .iso if needed) to mount it; macOS reads UDF natively. Use Disk Utility to burn. 
Linux2 apps
mount (udf) / GNOME Disks (built-in)
Built-in sudo mount -t udf -o loop file.udf /mnt to mount as a loop device, or right-click > Open with Disk Image Mounter. Technical details
deep spec| File system | UDF (Universal Disk Format), defined in ECMA-167 |
| MIME type | application/x-udf-image |
| Volume recognition signature | NSR02 (UDF 1.x-2.x) or NSR03 (UDF 2.50+) at byte offset 32,768 (sector 16) |
| Sector size | 2,048 bytes for optical media; 512 bytes supported for hard drives and flash storage |
| UDF specification versions | 1.02, 1.50, 2.00, 2.01, 2.50, 2.60 |
| Filename character set | OSTA Compressed Unicode (CS0), a variable-length UTF-16-based encoding |
| Maximum filename length | 255 characters (UTF-16 encoded) |
| Maximum single file size | Up to 2⁶⁴ − 1 bytes; the Information Length field in the File Entry descriptor is 64 bits wide across all UDF revisions |
| Packet writing | Supported; enables incremental writes and in-place deletion on rewritable media such as DVD-RW and BD-RE |
| UDF Bridge | Optional dual-format layout combining UDF and ISO 9660 (CD001) signatures in a single image for maximum compatibility |
| Primary use cases | DVD-Video and Blu-ray Disc mandatory file system; optical disc image archiving and interchange |
| OS native support | Windows XP and later (built-in); macOS 10.4 and later; Linux via kernel UDF driver |
| File metadata | Creation, modification, access, and backup timestamps; Unix-style permissions and access control lists (UDF 2.01+) |
| Encoding | Binary; raw sector data written in 2,048-byte logical blocks matching physical optical disc sectors |
| Released | UDF spec from OSTA, 1995; based on ECMA-167. Common for DVD/Blu-ray images. |
| Open standard | Yes · royalty-free |
| Specification | www.osta.org |
UDF conversions
Possible
.UDF .ISOGUIDE
Yes - in most cases renaming the file from .udf to .iso is all you need.
How to convert
.UDF .IMGGUIDE
Yes - use PowerISO or AnyBurn to rewrite the UDF image as IMG in one step.
How to convert
.UDF .BINGUIDE
Yes, a UDF disc image converts to BIN/CUE -- AnyBurn (free) does it in a few clicks on Windows.
How to convert
Not possible
Community Q&A
asked by users
R
Frequently asked questions
How do I open a UDF file on Windows?
Rename it to .iso and right-click > Mount (Windows 8 and later mount UDF images natively as a virtual drive), or open it with 7-Zip to extract the files. No special software is required.
Is a UDF file the same as an ISO?
Almost. Both are optical-disc images; the difference is the internal file system (UDF vs ISO 9660). DVD and Blu-ray images use UDF but are usually still named .iso, and the two are largely interchangeable for mounting and burning.
How do I extract files from a UDF image without burning it?
Open the .udf file with 7-Zip (Open archive) and copy the files out, or mount it as a virtual drive and browse it like a disc.
How do I burn a UDF image to a DVD or Blu-ray?
Use ImgBurn's 'Write image file to disc', or right-click and choose Burn disc image on Windows / use Disk Utility on macOS, with a blank disc of sufficient capacity.
What is a Windows NT 'uniqueness database' UDF file?
A different, obsolete use of the same extension: a plain-text file used with Windows NT/2000/XP unattended Setup to apply per-computer settings during automated installs. It has nothing to do with disc images and is no longer used (replaced by unattend.xml).
Why won't my UDF file open directly?
Some tools save the image with the .udf extension that Explorer doesn't auto-associate. Rename it to .iso and mount it, or open it with 7-Zip.
References
1Wikipedia - Universal Disk Formaten.wikipedia.org
2Microsoft (legacy) - Unattended Setup Uniqueness Database File (UDF)learn.microsoft.com
