The short answer
- DMG is an aggregate disk-image format that stores files and folders in a mountable image, with representations that can differ in compression, writability, encryption, and storage structure.
- On a Mac, double-click the DMG. macOS mounts the image and shows its disk icon on the desktop and in the Finder sidebar; open that mounted disk to access the contents. An encrypted image also requires its password. DMG is a macOS disk-image format, and Apple's documented opening, verification, and conversion methods use a Mac. For dependable access from Windows, use a suitable Mac.
- Treat a downloaded DMG as a container whose contents may include executable code. A Developer ID signature and notarization provide validation signals, and a signed disk image lets Gatekeeper validate the image and its contents when first mounted, but these signals do not prove that the file is trustworthy. Eject an encrypted image after use because its documents remain accessible to anyone using your account while it stays mounted.
DMG File (Apple Disk Image): How to Open, Verify, and Convert It
Apple Disk Image was developed by Apple Computer, Inc. and is identified by PRONOM as fmt/1071. A DMG can package files, folders, applications, scripts, libraries, or other executable content. Its published MIME type is application/x-apple-diskimage, and PRONOM does not publish a byte signature for it.
Disk Utility can convert a DMG's representation, changing properties such as compression, encryption, writability, or storage structure. Converting a read/write image to UDRO or ULFO makes the resulting image read-only. This does not mean that the files inside have been converted to different file formats.
How to open it
On a Mac, double-click the DMG. macOS mounts the image and shows its disk icon on the desktop and in the Finder sidebar; open that mounted disk to access the contents. An encrypted image also requires its password. DMG is a macOS disk-image format, and Apple's documented opening, verification, and conversion methods use a Mac. For dependable access from Windows, use a suitable Mac.
On a Mac, double-click the DMG. macOS mounts the image and shows its disk icon on the desktop and in the Finder sidebar; open that mounted disk to access the contents. An encrypted image also requires its password. DMG is a macOS disk-image format, and Apple's documented opening, verification, and conversion methods use a Mac. For dependable access from Windows, use a suitable Mac.
Disk Utility can convert a DMG's representation, changing properties such as compression, encryption, writability, or storage structure. Converting a read/write image to UDRO or ULFO makes the resulting image read-only. This does not mean that the files inside have been converted to different file formats.
File safety
Treat a downloaded DMG as a container whose contents may include executable code. A Developer ID signature and notarization provide validation signals, and a signed disk image lets Gatekeeper validate the image and its contents when first mounted, but these signals do not prove that the file is trustworthy. Eject an encrypted image after use because its documents remain accessible to anyone using your account while it stays mounted.
An unknown sender, a name ending in two extensions, or contents that do not match the extension — check before you open it.
Converting it
Disk Utility can convert a DMG's representation, changing properties such as compression, encryption, writability, or storage structure. Converting a read/write image to UDRO or ULFO makes the resulting image read-only. This does not mean that the files inside have been converted to different file formats.
Convert a copy, never the original, then compare quality, metadata, and layout against what you started with.
Common questions
What is a DMG file and what is it used for?
A DMG is an Apple Disk Image containing files and folders in a mountable container. It may distribute documents, applications, scripts, libraries, or other content.
How do I open a DMG file on a Mac?
Double-click the DMG. macOS mounts it and displays a disk icon on the desktop and in the Finder sidebar; open that disk to reach the files.
Can I open a DMG file on Windows?
Not with any method Apple documents. Opening, verification, and conversion use a Mac; for dependable access from Windows, use a suitable Mac.
Why will an encrypted DMG file not open after I forgot the password?
The password is required to open an encrypted image and view its files. Apple states that a forgotten password prevents access.
How do I verify whether a DMG file is corrupted?
Use Disk Utility's image verification. It recomputes the checksum recorded when the image was created; a match indicates that the image data has not been corrupted.
Should I use a read-only or read/write DMG file?
Choose read/write when the image must remain editable. Choose a read-only representation such as UDRO or compressed ULFO when later editing is not required.
How do I convert a DMG file with Disk Utility?
Use Disk Utility's image conversion to change representation, encryption, compression, writability, or storage structure. The conversion changes the image, not the file formats stored inside it.
What happens when I convert a DMG file to read-only format?
Converting a read/write image to UDRO or ULFO removes subsequent editability from the resulting disk image.
Is a downloaded DMG file safe to open?
Do not assume so: a DMG can contain applications, scripts, libraries, and other executable content. A Developer ID signature, notarization, and Gatekeeper validation are useful trust signals, but do not prove the file is trustworthy.
Does a valid DMG checksum prove the file is safe?
No. A matching checksum indicates that the image data has not been corrupted since the checksum was recorded; it does not establish that executable contents are trustworthy.
What is the difference between a DMG made from a whole disk and one made from a folder?
An image of an entire disk or connected device includes its data and free space. An image made from a folder or device contents copies the contents without copying the device's free space.
How do I protect access to an encrypted DMG after opening it?
Eject it after use. While it remains mounted, anyone with access to your computer account can use its documents.
Related formats
Sources
PRONOM Format Details: Apple Disk Image (fmt/1071) · The National Archives (UK) PRONOM format registry
Create a disk image using Disk Utility on Mac · Apple Support
Convert a disk image to another format using Disk Utility on Mac · Apple Support
Verify that a disk image's data isn't corrupted using Disk Utility on Mac · Apple Support
Packaging Mac software for distribution · Apple Developer Documentation
Code Signing Tasks · Apple Developer Documentation
Technical Note TN2206: macOS Code Signing In Depth · Apple Developer Technical Note