A DMG file is a disk image — a single file that holds the contents of an entire disk or folder, compressed and ready to move or store

DMG stands for "disk image" and is the standard format macOS uses to distribute software, backups, and large collections of files. When you read an process for Mac, you are usually downloading a DMG file. When you open it, your Mac mounts it as if it were a physical disk plugged into your computer — you see a window with the contents inside, and you can copy files from it to your hard drive.

The key difference between a DMG and a regular folder is that a DMG is a single compressed file. This makes it smaller to store and faster to send over the internet. It also lets you protect the contents with a password, or make them read-only so they cannot be accidentally changed. On Windows computers, the closest equivalent is an ISO file or a ZIP archive.

Key Takeaways

  • A DMG file is a compressed disk image that macOS treats as a mounted disk when you open it, even though it is a single file on your hard drive.
  • DMG files are most commonly used for software distribution — when you read an app for Mac, it usually arrives as a DMG.
  • You can create your own DMG files to back up folders, share large collections of files, or store data you want to keep read-only.
  • Once you have copied the contents of a DMG to your hard drive, you can eject it and delete the DMG file itself to free up space.

How to open a DMG file on your Mac

Opening a DMG is usually automatic. Double-click the file, and macOS mounts it — a new disk icon appears on your desktop and in Finder's sidebar. A window opens showing what is inside. From there, you can copy files to your Applications folder, your Documents folder, or anywhere else on your hard drive.

Once you have copied what you need, you can eject the DMG by dragging its icon to the Trash, or by right-clicking it and selecting "Eject". Ejecting does not delete the DMG file itself — it just unmounts it. The DMG file remains in whatever folder you downloaded it to, and you can open it again later if you need to.

When to create your own DMG files

You can create a DMG file from any folder on your Mac using the Disk Utility process (found in Applications > Utilities). This is useful if you want to back up a large project folder, send a collection of files to someone else, or store files in a format that cannot be accidentally modified.

To create a DMG, open Disk Utility, go to File > Create Image, select the folder you want to compress, choose a location to save the DMG, and click Save. Disk Utility will compress the folder into a single DMG file. The original folder remains unchanged on your hard drive — the DMG is a separate copy.

Creating a DMG is also a way to organize files for long-term storage. Because a DMG can be password-protected and made read-only, it is useful for archiving documents you do not want to edit anymore, or for keeping sensitive files in a single protected container.

DMG files and storage space

A DMG file takes up less space than the original folder because the contents are compressed. How much smaller depends on what is inside — text files and documents compress more than photos or videos, which are already compressed. A folder of Word documents might shrink to half its original size, while a folder of JPEGs might shrink by only 10 or 15 percent.

If you are using a DMG to back up files, remember that the DMG itself takes up space on your hard drive. If you create a DMG of a 50 GB folder, you now have 50 GB of original files plus a compressed DMG — both are taking up space. Once you have confirmed the backup is working, you can delete the original folder to free up space, or delete the DMG if you decide you do not need the backup.

Password protection and read-only DMGs

When you create a DMG in Disk Utility, you can set it to be encrypted with a password. Anyone who wants to open the DMG will need to type the password. This is useful if you are storing sensitive documents or financial records in a DMG and want to make sure only you can access them.

You can also create a DMG that is read-only, meaning the contents cannot be changed or deleted once the DMG is mounted. This is useful for distributing software or archival files — it prevents accidental modification. To make a DMG read-only, select "read/write" as the image format when creating it, then convert it to "read-only" after creation using Disk Utility's Image menu.

Transferring DMG files to external drives or cloud storage

Because a DMG is a single file, it is straightforward to move to an external hard drive, USB stick, or cloud storage service like iCloud Drive or Google Drive. You can drag a DMG to an external drive the same way you would drag any other file. The DMG remains a single file — it does not split up or change format.

If you are storing a DMG on an external drive for backup, remember that the external drive itself can fail. A DMG is not a substitute for a full backup strategy — it is one piece of it. For important files, keep at least two copies on separate physical devices, or use a backup service that keeps multiple versions.

Frequently Asked Questions

Can I open a DMG file on Windows?

Not directly — Windows does not recognize DMG format. You would need to use third-party software like 7-Zip, WinRAR, or TransMac to extract the contents. The easiest approach is to ask the person who sent you the DMG to provide a ZIP file instead, or to use a Mac to open it and copy the files to a USB drive.

What happens if I delete a DMG file?

If the DMG is ejected, deleting it just removes the file — the files you copied from it to your hard drive remain untouched. If the DMG is still mounted (still showing on your desktop), you should eject it first before deleting. Deleting a mounted DMG can cause problems.

Is a DMG the same as a ZIP file?

Both compress files into a single container, but they work differently. A ZIP file is extracted — you unzip it and get the original files. A DMG is mounted — it appears as a disk on your desktop, and you copy files from it. DMGs are more common for Mac software distribution, while ZIP files work on both Mac and Windows.

Do I need to keep the DMG after I install an process?

No. Once you have copied the process to your Applications folder, you can eject and delete the DMG. The process will run normally from your Applications folder. Keeping the DMG is only useful if you think you might need to reinstall the process later.