Open a .dmg file without a Mac
You have a .dmg and no Mac. Drop it in — it is opened in your browser, and you get to see and extract what is inside. Nothing is installed and nothing is uploaded.
Be clear about what this can and cannot do: a DMG usually contains a Mac OS X or macOS application, and those cannot run anywhere but a Mac. Opening the image to get at documents, images, drivers or installers inside it is a different and very achievable thing, and that is what happens here.
Everything happens in your browser. Your file is never uploaded anywhere.
What a DMG actually is
A disk image: a file containing a complete filesystem, which macOS mounts as though it were a physical disk. Apple has used several container formats under the name over the years, and which one you have decides how easy your life is.
UDIF is the modern one, used since Mac OS X. It is a structured container — a compressed data fork holding the filesystem in blocks, with a trailer (the XML property list known as the koly block) at the end describing them. Inside is usually an HFS+ or APFS volume. Compression is zlib, bzip2 or LZFSE depending on how it was made.
NDIF is the older Mac OS 8 and 9 format, and .smi is a self-mounting image — an NDIF image with a small Macintosh application attached so the recipient did not need any tools.
Some DMGs are also encrypted, and those genuinely cannot be opened without the password.
Why Windows cannot open one
Two reasons stacked on top of each other. Windows does not understand the UDIF container, and even after unpacking it, Windows cannot mount HFS+ or APFS. Both problems have to be solved, and most free tools that claim to open DMG files solve only the first, then present you with a raw volume nothing can read.
What works is a tool that unpacks the container and reads the enclosed filesystem — 7-Zip does a decent job on many DMGs for exactly this reason, and that same capability compiled to WebAssembly is what runs on this page.
And if it contains an application
Then you can see it, and you can extract it, and it will not run on Windows or Linux. A .app is a folder containing a compiled binary for macOS — Intel or Apple Silicon — plus its resources, and it needs macOS's frameworks underneath it. No emulator on this site or anywhere else makes that a Windows program.
This site emulates the classic Macintosh — System 6, System 7, Mac OS 8 and 9 — which is a completely different machine architecture from a modern Mac. If what you actually want is to run old Macintosh software, and the file you have is from that era, the classic loader is the page you want. The what runs page draws the line properly.
Questions people ask
Can I open a DMG on Windows?
You can open and extract one, yes — including on this page, with nothing installed. What you cannot do is run a macOS application that comes out of it.
Will this install the app for me?
No. macOS applications only run on macOS. This extracts the contents so you can see and take what is inside.
My DMG will not open.
It may be encrypted, in which case a password is required and there is no way around it. It may also be an APFS image, which is newer and less widely supported than HFS+.
What is the difference between .dmg and .iso?
An ISO is a plain CD filesystem image with no compression or container. A DMG is an Apple container format, usually compressed, usually holding an HFS+ or APFS volume.
Is my file uploaded?
No. The extractor is WebAssembly running in your browser.
Can I run old Mac software instead?
Yes, that is most of this site. Classic Macintosh applications from the System 6 through Mac OS 9 era run here properly — start at the loader or the catalogue.