ThisCalculator

Everyday

Create macOS Big Sur ISO File: Full Guide

Updated · Standard formula, shown below

Create macOS Big Sur ISO File: Full Guide

To create a macOS Big Sur ISO file, you need a Mac running macOS, the Big Sur installer app, and a terminal command that converts the installer into a bootable ISO. This guide walks you through the exact steps, explains what the file is for, and covers common problems like missing installer errors and virtual machine issues.

What a macOS Big Sur ISO File Is

An ISO file is a single-file disk image that contains the complete contents of a disc or volume. For macOS, an ISO of Big Sur bundles the entire installer system—the recovery environment, the kernel, and the installation payload—into one file that can be mounted, copied, or used to boot a virtual machine. Unlike a USB installer, which is a physical drive, an ISO is portable and can be stored, shared, or attached to a VM without needing extra hardware.

The main reason people create a Big Sur ISO is to install macOS in a virtual machine (like VirtualBox, VMware, or QEMU) where the guest system needs a bootable CD/DVD image. It is also useful for creating a backup of the installer or for testing upgrades in a sandboxed environment.

Prerequisites: What You Need Before Starting

Before you run any commands, make sure you have the following:

  • A Mac running macOS 10.15 Catalina or later (the commands work on newer versions too).
  • At least 15 GB of free disk space (the installer is about 12 GB, plus the ISO conversion needs extra room).
  • Stable internet connection to download the installer from Apple.
  • Terminal app (found in /Applications/Utilities/).
  • If you plan to use the ISO in a VM, the VM software must support macOS guests (check the vendor’s documentation).

Step-by-Step: Downloading the Big Sur Installer

You cannot create an ISO without the full installer app. Here is how to get it:

  1. Open the App Store on your Mac, search for “macOS Big Sur,� and click Get. The installer downloads to /Applications/.
  2. If the App Store shows “Open� instead of “Get,� the installer is already present. Alternatively, use the softwareupdate --fetch-full-installer --full-installer-version 11.7.10 command in Terminal to fetch a specific version (this is the last Big Sur update).
  3. Wait for the download to finish. The file is named “Install macOS Big Sur.app� in /Applications.
  4. Do not run the installer—you only need the app bundle.

Step-by-Step: Converting the Installer to ISO

Once the installer is in place, open Terminal and run these commands one by one. Each command does a specific job, so do not skip any.

  1. Create a blank disk image of 12 GB (adjust size if your installer is larger):
    hdiutil create -o /tmp/BigSur -size 12000m -layout SPUD -fs HFS+J
  2. Mount the blank image:
    hdiutil attach /tmp/BigSur.dmg -noverify -mountpoint /Volumes/BigSur
  3. Copy the installer’s contents onto the mounted image:
    sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/BigSur --nointeraction
    Enter your admin password when prompted. This step takes 10–20 minutes.
  4. Unmount the image:
    hdiutil detach /Volumes/Install\ macOS\ Big\ Sur
  5. Convert the DMG to ISO format:
    hdiutil convert /tmp/BigSur.dmg -format UDTO -o /tmp/BigSur.iso
  6. Rename the file to have a .iso extension (the command creates .iso.cdr):
    mv /tmp/BigSur.iso.cdr ~/Desktop/BigSur.iso

You now have a bootable ISO file on your Desktop. The file is roughly 12 GB, so it will not fit on a standard DVD—use it with a VM or a large USB drive.

Common Problems and How to Fix Them

Several issues can trip you up during the conversion. Here are the most frequent ones and their solutions.

  • “createinstallmedia: command not foundâ€� – The installer app is either missing or not in /Applications. Re-download it and check the exact name (spaces need escaping with backslashes).
  • “Not enough spaceâ€� error – The blank image size is too small. Check the installer size (right-click, Get Info) and recreate the image with a larger size.
  • “Resource busyâ€� when detaching – Close Finder windows or any app that has the volume open. If it persists, use hdiutil detach -force /dev/diskX (find the disk number with diskutil list).
  • ISO does not boot in a VM – The VM must be configured for macOS guests (e.g., set the OS type to macOS 11, enable EFI). Also, some VM tools require the ISO to be attached as a CD/DVD drive, not a hard disk.
  • App Store shows “Openâ€� but the app is corrupted – Delete the installer and re-download. Check the integrity with codesign --verify if you are comfortable with terminal commands.

Alternatives to a Full ISO

If you do not need a single ISO file, consider these options:

  • USB installer – Use the same createinstallmedia command but point it to a USB drive (at least 16 GB). This is the official method for booting a real Mac.
  • Direct DMG – The .dmg file from step 1 is not bootable by itself, but you can skip the ISO conversion and use the DMG as a mountable volume for some VM tools.
  • Recovery partition – On a real Mac, you can reinstall Big Sur from the built-in recovery system without any external media.
  • Dual-boot with a partition – If you want to run Big Sur on a separate drive, use the installer app directly to create a bootable volume.

Safety and Legality Notes

Creating an ISO from Apple’s installer is legal for personal use, but you must have a valid Apple ID and the installer from the App Store. Do not attempt to bypass hardware checks or run macOS on non-Apple hardware—this violates Apple’s EULA and may not work reliably. Also, the ISO contains the full installer, so treat it like any large file: keep it in a safe location and verify the checksum if you downloaded it from a third party (which is not recommended). Always back up your data before creating a bootable image, as the process can erase the target volume if you point it to the wrong drive.

Frequently Asked Questions

Can I create a Big Sur ISO on Windows or Linux?

Not directly. The createinstallmedia tool is a macOS binary, so you need a Mac. However, you can download the installer on a Mac and then transfer the ISO to another OS for use in a VM.

How long does the conversion take?

Downloading the installer (about 12 GB) can take 30 minutes to several hours depending on your connection. The createinstallmedia step takes 10–20 minutes, and the final conversion is usually under 2 minutes.

Will this ISO work on a real Mac if I burn it to a DVD?

No. The ISO is about 12 GB, which exceeds a DVD’s 4.7 GB capacity. It is intended for virtual machines or USB drives (if you convert it back to a DMG or use a tool to write it to USB).

What is the difference between an ISO and a DMG?

A DMG is a macOS-native disk image that can be mounted directly. An ISO is a generic disc image that most operating systems and VM tools recognize. The conversion simply repackages the same data into a different container.

Do I need to disable SIP (System Integrity Protection) to create the ISO?

No. The commands run in user space and do not require disabling SIP. You only need an admin password for the sudo step.

Can I use this ISO to upgrade an existing Mac?

No. The ISO is a bootable installer, but you cannot “upgrade� a running Mac by mounting it. To upgrade, run the installer app directly or boot from a USB installer.

Related

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.