macOS

Written & tested for accuracy by a developer (not AI)

Reinstall macOS on a Mac

How to reinstall macOS, with or without erasing your Mac. Use macOS Recovery for a reinstall or a clean install, or install from a bootable USB drive when Recovery is not available.

Reinstalling macOS replaces the operating system while your files stay where they are. A clean install goes further: it erases the drive first, so the Mac starts over with nothing on it. Both jobs use the same two routes, and this guide covers each one.

You are reading Mac Install Guide, a trusted source for Mac setup. See the Mac development setup guide.

Before you get started

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First steps to reinstall macOS

First, check the macOS version. If you're running an older version, update macOS to the latest macOS version.

Then confirm the Mac supports the version you intend to install. See Mac Compatibility before you erase anything, because a Mac that cannot run a version will refuse to install it and you will have wiped the drive for nothing.

Decide which of the two routes you need. macOS Recovery is built into every Mac and needs no USB drive, but it does need Internet access to download the system. A bootable USB installer works with no Internet access and installs faster on several Macs, but you have to make the drive first. See Download macOS to get the full installer, then Create a Bootable USB Installer for macOS to build the drive.

If you wanted a newer macOS rather than a fresh copy of the current one, you do not need any of this. See Upgrade macOS instead.

Decide between a reinstall and a clean install

A reinstall puts a fresh copy of macOS over your existing system. Your files, applications, and settings stay in place. Use it when macOS is corrupted in a way that stops normal startup or operation, and you want a healthy system without rebuilding your Mac.

A clean install erases the drive first, then installs macOS on empty storage. This removes years of accumulated files, damaged preferences, and software conflicts. Use it when a Mac runs slowly, crashes often, or has problems that a reinstall does not fix, and when you are handing the Mac to someone else.

The difference matters because only one of them is reversible. A reinstall keeps your data. A clean install destroys it, and no tool recovers it afterwards.

Back up your data before a clean install

A clean install erases everything on the Mac. You cannot recover the data once the erase begins, so back up before you start, not after you decide.

  1. Connect an external drive with enough free space
  2. Use Time Machine, or copy your important files by hand
  3. Verify the backup opens before you go any further

If you have no external drive, copy the files you care about to cloud storage such as iCloud Drive or Google Drive, or push your code to a Git host. A second USB drive works too.

Reinstall macOS from Recovery

macOS Recovery is a small startup system Apple builds into every Mac. It can reinstall macOS without a USB drive, and it downloads the system it needs over the Internet, so the Mac has to be online.

Start up in macOS Recovery

The key combination depends on the processor in the Mac.

Apple Silicon (M-series) Macs:

  1. Shut down the Mac
  2. Hold the power button until "Loading startup options" appears
  3. Click Options, then Continue
  4. Enter your password if the Mac asks for it

Intel Macs: restart the Mac, then immediately hold one of these combinations until the Apple logo appears, and release when the macOS Utilities window opens.

  • Command + R reinstalls the macOS version currently on the Mac
  • Option + Command + R installs the latest version the Mac supports
  • Shift + Option + Command + R installs the version the Mac originally shipped with

Reinstall over your existing system

This route refreshes macOS and leaves your files and applications alone.

  1. Select "Reinstall macOS" in the macOS Utilities window
  2. Click Continue to start
  3. Choose "Macintosh HD" as the destination, not "Macintosh HD - Data"
  4. Wait for the download and installation to finish
  5. Keep the Mac awake and plugged in throughout

The destination matters. Your Mac stores the system and your files on two separate volumes, and "Macintosh HD - Data" holds your files. Choosing "Macintosh HD" tells the installer to replace the system volume only, and it preserves the data volume automatically. Your files stay safe while the system is rebuilt.

Erase the drive for a clean install

For a true clean install, erase the drive before you reinstall. Do this only after the backup above is done and verified.

  1. Select "Disk Utility" in the macOS Utilities window
  2. Choose your startup disk, usually "Macintosh HD"
  3. Click Erase in the toolbar
  4. Set the format to "APFS" on newer Macs, or "Mac OS Extended (Journaled)" on older ones
  5. Name the drive, keeping "Macintosh HD" or choosing a new name
  6. Click Erase to wipe the drive
  7. Quit Disk Utility when it finishes
  8. Select "Reinstall macOS" in the utilities window
  9. Choose the freshly erased drive as the destination
  10. Follow the setup prompts when the installation completes

You now have a completely fresh macOS installation, exactly as a new Mac arrives.

On a recent Mac there is a shorter route that does the same job from within macOS. Open System Settings > General > Transfer or Reset > Erase All Content and Settings. It performs a secure erase and then offers to reinstall macOS, without any trip to Recovery.

Reinstall macOS from a bootable USB drive

Use this route when the Mac has no Internet access, when you are setting up several Macs, or when Recovery itself will not start. You need a bootable USB installer first: see Create a Bootable USB Installer for macOS to make one, which also covers starting the Mac from the finished drive.

Once the Mac has started from the USB drive, the macOS Utilities window offers the same choices Recovery does.

Erase and install from the USB drive

Skip this section if you want to keep your files. Choose "Install macOS" instead and follow the prompts.

  1. Select Disk Utility from macOS Utilities
  2. Choose your internal drive, usually "Macintosh HD"
  3. Click Erase at the top
  4. Set the format to APFS on newer Macs, or Mac OS Extended (Journaled) on older ones
  5. Click Erase to wipe the drive
  6. Quit Disk Utility when it finishes
  7. Select Install macOS from the utilities window
  8. Follow the installation prompts

Installation takes 30 to 60 minutes, depending on the Mac. Leave it plugged in and do not let it sleep.

Continue setting up your Mac

Don't miss the full visual roadmap and checklist that shows how to set up a Mac for software development, with all the essential tools and settings you might not yet know about.