How to Reinstall Mac OS

A clean install of the OS may get your Mac working properly again

Photography depicity MacBook Pro 2018, frontal view, dark background

Razvan Nitoi/Getty Images

Reinstalling macOS will fix just about any problem you have with the operating system, but it's often overkill. It's a good option if your computer will no longer boot, or there are system-level problems you can't resolve. It's also useful if you've borked your installation so badly that you're better off returning to square one. If you're careful and lucky, you can even reinstall macOS without erasing your hard drive.

Choosing the Correct Way to Reinstall macOS

Your first reinstallation choice should always be to reinstall macOS without erasing your boot drive. This preserves your user files by replacing macOS system files with known-good versions, recovering your system from whatever software problem was causing an issue. If this doesn't fix your problem, then you can erase your boot drive and perform a fresh reinstall. Only use a bootable USB installer to reinstall macOS if you can't boot into Recovery at all.

How to Reinstall macOS Using Recovery Mode

The standard method for reinstalling macOS is through what's called Recovery. This special mode boots from a separate recovery partition on your boot drive, which macOS creates at the time of installation.

You're unable to change the partition you boot from. In order to unlock the boot partition to make changes, macOS boots from the recovery partition. In the recovery partition, you're free to make changes and adjustments to your primary boot partition, including completely erase it or reinstall macOS.

To boot into Recovery Mode, restart your Mac. Hold down Command+R as soon as your Mac shuts off and while the computer starts up. Release the keys when you see the Apple logo, a progress wheel, or a password prompt.

If your Mac requires a firmware password, you'll need to enter it before you can successfully boot into recovery mode. This typically happens with Macs administered by an organization with a mobile device management policy. If you don't know your firmware password and can't get it, Apple may be able to unlock your device for you, so long as you can prove you own it.

How to Reinstall macOS Without Erasing Everything

If your macOS installation is causing problems, you don't have to wipe the disk completely and start over from scratch. In fact, macOS can make an effort to reinstall only the system files, replacing any corrupted or damaged system files and, hopefully, returning your system to a working state.

This is the default method for reinstalling macOS. It's also the first method you should try since it's the least destructive. It might not fix your problems, but it's also not a nuclear option.

Screenshot of the macOS Recovery Utilities with Reinstall macOS selected.

To do so, boot into Recovery Mode and select Reinstall macOS from the macOS Utilities application, then select Continue to begin the installation process. Follow the on-screen instructions to reinstall macOS.

How to Erase and Reinstall macOS

Wipe your hard drive and start completely clean with an erase and reinstall. This will permanently delete all the files on your boot drive, so be sure you have a backup before proceeding. First, we will format with Disk Utility, then reinstall macOS.

  1. Boot into Recovery Mode and click Disk Utility from macOS Utilities.

  2. Select your boot drive in the left-side pane.

  3. Click Erase to reveal options for erasing your boot drive.

  4. If you are installing Mojave or later, choose Apple File System (APFS) as your format. If you are installing High Sierra or earlier, choose macOS Extended (Journaled) as your format.

  5. Give the drive the same name it had before erasure. The default name is "Macintosh HD."

  6. Click Erase to erase the drive completely.

  7. Close Disk Utility to return to the macOS Utilities app.

  8. Click Install macOS from the menu and follow the on-screen instructions to proceed with a typical installation.

Other Recovery Mode Options

By default, Recovery Mode enables the reinstallation of your macOS installation with the version of macOS installed on your Mac. But you can also access different installation tools that install different versions of macOS. You access these modes by holding different modifier keys while booting into Recovery Mode:

  • Command+R: Boot into Recovery Mode and reinstall the latest version of macOS on your Mac.
  • Option+Command+R: Boot into online recovery mode, which downloads the latest compatible version of macOS and installs it on your Mac. If your recovery partition is damaged or can't reinstall the version of macOS currently on your Mac, this network recovery can restore the installation with a fresh file download.
  • Shift+Option+Command+R: Boot into recovery mode to install the version of macOS that originally shipped with your Mac or the closest available version.

Reinstall macOS Using a Bootable Installer

If your Mac has become so non-functional that you can no longer boot into recovery mode, never fear. You can reinstall macOS from a bootable USB installer. It's a slightly complex process that requires at least one working Mac.

It's wise to create an up-to-date bootable installer in advance as an emergency repair tool.

You'll need a full version of the macOS installer and a USB drive formatted for macOS Extended and with at least 12 GB of space. Use disk utility to properly format your USB before proceeding, if needed.

  1. Download the appropriate macOS installer from the Mac App Store; either macOS High Sierra or macOS Mojave.

    If your hardware isn't compatible with Mojave or High Sierra, you can try an earlier version of macOS.

  2. After the installer automatically launches, quit the installer from the menu or press Command+Q.

    Screenshot showing how to quit the macOS Mojave installer
  3. Open Terminal and run the following command. Be sure to replace "USB" with the name of your formatted USB drive:

    sudo /Applications/Install\ macOS\ --volume /Volumes/USB

    If you're not installing Mojave, change the path to the correct name.

  4. Enter your system password when prompted and press Return to authenticate the process.

  5. Type "Y," then press Return to confirm the installer creation. This will erase the USB drive and write the bootable installer image to the USB drive.

    Screenshot showing the installmedia command runing on macOS Terminal
  6. When the process is completed, reboot your Mac while holding down the Option key to enter the boot selection menu.

  7. Select your USB drive from the boot selection menu.

  8. Follow the on-screen instructions to format and reinstall macOS on your primary hard drive.