In this article we're going to show you how to perform a clean install of macOS. This is often a handy option if your Mac is slow or otherwise misbehaving.
We'll start by showing how to do this for the latest version of macOS (macOS Mojave, at time of writing), before moving on to the far tricker business of installing a clean copy of High Sierra, Sierra, El Capitan, Mojave, Yosemite or some earlier version of OS X. If that's what you're looking for, it may be worth taking a look at another article where we focus on that entirely: How to download and install old versions of OS X on a Mac.
What you need
You'll need an 15GB or larger removable USB flash drive to hold the installer file for our chosen version of macOS. (We're going to erase it, so make sure it doesn't contain any valuable data.) You will also need admin privileges.
Before you start, remember to back up, just in case something goes wrong. Full instructions here: How to back up a Mac.
Ready? Let's get started.
Format the drive
We're going to re-download the installer file for our chosen version of macOS and store it on a USB stick or external drive. The drive you use needs to be formatted as a Mac OS Extended (Journaled) volume with a GUID Partition Table.
To format the drive, go to Applications > Utilities and open Disk Utility.
Select the drive and click Erase. (The name of the disk needs to be "Untitled", if the Terminal commands below are to work, so rename it if necessary.)
Select Mac OS Extended (Journaled) as the format type.
If your Mac is using APFS (which it may be if you are running High Sierra or Mojave), select APFS from the list of options.
If it's available choose GUID Partition Map from the Scheme options.
Get the Mojave install file
If you're reinstalling the latest version of macOS, our next step is easy: just go to the Mac App Store and search for its name. At time of writing, that means macOS Mojave, which you can jump to via this link.
Hit Download or Get (depending on the version of the Mac App Store you are using).
You may get a warning message at this point, telling you that this version of macOS 'is already installed on this computer. Would you like to continue?' Click Continue, then enter your Apple ID and password. The installer file will be multiple GB, Mojave is 5.7GB, for example, so downloading can take a while.
When it's finished downloading the installer will launch automatically, but we don't want to use it yet, so press Command + Q to quit out of the installer.
Find the installer file (if will be located in the Applications folder, or you could search for it using Spotlight (command + Space and start typing Mojave).
If you are just wishing to install Mojave, you can skip the next step, where we'll be showing how to dig up the installer file for an older version of Mac OS X.
Get the install file for an older version of Mac OS X
For older versions of Mac OS X it's a bit harder to get the install file: Apple doesn't offer downloads of outdated OS versions through the normal store front, and searching for the name won't find what you're looking for.
The installers for some versions of macOS can be found in the Purchased section of the App Store. Log into the Mac App Store with the Apple ID you used and click Purchased in the menu bar along the top. Scan down the list of apps you've purchased or download for free (which is in order of when you downloaded them) and find the name of your chosen version: El Capitan, Yosemite or whatever. Click download.
However, there are a few reasons why this may not work for you:
- If you have already installed Mojave on your Mac you cannot get old versions of the Mac OS from the new Mac App Store.
- If you are running High Sierra you will not be able to get Sierra via the App Store.
- You won't find the version of macOS your Mac shipped with in the Purchased section of the App Store.
- And you won't find versions of macOS in the purchased section if you never installed that version.
For more advice on getting hold of older editions of Mac OS, see How to download and install old versions of OS X on a Mac.
Create a bootable USB disk
Next we need to create a bootable USB disk so that we can install a new copy of macOS on the Mac from the flash drive. We've got full instructions on how to do that here - How to make a bootable Mac OS X install drive - but we'll run through the basics again. We're going to create the bootable drive using Terminal, which you can open by pressing Command + Space bar and typing Terminal.
Connect the removable drive to your Mac, and make sure it's called Untitled - rename it if necessary.
Create a bootable USB disk: Terminal
Open the Terminal application on your Mac (find it by searching using Command + Space and typing Terminal)
Now copy and paste the following text into Terminal:
sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled -- /Applications/Install\ macOS\ Mojave.app
Enter your password (you won't see the password appear).
When the bootable drive is prepared you will see the word Done. It can take a while.
Install from installer drive
Now we'll install a fresh copy of macOS from the installer drive.
With the boot drive connected, start up - or restart - your Mac while holding down the Option key (also known as Alt). This will take you to Startup Manager. Choose to install your chosen version of macOS from the external drive.
Select 'Disk Utility' and your hard drive, then Erase.
Go back to the main menu and choose Install macOS.
Once installation is complete, you can restoring apps and settings from your backup, or download them again manually. Remember that you won't want to recover a Time Machine backup if you don't want to copy over all the setting and preferences associated with your old system.