iCloud User Guide
-
Welcome
-
Messages
-
More resources

Check available email storage in iCloud
When you set up iCloud, you get 5 GB of free storage. See What is iCloud storage? You can check available iCloud storage by using your iPhone, iPad, iPod touch, Mac or Windows computer.
Do any of the following:
On iCloud.com: see Check your iCloud storage usage online or on any device.
On your iOS or iPadOS device: go to Settings > [your name] > iCloud, tap Manage Storage, then scroll down to see the storage size for Mail.
On your Mac: choose Apple menu
> System Preferences, then do one of the following:
macOS 10.15 or later: click Apple ID, click iCloud, then click Manage.
macOS 10.14 or earlier: click iCloud, then click Manage.
On your Windows computer: open iCloud for Windows, then click Storage.
If you need more storage, you can upgrade your storage or remove stored items to make more space available (see the Apple Support article Manage your iCloud storage).