I use 120 MB of iCloud storage space for iCloud Backup of my iPad & iPhone. There is still 4.8 GB free from 5 GB, but I have a notification in iPad's backup window that I'm lacking space to back up to iCloud and would rather not buy more space to back up today.
This could easily be explained as the 120 MB of used data is likely for the data all the apps store in iCloud. Contacts, calendars, specific third party apps can have storage in iCloud. You can determine this by looking at the Manage Storage pane in settings app (see below).
The warning you describe comes up when the expected size of the next backup is greater than the available free space.
You can reduce the size of the expected backup on iOS (updated for iOS 16) as follows:
- open the Settings app
- tap your AppleID/iCloud avatar (first item, top of list)
- tap iCloud
- tap Manage Account Storage
- tap Backups
- tap the name of the device in question (it usually says this iPod touch, this iPhone or this iPad to help if you have many devices connected to iCloud)
- look at Next Backup Size
- toggle off any unwanted apps from the list below
If that is less than the available space, you may need to contact Apple support to have your account fixed on the server side. In most cases, you can toggle off the largest few items and then attempt a backup when the estimate is less than the available space.
Once the backup is complete, you can tune the backups or manage your space as needed on iCloud.
On iOS 15 I did have an open case with Apple. I have a 256 iPhone that refuses to back up to iCloud or a Mac with out of space errors and it’s not possible since the phone has less total space than either destinations have free. Once I enabled my support pin with them, it did show podcasts needed to back up 590 GB of data (on a 256 phone) so I disabled that from backup and then it backed up successfully. Once the "partial" backup was done, I re-enabled podcasts app to backup and it had a reasonable amount of space (less than 500 MB) and the situation was sorted while preserving all data to iCloud.
In my case, I have the problem that iCloud estimates my data usage to be 10.9GB, while I have chosen to back up maybe only 1-2GB. If I turn, for example, camera roll off or on again, it doesn't matter.
iCloud estimates this wild, high, far too much space to make a backup, and I even deleted my previous backup and started new, to be sure to 'reset' everything, but it doesn't help!
My iPad only uses 2.2 GB, I have 2.8 left for the phone, and select 'no backup' for most, but the mail and a few programs like Skype and so on, altogether far less than "10.9" GB, probably only 1-2GB.
This problem seemingly happened before, the iCloud backup grow out of control, like a classical Windows installation.
However, I now see that, as I deactivate more and more smaller applications, of a few kb size of their data, that the total usage decrease much more than these few kb which are stated for each app.
This means that Apple seemingly underreports how much data they backup, from each app, or maybe the system counts too much, I don't know.
It is not possible to deactivate all the apps at once, in the hope to get down to the 17MB which is stated on the first page, (e-mail and settings), so there will be a lot of job to make this work again... :-/
The solution where you disable certain data from being backed up in iCloud is not really a solution but merely a workaround. Unfortunately, the real solution would imply you paying for iCloud's yearly subscription charges.
Or keep your iPhone backup out of the cloud an on your PC instead: There is not enough iCloud storage available. How to backup iPhone
Alternatively, keep the Camera Roll under iCloud backup settings disabled as that eats lots of space and back up your iPhone to your PC via iTunes. This way you don't lose the backup of your Camera Roll.
Are you sure you only need 120 MB for iCloud Backup?
Check the size of the 'next backup' in iCloud Backup on your device. Make sure that it's less than the free space of your iCloud account.
Are you checking the storage of the correct iCloud account?
Make sure that you're looking up the free space of the correct iCloud account if you have multiple. Only the main iCloud account can be used for iCloud Backup — accessible through Settings → iCloud.