iPhone 'iCloud Backup Failed' — How to Fix It

What it means

'iCloud Backup Failed' (or 'The last backup could not be completed' on the iCloud settings screen) means the iPhone tried to back up overnight or on demand and couldn't finish.
The usual causes are not enough iCloud storage left, the iPhone going off Wi-Fi or off charge before the backup completed, or — occasionally — a corrupted item in the backup that needs to be skipped.
Your existing backups are still there; this only affects the new one.

Affected Models

All iPhone models with iCloud Backup turned on
Same behaviour on iPad and iPod Touch
Most common on the free 5 GB iCloud tier, since one full backup usually exceeds that

Common Causes

Not enough free iCloud storage to save the new backup
iPhone wasn't on Wi-Fi long enough overnight
iPhone wasn't on the charger for the automatic nightly backup
Backup of a specific app (Photos, Messages, a giant game save) is failing
iCloud server slowness or a temporary Apple outage
Apple ID signed out of iCloud, or Apple ID password recently changed
iOS or iCloud settings need an update

How to Fix It

Check iCloud storage.

Settings > [your name] > iCloud > Manage Account Storage (or Manage Storage).
If 'Available' is less than your iPhone's used storage, the backup will never fit.
You can either free up iCloud space (next step) or buy more iCloud+ storage from Settings > iCloud > Upgrade — 50 GB is roughly £1 / $1 a month and fits almost any iPhone.
Free up iCloud space by choosing what's backed up.

Settings > [your name] > iCloud > Manage Account Storage > Backups > [this iPhone] > scroll down to 'Choose Data to Back Up'.
Turn off big apps you don't need in a backup (delivery apps, podcast apps, anything that re-downloads its data anyway).
You can also delete an old backup from a phone you no longer use to free space straight away.
Connect to Wi-Fi, plug in, and back up manually.

Plug the iPhone into power, connect it to a good Wi-Fi network, and lock the screen.
Settings > [your name] > iCloud > iCloud Backup > Back Up Now.
Watch the progress for a couple of minutes — if it starts and runs, leave it; a 64 GB iPhone can take an hour or two on the first try.
Restart the iPhone.

Press and hold the side button + a volume button until the power slider appears, drag it, wait, then hold the side button to turn on.
A proper restart clears stuck iCloud state.
Try Back Up Now again after.
Sign out of iCloud and back in (with care).

Settings > [your name] > Sign Out at the very bottom.
When asked, keep a copy of your contacts and calendars on the iPhone — that way nothing is lost during the sign-out.
Sign back in.
This rebuilds the iCloud token, which fixes stubborn 'Backup Failed' loops that survive restarts.
Check what's slowing the backup.

If Back Up Now starts but stalls partway, look at the Estimated Time.
If it's massive, something specific is the problem — usually Photos with thousands of new shots that aren't yet in iCloud Photos.
Turning iCloud Photos on (Settings > Photos > iCloud Photos) takes photos out of the backup and uploads them separately, which often unblocks the backup.
Update iOS, then try a clean backup.

Settings > General > Software Update — install anything pending.
Then plug in, connect to Wi-Fi, lock the screen, and leave it overnight.
If backups fail every night on a fully updated iPhone with plenty of iCloud space and a solid Wi-Fi connection, contact Apple Support — a corrupted backup at the iCloud end occasionally needs them to reset it from their side.

Frequently Asked Questions

Will my iPhone be lost if iCloud Backup keeps failing?

No.
Your previous successful backups are still in iCloud — only the new one didn't go through.
You can see the date of the most recent backup at Settings > [your name] > iCloud > iCloud Backup; if it shows a date from earlier this week, you still have a working safety net.
Photos backed up via iCloud Photos (a separate system from device backup) are also unaffected by a failed device backup.
Fix the failed backup quickly, but you're not in immediate danger of losing data, especially if you also have a Photos sync or an occasional computer backup.