The message arrives, you dismiss it, and it comes back for the next eighteen months. Most people ignore it until something stops working.
It is worth dealing with, because while it is showing, your phone has usually stopped backing anything up. That is the real cost, not the nagging.
Two different full messages
People confuse these constantly and they need different answers.
The phone itself is full. No room on the device. New photographs may not save and apps will not install.
Your online storage is full. iCloud or Google. The phone is fine, but the copy being kept for you has stopped. This is the more serious one, and it is the one people ignore longest.
Read the message properly to see which you have. It will say.
What is actually taking the room
Almost never what people expect. Before deleting anything, look at the list: on an iPhone, Settings, General, iPhone Storage. On Android, Settings, Storage. Both show you the biggest offenders in order.
Usually it is:
Videos. A minute of video is worth roughly two hundred photographs. Ten holiday videos can be the whole problem.
WhatsApp. Every picture and video anybody has ever sent you, including the ones from group chats you never look at. This is very often the single largest item on the phone, and people are astonished by it.
Podcasts and downloaded programmes. Anything downloaded to watch on a train and never deleted.
The safe order to clear things
Start with what cannot be lost, and stop as soon as there is room.
1. Downloaded television and podcasts. Nothing is lost. They can be downloaded again.
2. WhatsApp media. In WhatsApp, Settings, Storage and Data, manage storage. It shows the biggest items and the chats using most room. Clear videos from group chats first. Your actual conversations stay.
3. Apps you have not opened in a year. Delete them. If you want one back, it reinstalls in a minute.
4. Duplicate photographs. Everyone takes six of the same thing. Both phones now have a way of showing you duplicates.
5. Old videos, once you have copied anywhere you want to keep off the phone.
What not to do
Do not mass-delete photographs to solve an online storage message. If the two are linked, deleting on the phone deletes the backup too, and that is how people lose pictures for good.
Do not install a cleaner app. They range from useless to actively harmful and none does anything you cannot do in settings.
Do not empty the recently deleted folder in a hurry. It holds things for about thirty days, and that folder has rescued more people than any other feature on the phone.
When paying is the right answer
If it is the online storage that is full and the material is photographs of family, pay. It is usually somewhere between one and three pounds a month and it restarts the backup immediately.
People resist this on principle. I understand it. But an afternoon of deleting, repeated every six months, against a pound a month for the pictures to look after themselves, is not really a close call.
Bring the phone to the Friday clinic and we will look at what is actually using the space before anything is deleted. That order matters, because deleted is usually recoverable for about thirty days and not one day longer.