Updated
Quick fix
Open your storage settings to see what’s using space. The quickest wins are usually old videos, message attachments and apps you don’t use. Back up photos to the cloud before deleting them from the phone.
Step by step
See what’s using space
On iPhone: Settings > General > iPhone Storage. On Android: Settings > Storage (on some phones it’s under Battery & device care or Device care).
Delete or offload apps you don’t use
iPhone can Offload Unused Apps, which removes the app but keeps its data. On Android, uninstall apps you no longer use.
Clean up videos first
Videos take far more space than photos. Delete ones you don’t need, or back them up and remove them from the phone.
Back up photos before deleting
Use iCloud Photos, Google Photos or another backup so your pictures are safe in the cloud. Make sure the backup finished before freeing space on the phone.
Clear message attachments
Old photos and videos in text and chat apps add up. Many messaging apps let you review and delete large attachments.
Empty “Recently Deleted”
Deleted photos usually stay in a Recently Deleted or Trash album for a while. Empty it to get the space back now.
Restart
Restart the phone so storage numbers refresh.