A full phone won't install updates, won't back up and runs slowly. Freeing space takes a few minutes and these steps are ordered by how much you get back for the effort.
See what's using the space
On iPhone, open Settings, tap General, then iPhone Storage. On Android, open Settings, then Storage. Both show a breakdown by category with the biggest offenders first. Start there rather than guessing.
Photos and video, almost always the biggest
- Turn on cloud photo storage so full resolution copies live in the cloud and lighter versions stay on the phone.
- Empty your recently deleted album. Deleted photos sit there for up to 30 days and still take up space.
- Delete long videos you've already saved elsewhere. A few minutes of 4K video is larger than hundreds of photos.
Apps you don't use
Your storage screen lists apps by size and shows when you last opened each one. Delete anything you haven't touched in months. iPhones can also offload unused apps automatically, which removes the app but keeps its data for if you reinstall.
Downloads and cached files
Streaming apps store downloaded music, podcasts and shows. Open each one and clear its downloads. Your browser's downloads folder is often forgotten too.
Messages
Years of photos and videos sent in messages add up quickly. Both iPhone and Android can delete message attachments over a certain age, or drop old conversations entirely.
If it fills up again straight away
Something is downloading automatically. Check your photo settings, your podcast and music apps, and any app set to sync large files.
If you still can't free enough
Backing up to a computer and then removing photos from the phone gives you the most space back in one go. See the article on backing up your phone.