How to free up space on your Mac (and fix “startup disk almost full”)

First, see where the space went

Open System Settings, General, Storage for Apple’s overview. It groups things into broad categories, and a large share often lands in System Data, which isn’t one thing you can delete. A disk space analyzer shows the actual folders instead. Here’s how the main ones compare.

Safe ways to free up space

• Empty the Trash. Files in the Trash still use space until it’s emptied.
• Clean out Downloads. Installers (.dmg, .pkg) for apps you’ve already installed, old zips and duplicates pile up here for years.
• Delete old iPhone and iPad backups. In Finder, select your device, choose Manage Backups, and remove the ones you don’t need. Each can be many gigabytes.
• Remove apps you don’t use, along with the data they keep in your Library. Dragging an app to the Trash leaves that data behind.
• Clear caches of apps you no longer use. Caches of apps you still use come back as you use them. See how to clear cache on Mac safely.
• Move or delete big media. Old screen recordings, exported videos and photo libraries you’ve backed up elsewhere.
• Developers: build folders and tool data. Xcode’s DerivedData, old simulators, idle node_modules, Docker images and AI models can each take tens of gigabytes. Clear tool-managed data with the tool itself (for example docker system prune). See developer caches.
• Local Time Machine snapshots. macOS removes these by itself when space runs low; tmutil listlocalsnapshots / shows them.
• Optimize Storage. In Storage settings, Apple can keep files in iCloud and download them when needed.

What not to do

Don’t delete things inside /System, /private or /Library because a list online said they’re safe, and avoid “cleaners” that delete instantly instead of using the Trash. Clear specific, known things, and keep a way back.

How DiskApp helps

DiskApp scans your Home folder or the whole Mac, shows every folder by what it really holds, and flags caches, idle build folders and leftovers of deleted apps only when a rule can prove what they are. Scanning is free. Clearing moves things to the Trash, so Put Back works until you empty it.