Mac startup disk full
The space usually goes to a few large things you forgot about. Find those before deleting anything small.
What's usually going on
Old iPhone backups, downloads, video files, the Photos library, and mail attachments are the usual culprits. A nearly full Mac runs slowly and can fail to install updates, so it's worth sorting out rather than living with.
Worth trying first
See what's using the space.
Open the Apple menu, System Settings, General, Storage. Wait for the bars to fill in. The categories and recommendations show where the space has gone.
Clear Downloads and the Trash.
Sort the Downloads folder by size and delete installers and files you've finished with. Then empty the Trash; nothing is freed until you do.
Remove old iPhone backups.
In Storage, look for iOS Files. Backups of phones you no longer own can take tens of gigabytes. Keep the most recent backup of your current phone.
Let iCloud help, carefully.
Optimize Mac Storage keeps full photos and documents in iCloud and smaller copies on the Mac. It only works well if your iCloud plan has room.
Move big archives to an external drive.
Old video projects and photo archives can live on an external drive, as long as they are also backed up somewhere else.
When to hand it over
If the space keeps disappearing, the Photos library is the problem, or you're not sure what's safe to delete. We'll clear it remotely and make sure nothing irreplaceable goes with it.
Your own technologist, on call.
Tell us a little about your setup. We'll take it from there.