See how much storage is left

System Settings → General → Storage shows free space and what is using the rest. One Terminal line prints it too: df -h / shows the size of the disk and how much is available.

Without Terminal

Open System Settings → General → Storage.
The bar at the top shows what is used and what is free.

In one command

Free space on the main disk:

df -h /

Or just ask Titan

How much space is left, and what is using the most? Do it with Titan

Titan is an AI that lives on your computer: tell it what you need and it does it while you watch — you press Return, nothing runs by itself. What is Titan?

Titan isn’t on this computer yet. Get Titan, then press the button again — the task will be waiting, typed in.

Related

Find what is taking up disk space Clean up old files in Downloads