Does macOS warn me when disk space is low?

Only when it’s nearly gone, and it doesn’t say what filled it. An alert at a level you choose gives you time to act.

How does it forecast when the disk fills?

From a daily reading of free space. It only says a number when the recent days fall on a clear line downwards, and starts again after a cleanup frees space.

Why is my Mac storage filling up by itself?

Usually caches, iCloud copies, local snapshots or an app writing data in the background. What Grew This Week names the folders, and our guide explains the usual causes.

Is the alert on by default?

No. It stays off until you turn it on in Settings, and macOS asks once for permission to show notifications.

Peekaboo · Disk space monitor

Know your Mac is running out of space before it does

“Your disk is almost full” usually arrives in the middle of an export. Peekaboo tells you earlier, at a level you choose, and shows which folders have been eating the space.

Download free trial14 days free · macOS 14+ · Added in 0.1.0 · Updated November 7, 2026

What it does

An alert at your level: Pick 5, 10, 20, 50 or 100 GB. One notification when free space drops below it, not a stream of them, and another only after space has recovered and dropped again.
Roughly when it fills: When the trend is clear, the alert adds a sentence like “At this rate it fills up in about 12 days”, worked out from recent daily readings since the last cleanup.
What Grew This Week: Home shows the folders that grew most over the last seven days, with a month’s growth once there is one. Click a folder to open it in Disk Usage.
Readings stay on your Mac: The daily folder sizes behind it are kept on this Mac for 90 days and never sent anywhere.

How to use it

1Open Settings › Cleanup and turn on Notify me when space is low.⌘,
2Choose the level, and allow notifications when macOS asks.
3When it fires, check What Grew This Week on Home, then Clean Up.⇧⌘K

When another app fits better

If you want free space on screen all the time, a menu bar monitor such as [iStat Menus](/alternatives/istat-menus/) or [Stats](/alternatives/stats/) (free) shows it constantly; Peekaboo’s menu bar readings cover processor, memory and network. To warn you a drive is failing rather than filling, [DriveDx](/alternatives/drivedx/) reads its health data.

Questions

Does macOS warn me when disk space is low?: Only when it’s nearly gone, and it doesn’t say what filled it. An alert at a level you choose gives you time to act.
How does it forecast when the disk fills?: From a daily reading of free space. It only says a number when the recent days fall on a clear line downwards, and starts again after a cleanup frees space.
Why is my Mac storage filling up by itself?: Usually caches, iCloud copies, local snapshots or an app writing data in the background. What Grew This Week names the folders, and our guide explains the usual causes.
Is the alert on by default?: No. It stays off until you turn it on in Settings, and macOS asks once for permission to show notifications.