Your system has run out of application memory

What the alert says

The alert is a window titled Force Quit Applications, headed Your system has run out of application memory, with the line To avoid problems with your computer, quit any applications you are not using. It lists your open apps, may mark some of them (paused), and has two buttons: Force Quit and Resume. Apple's help pages do not describe this alert by name, so what follows is Apple's general advice on memory, applied to it.

First: quit what you are not using

1. Save your work in the apps you need.
2. Quit the apps you are not using: switch to each one and press Command-Q, or choose Quit from its app menu.
3. Only if an app will not quit, select it in the alert, or in the Force Quit window (Option-Command-Esc, or Apple menu → Force Quit), and click Force Quit.

Apple's advice is to try a normal quit before forcing one, because an app that quits normally can do what it needs to first, such as saving changes.

Then: see what is using the memory

1. Open Activity Monitor (it is in Applications → Utilities, or search for it with Spotlight).
2. Click Memory.
3. Look at the Memory Pressure graph at the bottom of the window, and at the apps at the top of the list.

Apple's reading of the graph:

• Green: your computer is using all of its RAM efficiently.
• Yellow: your computer might eventually need more RAM.
• Red: your computer needs more RAM.

Beside it, Compressed is memory macOS has compressed to make more RAM available, and Swap Used is space on your startup disk being used to swap unused data to and from RAM. A low free-memory figure on its own is not the problem: Cached Files is file data macOS keeps in otherwise unused memory to improve performance.

If it keeps coming back

• Free up space on the startup disk. Swap lives there, and Apple lists a startup disk without enough free space as a reason a Mac runs slowly. See Mac storage full.
• Check the app you were using. Apple's advice is that an app may need more memory than your Mac has easily available; check its documentation for its system requirements.
• Watch the pressure graph while you work. If it sits in red with only the apps you need open, Apple's verdict is that the Mac needs more RAM.
• Restart. A restart closes everything and gives the swap files back to the disk.

What does not help

A memory purge or "RAM cleaner" empties the file cache so the free-memory figure jumps, and macOS reads that cache straight back. It does not touch the memory apps are actually using, which is what this alert is about. See does purging RAM help.

Where the app comes in

RefreshYourMac's memory reading, at the top of its window in both editions, shows macOS's own memory pressure: normal pressure, under pressure or out of memory, with how fast memory is moving to and from swap. In the download edition, clicking it lists the apps holding the most memory, with an app's helper processes counted together, and marks the ones that look not in use. Each has Show and Quit; Quit asks the app rather than forcing it, so unsaved work gets its own dialog, and nothing is quit for you. The Mac App Store edition shows the reading but not the list, because the sandbox does not allow it. Both say when a restart would give a gigabyte or more of disk back. More on CPU and memory, and for a slow Mac in general, why a Mac runs slowly.