Open Activity Monitor on almost any Mac and memory looks nearly full. That’s normal, and it’s the reason “free memory” is a poor guide to whether your Mac is struggling. The figure to watch is memory pressure.
Why macOS keeps memory full
Memory that holds nothing does no work. So macOS fills spare memory with recently used files and data it may need again (the cached files figure). When an app asks for memory, macOS drops some of that cache instantly. Full memory with plenty of cache is a Mac working as designed.
What happens when memory really runs short
macOS makes room in stages, each more expensive than the last:
- Drops cache. Free and immediate.
- Compresses memory. Data apps aren’t using right now is squeezed in place. It costs a little CPU and is usually unnoticeable.
- Swaps to disk. Data is written to the startup disk and read back when needed. This is what you feel as beachballs and slow app switching.
Memory pressure measures how far down that list your Mac has gone. It’s the figure macOS itself uses when it decides it must terminate apps.
How to read it
In Activity Monitor, open the Memory tab. The Memory Pressure graph at the bottom is coloured:
- Green: enough memory, whatever the other figures say.
- Yellow: macOS is compressing heavily and may be swapping. You may notice short pauses.
- Red: macOS is swapping hard. This is when a Mac feels slow.
Two more figures on the same tab help: Swap Used (how much has been written to disk) and Compressed. Swap used on its own isn’t a problem; swap that keeps climbing while you work is.
How to bring pressure down
- Sort the Memory tab by memory, and quit or close what’s at the top. Browser tabs add up quickly.
- Look for an app that keeps growing over hours: that’s a leak, and restarting the app fixes it.
- Keep free disk space for swap. A nearly full disk turns pressure into stalls; see the other causes of a slow Mac.
What not to do
Don’t use a memory cleaner to push “free memory” up. It forces macOS to discard its cache, which your apps then read back from disk. Pressure barely moves, and the Mac is slower for a while. More on why RAM cleaners don’t work.