Activity Monitor is a very useful free utility provided by Apple as part of macOS.
It is handy to leave it running and change the dock icon to monitor your CPU history. Until you need more info, the app window can either be hidden (using the Hide command from the Dock), or simply closed.
In recent versions of macOS, the Memory tab of Activity Monitor shows a simple graph of "memory pressure" which lets you know whether your Mac needs more RAM. If the graph is green, the Mac has sufficient RAM. If yours does need more RAM, get at least as much as the figure for "Swap Used".
- Check if your Mac needs more RAM in Activity Monitor – Apple Support page
- Does my mac need more memory? – Macintosh How To. Includes images of older versions of macOS / Activity Monitor
- How To Use Activity Monitor: Master Class – blog by Roland Banks
This page was added by Neville Reid in December 2016, and expanded in January 2020.