The total amount of memory currently installed in your PC will usually be located in the BIOS setup for your system. Alternatively, your computers operating system has an option which displays the total memory installed.
Windows 10
Use the Task Manager. Right-click your taskbar and select “Task Manager” or press Ctrl+Shift+Esc to open it. Click the “Performance” tab and select “Memory” in the left pane. If you don’t see any tabs, click “More Details” first.
The total amount of RAM you have installed is displayed here. The Task Manager will also tell you the type of RAM: Speed, form, and how many slots (see image below) You can use this information to match new ram or work out if your computer is upgradeable.
Windows Vista and Windows 7
Right mouse-click on the "Computer" menu item in the Windows Start Menu and select "Properties". The total memory will be displayed next to the "Installed Memory (RAM)" details.
Windows XP and earlier
Right mouse-click on the "My Computer" icon and select "Properties." The total memory is displayed under the "General" tab.
Apple Macintosh computers
Click "About This Mac" or "About This Computer" from the Apple menu or Apple symbol in the top left corner of your Desktop. The total memory amount will be displayed in the app window.