How do you choose a RAM module for your Apple iMac?
SODIMM form factor
SODIMM is a compact form factor RAM memory. It's more compact than DIMM and also the only form factor that fits in a iMac. This is the first step you should consider when choosing RAM for your iMac.
Note the DDR standard
There are currently 2 RAM standards available: DDR3 and DDR4. This indicates the speed of the memory. iMacs from before 2017 have DDR3 RAM, iMacs from 2017 and later have DDR4. Pay close attention to this when selecting RAM for your Apple iMac.
Clock speed of the RAM
The clock speed also plays a role in the speed of the RAM. This is expressed in Megahertz (MHz) and must match the speed that your iMac supports. If you want to replace the memory of a iMac that, for example, supports 1600MHz, you must install RAM modules with a speed of 1600MHz. A higher or lower speed doesn't work, so make sure that the speeds are the same.
Check how much space you have for extra memory
Most iMacs have 2 memory slots. If one is in use, you can upgrade the memory with the same RAM module as the one that's already in your iMac. The RAM is often divided into 2 slots by default, so when upgrading your iMac, you have to replace both RAM modules.