Looking to increase my iMac's RAM
The official RAM specifications are given by Apple here:
https://support.apple.com/en-us/HT201191

Using the list, you need to


  Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:
  
  
  PC3-12800
  Unbuffered
  Nonparity
  204-pin
  1600MHz DDR3 SDRAM
  


Note that only 32 GB of RAM is officially supported.