The iMac (Mid 2011) computers have four SDRAM slots in the bottom of the computer and come with at least 4 GB DDR3 SDRAM installed as two 2GB DIMMs into two slots.
The maximum amount of RAM you can install in your computer is 16 GB (a 4 GB SO-DIMM in each slot).
You can use any Small-Outline dual inline memory modules (SO-DIMMS) that meet all of these criteria:
PC3-10600
Unbuffered
Nonparity
204-pin
1333 MHz
DDR3 SDRAM
You can use RAM module sizes of 2 GB or 4 GB in each slot.
DIMMs with any of the following features do not work in the iMac (Mid 2011):
Registers or buffers
PLLs
ECC
Parity
EDO RAM
Always follow the upgrade instructions for memory installation as written. Never attempt to remove any screws that are not listed in the upgrade instructions.