Apple iMac (21.5-inch and 27-inch, Mid 2011) - Manual (Page 38)

38 Chapter 3 Boost Your Memory
Your iMac comes with at least 4 gigabytes (GB) of Double Data Rate 3 (DDR3) Dynamic
Random Access memory (DRAM) installed as two 2 GB memory modules. You can
replace or install additional 2 GB or 4 GB memory modules for up to a maximum of
16 GB. Memory modules must meet the following specifications:
 Small Outline Dual Inline Memory Module (SO-DIMM) format
 1333 MHz, PC3-10600, DDR3 compliant (also referred to as DDR3 1333)
 Unbuffered and unregistered
WARNING: Apple recommends that you have an Apple-certified technician install
memory. See “AppleCare Service and Support” on page 63 for information about how
to contact Apple for service. If you attempt to install memory and damage your
equipment, that damage isn’t covered by the limited warranty on your iMac.