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I recently came into possession of a 27 inch mid 2011 iMac. It only has 4GB RAM and I have been pricing compatible memory modules online.

I then remembered I also have a 13 inch mid 2012 MacBook Pro that was damaged some time ago. It has 8GB RAM that I have no reason to suspect has any faults as it worked fine before the damage which was not anywhere near the bottom case. So I thought I would investigate the types of memory modules the iMac and MacBook Pro use.

  • I found the iMac uses 204 pin PC3-10600 (1333 MHz) DDR3 SO-DIMM modules.

  • I found the MBP uses 204 pin PC3-12800 (1600 MHz) DDR3 SO-DIMM modules.

Obviously I'm thinking that if I can use the MacBook Pro RAM in the iMac I can save some money and it's a good way of recycling.

I understand this RAM would not operate at 1600 MHz in the iMac. My questions are:

  1. Would the MBP RAM work okay otherwise?
  2. If it would work, does it make a difference if I add them to the original RAM so I have 12GB in total, or will I have to remove the old modules and just use the 8GB instead?
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While I am not a fan of of mixing and matching memory, Crucial, a manufacturer of quality memory components, has this on their website:

Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory. However, your system will operate at the speed of the slowest memory module.

So, technically speaking, the answer to both your questions is, yes it will work and no it won't make a difference as you can mix and match the different speeds.

They also go on to say...

Keep in mind, that the right memory for your computer is the kind of memory it was designed to take. Check your system manual or look up your system in the Crucial Memory Advisor or System Scanner tools to find the memory guaranteed to be 100 percent compatible or your money back!

Well, the "system scanner tool" is for PC's but the rest of the statement is absolutely true. The last part about "money back" is why I only buy from reputable manufacturers like Crucial (PNY and Kingston are excellent choices as well).

That said, according to Everymac.com, the maximum supported memory configuration is 32GB1. Personally, I would (at minimum) sell the existing memory that you have (4GB modules are going for around $20USD on eBay) and instead buy a 16GB kit so you can max out your memory in your iMac either today or at a later date.

My personal iMac runs with 32GB of RAM which allows me to virtualize a Windows 10 machine, and two FreeBSD servers (one for development/testing and the other for a Plex media server). I am a firm believer in the old axiom "it't better to have and not need than to need and not have."

1 Everymac.com: Officially, this model supports four 4 GB modules -- one in each slot for a maximum of 16 GB of memory. However, third-parties have discovered that it actually will support 32 GB of memory using four 8 GB modules.

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The MBP RAM should work just fine on the iMac. The instruction on how to install the memory in the iMac is here. To mix them, make sure you put the pair that came with the iMac in a bank, and the one that came with the MBP into the other (don't mix them basically). If you never touched the memory on the iMac before just put the one on the MBP in the empty slot.

Also for reference your iMac supports up to 16GB.

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  • No, the iMac definitely has 4 slots. So, do you know if can I mix them to get 12GB, with all running at the lower 1333MHz?
    – user205310
    May 13, 2017 at 7:27
  • @Little.Eden OK, never mind... used that longtime ago, didn't remember that well. It should work together.
    – Tom Shen
    May 13, 2017 at 8:35

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