Installing RAM in a 27-inch late-2013 iMac

[NDS]

I would like to increase the RAM in my iMac (it currently holds the standard 2 x 4GB cards with 2 empty slots). I have found some 16 GB cards for a good price and would like to increase the RAM to support my graphics programs. I know that it can only support 32GB (as per apple website).

1. Do you have to use all 4 slots if you increase the RAM?

2. Will it still work if I remove the 2 x 4GB cards and replace with 2 x 16GB cards?

3. Can I add just one 16GB card and leave one slot empty?

Thank you

 

[Mikael H]

I would like to increase the RAM in my iMac (it currently holds the standard 2 x 4GB cards with 2 empty slots). I have found some 16 GB cards for a good price and would like to increase the RAM to support my graphics programs. I know that it can only support 32GB (as per apple website).

1. Do you have to use all 4 slots if you increase the RAM?

2. Will it still work if I remove the 2 x 4GB cards and replace with 2 x 16GB cards?

3. Can I add just one 16GB card and leave one slot empty?

Thank you

EDIT:
While typing out a reply, I forgot that the question was about an older model iMac. Disregard what I wrote about memory sizes; @keysofanxiety may very well be correct in that 16 GB sticks may be unsupported on this particular model Mac.

Original reply:
You can use all four slots, but you don't have to.

You definitely should use pairs of memory sticks even though using only one stick might work.

The computer should work a bit more efficiently if you use identical pairs of memory sticks across the banks than if you use an odd pairing like 2x4 +2x16 GB in the slots. Note that if your use case is extremely memory-intensive, you may still be better off with 40 GB than with 32 GB memory, if the alternative is that your computer has to swap stuff that doesn't fit in RAM to disk. The easy way to check this: Populate the slots with 2x16 GB sticks and do what you usually do while keeping an Activity Monitor window in a corner of your screen with the Memory view visible. If your memory pressure stays in the green, you're good to go. If, in a year or two, your memory pressure starts going red, get yourself another 2x16 sticks with the same specs.

 

[keysofanxiety]

IIRC the controller on this model will not support/read 16GB SODIMMs, much like the 2012 cMBPs. So basically 2x16GB won’t work at all and it’ll need to be 4x8GB instead.