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I have 16GB of RAM installed on my iMac 27" 5K i7 2017 (2 x 8GB).

There are 4 slots in total and I would like to add another 16GB.

I don't know if it's best to buy 2 x 8GB or 1 x 16GB?

  • Check out Mactracker... – Solar Mike May 11 at 15:25
  • Thx for your comment, I know which module(s) I'd have to buy, I just want to make the best choice regarding performances. :-( – Dakta Moriamé May 11 at 15:32
  • Define best, please. for me, leaving a slot free and fewer modules is best. – bmike May 11 at 16:10
  • Best as @benwiggy 's answer states, paired memory works better than single module. – Dakta Moriamé May 11 at 16:34
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Your model, like most Macs, supports memory interleaving, which means that paired memory may work better than single modules.

However, Apple's own guidelines for installing memory in iMacs make no mention of this, and the real-world effect may be minimal, (as mentioned in comments)

  • It’s hard to benchmark interleaving differences let alone run a Mac where that benchmark makes a real world difference. More RAM is orders of magnitude better value than faster memory since 2013 or so in Mac hardware and even RAM isn’t as important as proper software design / ssd. – bmike May 11 at 16:17
  • hmmm... this doesn't make it easier... – Dakta Moriamé May 11 at 16:36
  • Let me find some related posts. apple.stackexchange.com/questions/285828/… and apple.stackexchange.com/questions/206439/… might be useful. I can’t locate the one I recall with actual benchmarks showing pairs being slower or a wash. – bmike May 11 at 17:24
  • @bmike Thanks for the links! I will upgrade with 1 module of 16GB and I'll still have a free slot just in case... – Dakta Moriamé May 11 at 17:50
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After reading the posts that @bmike posted I made up my mind and I'll opt for one module of 16GB and I'll have one memory slot still available just in case.

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