A1418 / EMC 3069 / 2017 / 3.0 GHz quad-core i5, 3.4 GHz quad-core i5 or 3.6 GHz quad-core i7 Processor. Released June 8, 2017.

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Single RAM slot upgrade

How does the memory bus look like in the 21,5” iMac? It has got two slots. Can I upgrade those individually or do they have to be in pairs. My guestimate of course is, that they have to pair up, but what are the facts?

Reasoning behind the question: one 16Gb stick is same price as two 8Gb and would add up to 20Gb when one 4Gb stick would stay in the mac. Also at a later point I could upgrade it to 32Gb, by buying one additional 16Gb.

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Yes your machine will take one 8 and one 16 GB stick. However, after you look at the difficulty involved in installation (you have to pull the logic board), you may want to wait until you can afford to do the full upgrade 32GB at one time.

iMac Intel 21.5" Retina 4K Display (2017) RAM Replacement

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One stick would work fine. I would only do so if you plan adding another one later as dual channel memory allows faster and better performance as the loads are split, rather than one stick doing all the work.

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So that would mean that I would half the bandwidth with just one slot? But then if it is dual channel split across those two slots, then the configuration with one 16Gb plus one 4Gb wouldn't work, correct?



No, no. You could do 16 and 4.

Go ahead if you want, then later upgrade. Just seems funky. Just make sure it's for your mac :)

I kind of previously thought you wanted to do one stick.

The speeds would the same eitherway, not split. One stick or not. You kind of misunderstood.

It just eases the work load and can make it run better in some instances. And if one stick fails at least the other works.

Imagine one strong person carrying wood and loading but instead two people with less strength individually but add up to the same as that one person. One carrying wood and one loading.

So if you want clarification, you could go for it.


You lose interleaving with two different RAM sizes

@justbelikebrett The reason there are a lot of Mac question on this site is because it was built and is hosted by a company that started out as a Mac parts supplier and still is.


Using two different ram sizes can work and be fine as long as its the same speed, slot size and voltage.

You may be downsized to single channel memory speed like that but would work until he replaced it.

And nice to know mayer. I still think Mac users have more questions as its so limited, and annoying to do what you want on it. Specifically upgrades, replacements and OS fixes.

Apple loves it their way haha.


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