27" iMac Upgrading Memory

[matteise]

Five plus years later, and outside of some display retention issues around the edge of the screen after long use, I still am a huge fan of my Retina 5K, 27-inch, Late 2015 iMac.

With that being said, I have found myself looking at memory upgrades. Needed? Maybe not, but also of some "why not" thought to it as well.

As I consider this though, I also have to admit that while I consider myself tech/software savy, hardware savy I am not!

My questions are as follows:

• If I were to purchase an additional 8 GB (2x4GB), can I simply add it to what I have already or do issues arise in mixing old / new memory?
• Assuming there are no immediate issues of mixing, the 4x4GB setup vs. a 2x8GB setup at ~1.75x the cost vs. a 4x8GB setup at ~ 3.5x the cost...?

I had been looking at OWC memory.

Thanks for any input with this!

 

[usagora]

I'm sorry this isn't a direct answer to your post, but I just went to OWC and I taken aback by how much more the memory cost for your iMac than mine (2020 5K). Anyone know what's up with that? Why does the 16GBx4 option for the 2015 iMac cost THREE TIMES as much as the same option for the 2019+ iMacs? Heck, you can 128GB for the 2019+ for about 64% of the cost of the 64GB on the 2015 😮

 

[usagora]

Ok, so more at your actual questions, I found this on Apple's website:

Install memory in an iMac - Apple Support

Get memory specifications and learn how to install memory in iMac computers.

support.apple.com

So mixing memory shouldn't be an issue, but Apple recommends minimizing the capacity difference between the two channels. In other words, they'd say avoid having 2x4GB in channel A and 2x16GB in channel B, I assume. OWC also says, "All installed memory should be the same speed for best performance." So if you DO plan to use your existing RAM along with two new DIMMs, check the MHz or your existing RAM and be sure to buy the same speed on OWC (either 1867 or 1600).

Not sure what your budget is, but if I were you, I'd probably be tempted to get the $164.99 32GB option (8GBx4) and just not use the old memory:

 

[wilberforce]

Slots 1 and 3 should have a matched pair, and slots 2 and 4 should have a matched pair, but the first pair does not need to match the second pair. For the late 2015 iMac, all sticks should be 1867 MT/s DDR3 SO-DIMMs.

I suggest just add something like this to the two open slots, for a total of 24GB:

Timetec 16GB KIT(2x8GB) Compatible for Apple Late 2015 iMac (27-inch w/Retina 5K Display) DDR3L 1867MHz / 1866MHz PC3L-14900 2Rx8 CL13 1.35V 204 Pin SODIMM Memory Module MAC RAM Upgrade for iMac 17,1

Timetec - Memory of a lifetimeCompatible with (But not Limited to):*Please click image for more compatible systems modeliMac 17,1 w/Retina 5K display (27-inch Late 2015)3.2 GHz Core i5 (MK462LL/A)3.3 GHz Core i5 (MK482LL/A)4.0 GHz Core i7 (BTO/CTO)Need to know if this part is compatible?Contact u...

www.amazon.com

This is what I did (on a 2014 iMac, which is the same except 1600 instead of 1867 MT/s), worked great.

 

[wilberforce]

I'm sorry this isn't a direct answer to your post, but I just went to OWC and I taken aback by how much more the memory cost for your iMac than mine (2020 5K). Anyone know what's up with that? Why does the 16GBx4 option for the 2015 iMac cost THREE TIMES as much as the same option for the 2019+ iMacs? Heck, you can 128GB for the 2019+ for about 64% of the cost of the 64GB on the 2015 😮

Because 16GB DDR3 SODIMMs are very unusual, very few manufacturers (possibly only OWC?) make 16GB DDR3 SODIMMs, so no/little competition.
Apple only officially supports 8GB memory sticks in the late 2015 iMac, although it appears 16GB sticks do in fact work. I would not recommend using 16GB sticks for the 2015 iMac, unless one has a special need for more than 32GB memory.

 

[ZebedeeG]

Been running my Late 2015 27" iMac with 24GB mixed Ram for a while now (2 x 4GB + 2 x 8GB), and it works perfectly...

I just stuck my new RAM into the empty slots alongside the factory RAM.

 

[mlody]

I would recommend going with 2x8 GB for $86.88 while keeping the stock memory in. I did the same few months ago and I am very happy. I can bet that 24 GB in total will be plentiful given that you were fine with the base for that many years