MacOS Catalina and maximum installed physical RAM

Question

Level 3

809 points

MacOS Catalina and maximum installed physical RAM

My 27” 5k (Late 2015) iMac came installed with 16Gb RAM installed in two slots. I have two more memory slots available for use. Apple tells me that, right now with Mojave installed, I may install a maximum of 32 GB RAM if I want to.

If Catalina is to be 64bit only, will this mean I could install a lot more RAM than 32Gb if I could afford it?

I understand that theoretically 64bit computers can access about 16 exabytes of RAM.

Or is my iMac prevented from addressing more RAM in some other way?

iMac Line (2012 and Later)

Posted on Sep 28, 2019 6:11 AM

Reply

Answer 1

Top-ranking reply

VikingOSX

Level 10

126,346 points

Sep 28, 2019 6:20 AM in response to El Deanio

Officially, the late-2015 iMac supports 4x8GB RAM modules, but it will take 4x16GB RAM modules from third-party suppliers such as OWC — with Mojave, or Catalina. I picked a Late-2015 27-inch model at random, so you would need to revise that link for your particular processor configuration.

Reply

Answer 2

macjack

Level 10

111,292 points

Sep 28, 2019 6:21 AM in response to El Deanio

If this is your Mac, you should be able to install 64GB RAM.

https://everymac.com/systems/apple/imac/specs/imac-core-i5-3.3-27-inch-aluminum-retina-5k-late-2015-specs.html

Catalina doesn't factor into it, Macs have been 64-bit for a long time. However, Macs are fussy about RAM. Both Crucial and OWC are reliable vendors with high quality RAM.

Reply

Answer 3

Niel

Community+ 2026

Level 10

746,121 points

Sep 28, 2019 6:16 AM in response to El Deanio

The 16EB figure is a theoretical limit; as of now, the largest modules available for your Mac are 16GB, for a total of 64GB.

(174783)

Reply

Answer 4

macjack

Level 10

111,292 points

Sep 28, 2019 7:31 AM in response to El Deanio

This one?

https://everymac.com/systems/apple/imac/specs/imac-core-i7-4.0-27-inch-aluminum-retina-5k-late-2015-specs.html

Reply

Answer 5

stedman1

Level 10

266,014 points

Sep 28, 2019 6:16 AM in response to El Deanio

32 gB is the maximum for the computer. The operating system doesn’t determine RAM capability.

Reply

Answer 6

dialabrain

Level 10

137,413 points

Sep 28, 2019 6:18 AM in response to El Deanio

Niel is correct. You can install up to 64GB.

Reply

Answer 7

El Deanio Author

Level 3

809 points

Sep 28, 2019 7:25 AM in response to El Deanio

Thanks all for your help.

I will look into installing 64gb sometime. My iMac is similar to the one described in the link except it has a 4Ghz i7 processor. The RAM is 1867 MHz DDR3 as described however.

Anyone know where the iMac ‘Model ID’ is now? Looking in 'About.../System Report/Hardware’, it no longer seems to be there. I’m running MacOS 10.14.6. I note that the serial number ends with 17. I wonder if this is a clue to the model ID?

Not sure about Catalina. I may stick with Mojave. For various reasons the majority of my favourite apps appear to be unlikely to be updated to 64bit.

Thanks again

Reply