[Graham Lee77]

I've got a Late 2015 27" Retina iMac with 16gb ram in currently in 4 x 4gb modules (originally bought as 8gb and bought two extra modules separately due to Apples extortionate prices).

Slowly but surely, finding 16gb isn't enough to run smoothly when running a VM Windows 10 environment and lots of other apps at the same time.

Looking to boost my ram up a bit more, but the question is can I change just two of the modules to 8gb each to get up to 24gb or do all 4 need to be the same size making it 32gb.

Thanks

[anon(9602380)]

I've got a Late 2015 27" Retina iMac with 16gb ram in currently in 4 x 4gb modules (originally bought as 8gb and bought two extra modules separately due to Apples extortionate prices).

Slowly but surely, finding 16gb isn't enough to run smoothly when running a VM Windows 10 environment and lots of other apps at the same time.

Looking to boost my ram up a bit more, but the question is can I change just two of the modules to 8gb each to get up to 24gb or do all 4 need to be the same size making it 32gb.

Thanks

You can change just two of them. I have an iMac currently running 24GB. Never any issues. Can't remember how I have them configured in the slots though. Thanks.

[Blackcash]

I also have a late 2015 27" Retina iMac. I upgraded the memory to 24GB.
The configuration is 2 4GB and 2 8GB. It's working fine like this.
Some day I'll switch out the 2 4GB to 8GB giving me the max memory of 32GB.

[Lee_Bo]

Should work fine with no problems. About the only difference you would see is if you ran a benchmark.

[bakron1]

I have two iMac 27 5K units and both have 24GB with 2 4 gig and 2 8 gig modules and no issues.