How much memory do you need in a 5K Retina iMac for anything from writing to video production?

Apple fills up their iMacs with far too little memory to be useful, so the first thing you want to do when you buy a new machine is think about installing additional memory. Apple’s memory isn’t cheap and iMacs – apart from the iMac Pro – can easily be upgraded after having purchased one with the standard 8GB installed. The question is how much you need to buy, given that even third-party RAM like that of Crucial isn’t for free. It all depends on what you plan out to do with your new iMac.

If you’re planning to use your machine to do some heavy lifting, like creating movies, compositing, or 3D design, you’ve probably bought one of the 27inch 5K Retina models. Perhaps you’ve purchased the top of the range iMac, but even if you went for the cheapest of the 27inch models, you can fill it up to 64GB. Apple says on its product page the 3.4GHz model can only be upgraded to 32GB of memory, but you can install up to 64GB with no problem at all. I know, as I currently have 40GB installed and everything works just fine.

Except for a slower CPU and lower-end GPU, the 3.4GHz model performs just as bad with its standard 8GB as the high-end models – i.e. slow with lots of file swapping going on behind the scenes. More memory solves the swapping problem because more and bigger files can be kept in fast memory. But that still doesn’t tell you how much you really need to install to avoid those swaps.

New iMacs are easy to upgrade. The iMac has a door that sits flush with the rest of the aluminium back, slightly below the power socket. The power socket itself has a button that, if you press hard enough, opens the door, meaning it will pop out and come loose. Once you remove the door entirely, you have access to the four memory slots of which two have a 4GB SODIMM module each.

The latest iMacs need SODIMM memory modules and normally you would need to figure out which speed they run at, but you can also buy your memory from Crucial, as I did, and use their web-based configuration assistant. The assistant is accurate and leads you to the correct modules using a Q&A approach. In my case, the assistant stated that I needed to buy Crucial premium memory 16GB DDR4 – 2400 SODIMM 1.2V CL17. There were no specific modules for my 3.4GHz model, so I trusted the assistant and got two of these modules. The cost, shipping included, was a fraction of what Apple charges.

To figure out how much memory really makes a difference, I installed the modules one by one, running tests in-between. I also decided to leave the two 4GB SODIMM from Apple in place. Finally, what seems like logical to me but isn’t for many people, is that it’s best to always install two by two modules of the same size, so 2 x 4GB and 2 x 16GB is fine, but 2 x 4GB, 1 x 16GB and 1 x 8GB probably isn’t. Leaving one slot empty, however, shouldn’t cause problems.

With the single 16GB module in place, Activity Monitor showed me that swapping was no longer a problem for many tasks, except for when I started opening multiple files in some apps – Skylum’s Luminar 2018 and Aurora HDR 2019, for example – or when I rendered a 15-minute video clip in Final Cut Pro X. Such tasks clearly required some more memory.

After installing the second 16GB Crucial module, there was no longer any swapping going on. Even when I had seven files open in Luminar and a couple in Aurora HDR while a render job was going on in the background, Activity Monitor reported that all was being handled by internal memory. Judging from the memory that was actually being used, I’d say 32GB would have been fine too.


If you have the money, fill up your Mac with the maximum amount it will take. I had an iMac for seven years and it still performed well – its age considering – due to the maximum amount of memory I installed when I bought it.

If you are on a tight budget – and who isn’t these days – then install one 16GB module on top of Apple’s 8GB. That will pull you through most of the more common tasks you might have to do. Because it’s so easy to install, add another 16GB module as soon as you find the extra money. Having 40GB on board turbo-charges even the 3.4GHz iMac and stops the need for swapping for almost anything you throw at it, including the rendering of shorter movies (15-minute clips are quite feasible) with not too many special effects added to them.

Once you start doing more with this machine, it pays off to install even more, but then the law of diminishing returns probably also kicks in action as both CPU and GPU are then becoming performance bottlenecks.

As for the brand you buy, I’d recommend not to save on third-party memory too much. Buy from an established brand, even if it’s a little bit more expensive than “white label” products. Seven years ago, when I upgraded my iMac to its full 16GB, I purchased Crucial memory and before that, I filled my Power Mac Pro with their modules too. From the day their online assistant became available, I’ve relied on it and never had any trouble; not in terms of modules arriving DOA, with timing or otherwise, so it seems their quality control is spot on.