Adding RAM to my new 27" 2021 iMac

In order to get the fastest speeds possible the new RAM (both 16 GB DIMMS) should be installed in slots 1 (1 is the very top slot) and 2 which is right below slot 1. The RAM that came pre-installed should be in slots 3 & 4. I did this on my 2020 (there is no such thing as a 2021 27" iMac" they were introduced in 2020) and can provide screen shots that show:

A) My method works.

B) this method shows that it yields the fastest speeds possible.

If you want the screen shot that show this please let me know.

Before you begin, only upgrade with Crucial or OWC RAM. If you are not, don’t upgrade.

Remove the original RAM, and install the new RAM, following these instructions: Install memory in your 27-inch iMac - Apple Support.

Cheers,

Jack

I always install new RAM in the two open slots, leaving the factory RAM. This has always worked without issue.

If you add 2X16GB to the factory 8GB, you'll have 40GB RAM.

iMac’s are picky when it comes to RAM upgrades.

Go to > https://eshop.macsales.com/shop/memory/imac-2019-27-inch and watch the Install Video.

I also only recommend purchasing your RAM upgrade from either OWC or Crucial.

As rkaufmann87 noted…

Your welcome. BTW, I added 2 16GB DIMMs from OWC (www.macsales.com). Please use only them or Crucial.com as your vendor for buying the upgraded RAM.

+1 to only using RAM from OWC or Crucial. and also, rkaufmann87 is giving the best advice on which slots the new modules position should be.