Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
I have 16GB of RAM installed on my iMac 27" 5K i7 2017 (2 x 8GB).
There are 4 slots in total and I would like to add another 16GB.
I don't know if it's best to buy 2 x 8GB or 1 x 16GB?
Your model, like most Macs, supports memory interleaving, which means that paired memory may work better than single modules.
However, Apple's own guidelines for installing memory in iMacs make no mention of this, and the real-world effect may be minimal, (as mentioned in comments)
After reading the posts that @bmike posted I made up my mind and I'll opt for one module of 16GB and I'll have one memory slot still available just in case.