2020 iMac 27" RAM Upgrade Question. Are all "2666Mhz" RAMs automatically "PC4-21333"

You can refer to the chart below, in terms of terminology.

DDR4 2666 has the Bus bandwidth of 21333 (mb/s).

The dices are in your hand if you plan to purchase memory modules of your choice.

And NO, all memory modules are not created equally --> not all compatible with Macs.

Theoretically all SO-DIMM PC4-21333 2666MHz 260-pin unbuffered non-parity memory is compatible, but it doesn't quite work that way in practice.

Still, if you stay away from the budget tiers/brands you will probably be OK... its just that Crucial and OWC have long consistent brand histories and are considered the lowest risk.

Please on purchase RAM from crucial.com or Mac sales.com, iMacs are very sensitive to using quality RAM and both of these vendors supply very high quality products at fair prices. They both offer lifer time warranties and have great service, in short there is no reason to not use them.

OtherWorldComputing specializes in Mac compatible memory and ships worldwide. You might think that RAM of the exact same specs would be compatible, but sometimes all RAM is not created equal. Go with a trusted Mac specialist that carries a lifetime warranty and guarantees Mac compatibility.

OWC is a US company, you will probably do better with Crucial if you are outside the USA. However OWC does have international distributors, please use OWC International Resellers to locate one near you.

Please order from Crucial.com NOT ALL RAM is for Macs, chances are high that what you saw in an online shop is for a PC!!!!!!!!!!!!!!!!!!! You have been advised.

OWC ships world wide. They do thorough testing of Mac models to ensure compatibility, guaranteed.