iMac 27" 2015 memory upgrade option

[Tapash]

Hi Guys
I am still running stock 8GB 1866Mhz memory and considering upgrading the memory.
Just wondering if I can upgrade my ram to a higher speed 2666MHz ram? Is that compatible?
If not, what is the highest speed ram I can put in?
Thanks

 

[TwoH]

1866 is the fastest it can run - 2133 and upwards is DDR4 and will not fit onto your 2015 logicboard due to the pin difference.

You should also worry more on the capacity than the frequency.

 

[Tapash]

Thank very much for clarifying.