I have a Dell Optiplex 7040 with 4 slots (2x dual channel).

I have:

  • 2x 4GB 2133 1Rx8
  • 2x 16GB 2400 2Rx8

May I insert them without any problem and any performance loss ?

Of course I will insert one pair per channel. At the end I'll get 2x4 + 2x16 = 48GB, any problem ? The computer is used as a server with Centos 7.

2 Answers 2


In general, you should be able to mix memory speeds. The memory controller should run the RAM at the lowest common speed. However, this does not always work. Some memory controllers just wont allow it. There is absolutely no harm in trying, either it will work, or it wont. If it doesnt, BIOS will report a memory error and halt the system.

  • That means my 16GB memory parts will work at 2133MHz instead of 2400Mhz. Then performance loss ?
    – None
    Jan 15, 2018 at 4:20
  • @Alexis_FR_JP to be honest, you will most likely not be able to see the difference between 2133mhz memory and 2400mhz memory. The difference is speed is negligible.
    – Keltari
    Jan 15, 2018 at 4:24
  • @Alexis_FR_JP at maximum memory speed efficiency, which is not really attainable, the faster memory will run less than 1% slower, 0.88875% slower to be exact.
    – Keltari
    Jan 15, 2018 at 4:36
  • Great, will try to install my 48GB then.
    – None
    Jan 15, 2018 at 4:50

there will be a slight performance loss because your 2400 MHz DDR will be used at 2133MHz (x88.9%). But this difference will largely be masked by cache memory (it will only appear in tranfers between cache and RAM), and it probably much smaller than the gain you will get from having much more RAM and won't need swapping when your programs use much memory.

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