Question What RAM speed should I get?

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Craig C2

Mar 17, 2019
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For i9-9900K (not OC), integrated graphics,
Asus Maximus XI Hero or Asus WS Z390 Pro,
unknown M.2 or SATA3 SSD, 256 GB,
4x4 GB or 4x8 GB RAM.

Intel says use 2666 MHz, but Asus says the boards go up to 4400 MHz. That is the part I don't understand.

If I'm not OCing, not a gamer, and want my parts to last as long as possible, which speed should I choose?
 
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Up to 2666 is the default speed supported by the motherboard.
Higher speeds are set using a XMP setting.
i9-9900K uses dual channel ram so it is best to buy a 2 x 8gb or 2 x 16gb ram kit depending on your needs.
Faster ram is better, but it also costs more.
Some apps do better with fast ram, others are not much impacted.
I might guess that something in the 3200 to 3600 speed range would be ok.
At equal speed lower latency(cas) is better.
 
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There are some processors and motherboards that will run quad channel mode.
That will require a 4 stick kit of matched ram.

9900K runs only dual channel mode regardless if the motherboard is populated with 2 or 4 matched sticks.

Since it costs more to match 4 sticks vs.2, a 2 stick kit will usually be cheaper.
 
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Single sticks run the same as dual sticks for the most part. The difference comes with usage and bandwidth. Dual channels doubles the available bandwidth, so for most apps you'll be fine with just one, but not knowing exactly which apps or games make better use, or even demand dual channel because of extensive bandwidth use, using just one stick can easily be a handicap.

Mixing and matching ram is never a good idea. Even if the ram is otherwise identical, on the inside it isn't. Different batches of silicon have slightly different properties, so you can even get 2 sets of ram, from the same store, off the same shelf, 1 right behind the other in the same container and still have them not work. Ram upgrade should be a non-thing, if you want more ram, buy the entire amount you want in a single package, it's factory tested to work, you adding ram makes you the tester and you can go through a hundred sticks and still not come out right.

That means a 2x8Gb kit to maximize potential usage and compatability, not a single 1x16Gb or 4x4Gb kit.
 
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Karadjgne said:
That means a 2x8Gb kit to maximize potential usage and compatability, not a single 1x16Gb or 4x4Gb kit.
Click to expand...
OK, I get that 2x8GB is better than 1x16GB, to take advantage of dual-channel mode.
And quad-channel isn't available on i9-9900k, but why would 2x8GB be better than 4x4GB? Just the price of matching four modules?
 
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Craig C2 said:
And I hear the boards nowadays are OK with just one stick, is that true? I could save the other for a futre upgrade.
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Yes, they will run, and, really not badly unless you are using integrated graphics.
But it is false to assume that you could add a second stick in the future.
Ram must be matched.
If you think you might need more ram in the future plan on buying a 2 stick kit and sell the old.
 
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1 stick puts little strain on the memory controller, 2 sticks in dual channel does strain the mc a little more but still well within acceptable tolerance, but by using 4 sticks you strain the memory controller the most as now it has to synchronize 2 sets of dual channel ram. This can necessitate slight voltage bumps not ordinarily needed, especially with a cpu at stock.
Also if 1 stick goes bad, chances of a replacement working with 1 original stick are far greater than with being compliant with 3 sticks. Still equals the same thing as adding new ram and mixing kits, but does offer better chances.
 
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Because you'll need more than 16GB at some point in the lifetime of this machine, so going with 2x8GB leaves 2 RAM slots open for an upgrade later without having to "junk" 2 sticks. You'll want to buy in pairs though, to take advantage of dual channel.

2x8GB DDR4-3200 costs about $80-85 on sale these days.
 
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Skylake memory controller runs at 2133MHz, so that's what Intel tested ram at and certified for its usage. Kabylake is 2400MHz. CoffeeLake is 2666MHz. Now you can definitely use ram that's slower than 2666MHz, the memory controller will run slightly cooler by 1°C or so on the cpu, but why would you? Intel certified 2666MHz for a nice 1:1 ratio of ram to memory controller for optimum performance / thermals. Anything faster than that is upto the motherboard vendor and considered OC, because there's a slim chance you'll either need to bump the ram voltage above stock value to get faster ram to work right, or OC the cpu to give the memory controller more resources to work with. Or maybe both.

On typical B, H boards there's a locked bios, no OC possible. So ram is limited to non-OC values of whatever the cpu is, 2133/2400/2666. Only a Z board allows for OC, so ram can be whatever the vendor has assigned as recognizable, which currently is upto 4000MHz+. But Intel makes no guarantees on stability or compatability, only on its recommended and certified speeds.
 
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Craig C2 said:
So, why does Intel limit its reccomendation to 2666?
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That is not a limitation.
It is the default speed at which ram is supported for booting into the bios.
The max ram speed is determined by the ability of the particular motherboard to implement higher ram speed.
That is done by overclocking ram, usually by invoking the faster ram speeds as defined by a XMP profile which is imbedded in the ram stick.
 
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