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How to Choose RAM: Speed (MHz/CL) vs Capacity (GB)

If you've ever shopped around for system memory (RAM), or even a full PC, you're probably aware that RAM has its own 'speed', like that of a CPU or GPU. So, faster the better right?! Not quite. There's a little more to it than that. So, keep reading to find out how to intelligently choose RAM for your computer.


Assumptions about RAM

As a general rule, the faster the speed, the better the performance of RAM. But, as we’ve stated above, there’s a little more to it than that. In fact, the fruits of high-end memory aren’t available without some trade offs - especially if you want to install a large capacity of RAM.

So, we’ve put together this guide which will take you through the specifications of memory and ultimately help you to decide, when you’re looking for RAM, whether you should be buying for speed or larger capacity.

Note - this guide has been written with regards to DDR4, however the general concept should translate to the upcoming DDR5 standard, just as it does with older standards such as DDR3.

RAM: Why Not Speed and Capacity?

For the most common RAM speeds out there, you’ll be able to get the capacity you want at a decent speed.

Unfortunately though, if you’re looking for a large quantity of memory at a high speed (for example 64GB at 3600MHz), this isn’t a realistic proposition.

Many people underestimate the resources required to pull off this combination of large capacity and high speed. Where it can be pulled off, sacrifices have to be made elsewhere in your system.

But, why is this the case? Well, this guide aims to provide you with the answer.

TLDR - if you want the answer, skip to the conclusion. However, if you want the full details, keep reading.

“More RAM means more speed” - Debunking a classic assumption

Time and time again we speak with customers who assume that the larger the RAM capacity, the faster the system.

This assumption is false and originated many years ago, back when computers and laptops were equipped with smaller RAM capacities. Back then, most operating systems and applications were generally using a larger proportion of the average installed RAM capacity for most machines, and therefore slowing the system down. To fix this, adding more RAM was usually the answer.

Today, however, most machines are fitted with 8 or even 16 gigabytes of RAM and usually run without any issues. The reason? The operating system’s RAM usage is proportionally lower on newer hardware. As a result, there’s no need to increase the system’s RAM.

Of course, this doesn’t apply to absolutely everyone.

For the average consumer, there’s a rule which will help them understand if they need more RAM:

Adding more RAM will only boost your system’s speed if it’s using all the capacity that is currently installed.

For anyone buying or building a PC right now, this is a great nugget of knowledge to retain. If you can calculate your RAM requirements before you buy (and therefore avoid buying more capacity than you need), you can allocate the money you save to something with a larger innate performance, which leads nicely onto the next section…

RAM: Speed and Latency (CL)

RAM can be purchased in a variety of speeds, with differing latencies.

Unless you’re a true enthusiast, there are mainly two key specifications of a RAM module/kit (other than capacity) that you should pay close attention to:

• Memory frequency (MHz).
• CAS Latencies (CL).

You need to look at both specifications in order to get a good indication of the RAM’s final performance. As a general rule, you should consider the following (for those of you who are interested, we’ve added the formula below):

The higher the speed, and the lower the CL, the better the overall performance.


RAM latency (lower the better) =
CAS Latency (CL) x 2000
Frequency (MHz)



So, the money you’ve saved from not purchasing that larger capacity RAM kit should be used on the best performing RAM right? Well, before you do that, let’s discuss the drawbacks of hyper-fast memory and whether all kits with faster speeds are worth the money.


Does speed really make a difference?

The short answer is yes. But, the long answer is - it depends on a few factors.

Your workload will ultimately decide whether speed will make a difference for you. For people who use CPU-heavy applications, faster RAM will certainly make a difference.Whereas home users don’t need to worry about RAM speed too much. But, what about gamers who fall in between these two groups of people? Well, that depends on the gamer’s choice of CPU.

Intel vs AMD CPUs


Intel CPUs aren’t massively reliant on the performance of memory while running, which might explain why RAM speed support has historically been rather limited outside of Intel’s enthusiast chipsets (Z-Series) and capped to 2666Mhz (at least until recently).

If you’re the owner of an Intel CPU we certainly suggest getting a good quality RAM kit, but the speed of that RAM isn’t as important. Save your money for other components or a RAM capacity upgrade if required.


In stark contrast to Intel, AMD’s more recent ‘Zen’ line of CPUs has RAM speed almost baked into the architecture of the CPU.

AMD’s infinity fabric technology uses the speed of the RAM to pass information across sections of the CPU. This means that better memory will serve to boost the CPU performance as well as helping in those intense applications we mentioned earlier.

There is another factor to consider which is the FCLK speed and the ratio between that and the RAM itself - but that’s a topic for a separate article.

But, in summary, a high-performance 3600MHz memory kit is ideal for AMD Ryzen CPUs.


Did you expect overclocking to appear in a guide to RAM? Probably not, but to address the elephant in the room, most RAM speeds over 2933MHz are technically classed as overclocks.

This classification is usually set by your motherboard. For example, if you’ve ever checked your motherboard specifications, you might find that the speed of your average gaming RAM (3200MHz) is marked with a little ‘OC’ (overclocked) sign.

Whenever you activate the XMP or DCOP memory profile in your BIOS, you’re loading a preset bank of BIOS settings specifically for your memory which matches the advertised speed and CL. Once you save these settings, you could well be running your RAM above factory specifications.

The problem?

Overclocking any component above factory specifications has the potential to cause system stability issues, or even a boot failure. The further you push your system, the higher the likelihood of instability becomes.

This is the reason you should take a more measured approach with memory and review your choice of motherboard and CPU, rather than just buying the fastest RAM kit available. Just as you wouldn’t buy a budget motherboard to overclock a CPU, the same concept should be considered when it comes to RAM (but with slightly less significance).

Motherboard QVL/Memory support lists

Many manufacturers list specific RAM kits as ‘verified’ with their products, meaning that the manufacturer has tested the motherboard model in question with a specific RAM kit and has confirmed full support for that kit, at its advertised speed and CAS latency.

CCL advises that you purchase RAM listed on your motherboard’s QVL where possible, for the best compatibility. However, this is almost always impractical given the availability of exact RAM kits at any given time.

Module quantity and capacity

Now that we understand that high-speed RAM runs your system in an overclocked state, we can begin to piece together why people should ‘choose’ between speed and capacity.

Just like memory speed and CAS latency, module quantity and capacity are both factors that, when increased, will add to the stress of an overclocked system.

Luckily, the greater availability of RAM can help offset this problem slightly, as kits with larger individual modules can help eliminate the added stress of having many RAM modules installed at once. Larger RAM kits do this by compressing the capacity into a lower number of modules.

This isn’t a guaranteed failsafe, but it can sometimes make the difference between a system successfully booting or not.

Achieving stability

Speed, CAS latency, module size, and module quantity; in order to avoid running into problems you should balance these factors when considering your purchase.

For example, 16GB of 3600MHz CL16 memory is much more likely to be stable than 32GB of the same modules, even if the settings in BIOS remain the same.

Consider another example - you may want to run 64GB of RAM at 3600MHz, but to get it to run properly you need to lower the speed to 3000MHz.

We can condense this balancing act into a simple choice triangle (although speed and CL can technically be put in the same class - hence the title of this article):


Pick Two of Three - Speed, Low CL, or Capacity

What if you want it all?

It’s certainly possible!

A high-end motherboard features a more reliable power delivery design and a thicker PCB, which can be used to supply cleaner voltage to the RAM and regain some of, if not all, of the stability that is lost when applying a RAM overclock.

Generally, we find that motherboards of this class are in excess of the £250 mark, but that’s not always the case. The best thing is to do some research on your board. Freel free to contact our sales team or check out our motherboard buying guide; either way, we’ll be happy to help you choose.


So, to bring everything together, we can conclude that your choice of memory can be condensed into two, easier to understand, components:

Performance (speed) vs Stability (capacity)

You should keep in mind our earlier reference to Intel vs AMD CPUs as this will affect consumers differently based on their choice of CPU. We suggest that unless you’re a real computing enthusiast, you should stick with stability (unless you want the occasional headache of system crashes).

For your convenience we’ve included a handy combination table below that will give you a rough idea of what speeds you can generally achieve whilst retaining solid stability using a mid-range motherboard and chipset (example used is an MSI MAG B550M MORTAR motherboard; results may differ from board to board):


    Total RAM capacity
    8 GB 16 GB 32 GB 64 GB
Number of modules installed 1 3600 MHz 3600 MHz 3200 MHz 2933/3000 MHz
2 3600 MHz 3600 MHz 3200 MHz 2933/3000 MHz
4+ 3200 MHz 3200 MHz 2933/3000 MHz 2666 MHz



For those of you who want both high speeds and capacity, a quality motherboard, knowledge of overclocking and a good amount of patience is what you’ll be needing.


We have a great range of motherboards available here, and can offer further sales advice via email or telephone.