What is CAS latency in RAM? -

Understanding what CAS latency is and how it works is important to deepening your knowledge of RAM. Whether you’re running the best DDR4 RAM or the best DDR5 RAM, this metric will become an important factor in deciding on your next memory modules.

Simply put, CAS latency is the amount of clock cycles that your RAM module takes to output data that your processor needs. It’s shown in varying ways, so the listings may not always be consistently showcased from manufacturer listings.

CAS latency is also known as timings, and can be displayed as a string of numbers denoting the total cycles. If this seems a bit too technical, don’t worry, I’m here to explain what these timings mean and what to look out for in your next RAM kit.

What is CAS latency?

Column Address Strobe (CAS) latency is a RAM timing that dictates the speed at which the memory modules can receive data from the processor. It’s measured in clock cycles, with a lower amount of cycles meaning the RAM is more responsive.

CAS latency is measured numerically, including ways such as CAS timings with a singular number representing the total number of clock cycles, or a CL figure, with the first number of a string showing the latency.

That’s where the differences in DDR5 vs DDR4 RAM come in. While DDR5 is faster and achieves higher frequencies, it will have a longer CAS latency. Traditionally, the latter usually has a latency of 16 (CS16) whereas DDR5 tends to have double this at CS32.

Different frequencies and capacities will have varying latency. This can be seen by looking at the varying CAS latency from top-end DDR5 kits. The Corsair Dominator Platinum RGB has different CAS latency depending on the variant, for instance.

The 32GB 6000MT/s kit has a CAS latency of 36 (36-38-38-76). However, the 64GB 6200MT/s version has a CAS latency of 32 (32-38-38-80). So things aren’t always consistent between models, even from the same line.

What are CAS Timings?

CAS Timings are a set of four numbers in a chain that show how long it takes for the RAM to respond to the CPU. The first of these four numbers is the CAS Latency (CL) which is how many clock cycles it takes for the RAM to cue the data for the processor to access.

The second number is the Row Column Delay (tRCD) which is how long the RAM takes to read the data; the third figure is the Row Precharge Time (tRP) which is the length of time for a new row of data to be readied, and the fourth number is the Row Active Time (tRAS), the minimum amount of time needed for an active row to respond.

If you look at a Corsair Dominator Platinum RGB DDR5-6000 RAM kit, it has listed CAS timings of 36-38-38-76. That means the CAS Latency (CL) is 36, the Row Column Delay (tRCD) is 38, the Row Precharge Time (tRP) is 38, and the Row Active Time (tRAS) is 76. By understanding these four figures, you’ll have a better understanding of RAM timings, and how they compare to other models.

PNY xlr8 gaming mako ddr5 RAM slotted into a motherboard

(Image credit: Future / John Loeffler)

Does RAM speed affect CAS latency and timings?

DDR5 RAM trades faster frequencies at the cost of increased latency compared to DDR4 RAM, however, there are differences between budget, midrange, and high-end RAM with their CAS timings. For example, the Corsair Vengeance DDR5 kit (one of the cheaper memory modules available) features a CAS latency of 40 (or CL40) with the full timings for the 5,200MT/s kit being 40-40-40-77.

That’s an additional four clock cycles (or 11.1%) longer than you’ll find with the Corsair Dominator Platinum RGB DDR5 model at CL36. This CAS latency is consistent with some of the fastest (and most expensive) DDR5 RAM, such as the G.Skill Trident Z5 RGB 32GB kit at 7800MT/s, which also has a latency timing of CL36.

However, the full CAS latency timings for the G.Skill Trident Z5 RGB are 36-46-46-125, which we can directly compare to the Dominator’s timing of 36-38-38-76. While the clock cycles are identical, the faster G.Skill RAM (in terms of frequency) has higher timings and, therefore, takes slightly longer to respond to your processor.

This is the trade-off for faster memory, as it’s something we’ve seen in the jump from DDR3 to DDR4 and from DDR4 to DDR5, it’s also true between budget, midrange, and high-end DDR5 configurations.

Corsair Dominator Platinum RGB DDR5 RAM inserted into a motherboard

(Image credit: Future / John Loeffler)

Does CAS latency impact performance?

While it’s generally agreed that lower CAS timings are better than higher figures, what does this mean for the likes of gaming and productivity performance?

Generally, DDR5RAM has higher timings than DDR4, but its increased frequencies make it superior for intensive gaming and productivity. However, the discrepancies in varying DDR4 and DDR5 models have an impact on your system’s overall performance, depending on the model.

Even the best RAM kits on the market may not make a massive difference when it comes to gaming performance. Depending on the different timings and latency, you may see anywhere from a 1-5% improvement (at best) whether you’re using the cheapest and slowest DDR5 RAM or the best that money can buy. Performance is dictated far more by your graphics card and processor than the speed of your RAM.

If you want to squeeze every possible drop of performance out of your gaming PC, then you may want to opt for the fastest latency DDR5 RAM with the lowest CAS timings available. Every manufacturer is different, and the same line of hardware will not necessarily be consistent.

That means it’s important to read up on not only the capacity and frequency of the memory but all four figures of the CAS timings as well. It’s unlikely to matter much to most people, but you should be aware of things all the same.

MemTest86

(Image credit: PassMark)

How do I test my RAM’s latency?

There’s no shortage of ways to test your RAM’s CAS latency and timings available online with most of them being free and easy to use. MemTest86 is (arguably) the most comprehensive tool for memory diagnostics tool that’s been a popular choice for over 20 years. With the program, you can check not only the exact timings and latency, but it can also look for corrupted data and abnormal behavior.

Alternatively, you can use a built-in tool that’s already equipped in your operating system, such as the Windows Memory Diagnostic. You can boot it up with the keyboard shortcut Windows + R and then Run, to type in “mdsched”.

You can also easily find it by searching for it manually in the start menu. It will analyze your system’s resources, track event logs, and troubleshoot any potential issues, but you may only need to use the Diagnostic Data Viewer for the essential information.

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