What is CAS Latency? CL and RAM Timings Explained

CAS latency is a technical term you’ll often come across when comparing different types of RAM, but it’s not always explained in the clearest way. In simple terms, CAS latency (Column Address Strobe latency) refers to the delay between when your system’s memory controller requests data from the RAM and when that data becomes available.

In this guide we'll break down how memory timings, CAS latency and speed actually work, and how to know what RAM you need to buy based on your needs.

What is CAS latency?

Before diving into the details, it’s important to understand that memory for PCs generally comes in two types: industry standard and overclockable. Industry standard memory conforms to the speeds, timings, and voltages set by the JEDEC standards body, and these are the specifications all computers are built to comply with.

Overclockable memory is built with more aggressive timings than industry standard memory and generally has lower CAS latency values, but often with higher voltages and other aggressive timings that exceed industry standard specifications. Overclockable memory generally only work in computers that are built to support modified speeds, timings, and voltages.

CAS latency (also known as CL) is a key timing value that is programmed into your RAM. It refers to the amount of time your RAM takes to start delivering data after the CPU asks for it. In practical terms, think of it like asking a librarian for a book; the number tells you how many seconds it takes before they hand it to you. The lower the number, the faster your system responds.

Let’s take our Kingston FURY Beast DDR5 6000MT/s RAM with a CAS latency of CL30. That means your system takes 30 clock cycles from the time the memory controller sends out a read command until the data becomes available, where a clock cycle is the unit of time your CPU and RAM use to coordinate tasks. And since the latency is measured in cycles rather than time, the actual delay in nanoseconds depends on the RAM’s clock speed.

How to calculate the total latency in nanoseconds?

Total latency (also referred to as true latency) is measured in nanoseconds (ns) and is the total amount of time for the data request from processor to memory to be complete. CAS latency (CL) is one important value, measured in clock cycles, but you also need to consider the RAM’s clock speed or data rate. This is because faster RAM completes each clock cycle more quickly. As a result, a higher CAS latency value on faster RAM can still produce a lower actual delay when compared to slower RAM with a lower CL number. For example:

• A RAM module running at 6000MT/s with a CL30 has a total latency of about 10 nanoseconds.
• Meanwhile, a faster RAM module running at 7600MT/s with a CL38 has a similar delay despite the higher speed.

This shows why looking at CAS latency alone doesn’t give the full picture. Two RAM kits with very different CL values can deliver similar performance depending on their speed.

To find the total latency of RAM in nanoseconds, you can use this formula:
CAS latency × (2000 ÷ RAM speed in MT/s) = total latency (ns)

Using the examples above:

• A 6000MT/s RAM kit with CL30 has an actual total latency of:
  30 × (2000 ÷ 6000) = 10 ns
• A faster 7600MT/s kit with CL38 results in:
  38 × (2000 ÷ 7600) is also 10 ns

This is why understanding memory speed and latency timings together is essential for choosing the right overclockable memory. Real-world performance depends on the balance between these two factors, not just on the CAS latency number alone.

The difference between CAS latency and RAM speed

When shopping for overclockable RAM, you’ll often see two important numbers: CAS latency and RAM speed. These two values reflect different aspects of memory performance, but they are closely connected, and both affect your system’s overall performance.

RAM speed (or data rate) tells you how much data your memory can transfer each second. It’s measured in mega transfers per second (MT/s). Simply put, the higher the RAM speed, the more data your system can move at once, which can improve performance in tasks like gaming, video editing, and multitasking.

But here’s the catch, higher-speed memory often comes with higher CAS latency. For example, DDR5 RAM runs at much higher speeds than DDR4, but its CAS latency numbers are higher as well. This doesn’t mean DDR5 is slower.

This is why you’ll sometimes see gamers or PC builders debating between faster RAM with higher latency or slower RAM with lower timings. The truth is that the best overclockable RAM choice depends on your specific use case and the balance between speed and latency. Sometimes, a slightly slower speed with lower latency can offer better real-world performance.

Memory timings breakdown: What the numbers mean

While CAS latency and speed are key factors, they’re not the whole story. To understand overclockable RAM performance fully, you also need to know about other memory timings that affect how quickly your RAM operates.

You’ll often see these timings represented as a string of three or four numbers in the memory modules’ specification, such as 36-38-38-80. These numbers are known as memory timings or RAM timings, and they represent the latency of different operations inside the RAM.

Let’s break down the example 36-38-38-80:

• 36 = CAS Latency (CL) – The number of clock cycles it takes for the RAM to start sending data after the processor asks for it.
• 38 = Row to Column Delay (tRCD) – The delay between activating a row and accessing the column within that row.
• 38 = Row Precharge Time (tRP) – How long it takes to close one row of memory before opening another.
• 80 = Row Active Time (tRAS) – The minimum time a row must remain active to ensure the data is fully accessed.

Sometimes you’ll only see the first three numbers (like 36-38-38) listed on product pages. This is a common thing; it just keeps things simple and focuses on the most important parts. The fourth number (tRAS) is used by your system and can usually be found in your BIOS or detailed spec sheets.

Memory timings, including CAS latency and the other three values, impact how quickly your RAM can respond to commands and transfer data. While the difference might be subtle in everyday tasks, they become more important in gaming, video editing, 3D rendering, and other high-performance computing tasks. Lower timings generally mean faster response, but they must be considered together with RAM speed for a complete performance picture.

What CAS latency is best?

There’s no one-size-fits-all answer when it comes to choosing the best CAS latency, or overclockable memory in general, because the ideal choice depends on what you use your system for. For example, if you’re building a gaming PC, doing video editing, or other high-performance tasks, choosing overclockable RAM with a well-balanced speed and CAS latency can help reduce delays and improve responsiveness. But for everyday use, the difference might be minimal and won’t significantly impact your experience.

Generally everyday users typically use industry standard memory in their computers, foregoing any risk to stability by tinkering with high-performance RAM.

Gamers and streamers may want to nudge the speed up and timings down within a few degrees of industry standards to garner improvements to frame rates but stick within a safe range of overclock speeds that don’t push the limits of their system hardware.

Content creators and high-end workstations may opt for a balance between high-performance and high-capacity. Higher capacity memory needed to handle the large amount of data in RAM may not be available in the same extreme speeds and low latencies as lower capacities, so optimizing a balance between the two is recommended. Applications for creators also tend to be more bandwidth hungry than latency.

Competitive gamers and other extreme enthusiasts may go for the bleeding edge of what their PC hardware can handle. Paired with a high-end processor model and motherboard engineered for performance, overclockable RAM speeds can be pushed to their limit of stability. Generally sticking with factory pre-tuned profiles programmed into the RAM is recommended, but some users may venture into the BIOS to tweak the speed, timings, and voltage at their own risk to get all they can out of their hardware configuration.