What is CAS Latency? DDR5 latencies explained

When buying DRAM, you’ll see information about their timings, like 34-42-42-96 or 40-40-40-77. Those numbers represent the DRAM kit’s latency, and these settings are stored on a chip on the memory sticks called the SPD (Serial Presence Detect), so you may see those letters at the same time. The first number is often the only one mentioned as “C” or “CL” or “CAS”, but what does CAS latency really mean? And what implications does a DRAM kit’s CAS latency have for its performance?

A DRAM module’s CAS (Column Address Strobe or Signal) latency is how many “clock cycles” it takes for the module to access a specific set of data in one of its columns to make it available to the output.

From an easier approach it’s how many clock cycles it takes for the DRAM to output data called for by the CPU. A DRAM kit with a CAS latency of 34, for example, takes 34 RAM clock cycles to complete this task. The lower the CAS latency, the faster it’ll be done for the same DRAM frequency.

While a DRAM’s frequency tells you how many mega transfers (1,000,000 data transfers) it can do in one second (a DDR5-7200 RAM module can conduct 7,200 mega transfers in a second for reference).

As explained before, CAS latency tells you the total number of cycles it takes for the RAM to send data, but you should consider the duration of each cycle to get a better idea of that DRAM’s overall latency.

While DDR5 RAM is newer with better storage density and power efficiency than DDR4, it tends to have higher CAS latency. DDR4 usually has a CAS latency of 16, while DDR5 will have a CAS latency of at least 32. However, because of its faster clock speeds, the newer standard has better performance overall.

Luckily, CORSAIR makes DDR4 and DDR5 RAM in basically every mixture of size and speed you could want. Our Memory Finder can help point you in the right direction if you need a few pointers.

The operations that these numbers indicate are the following: CL-tRCD-tRP-tRAS-CMD. To understand them, bear in mind that the memory is internally organized as a matrix. This is where the data are stored at the intersection of the lines and columns.

• CL: CAS Latency. The number of cycles it takes between the processor asking for data from the memory and returning it.
• tRCD: RAS to CAS Delay. The number of cycles it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.
• tRP: RAS Precharge. The number of cycles it takes between disabling the access to a line of data and the beginning of the access to another line of data.
• tRAS: Active to Precharge Delay. For How long the memory must wait until the next memory access request can be initiated.
• CMD: Command Rate. The time it takes between the memory chip having been activated and when the first command may be sent to the memory. Sometimes this value is not announced. It usually is T1 (1 clock cycle) or T2 (2 clock cycles).

If you're looking for the best-performance memory, you need to find one with high frequency and low CAS & timings.

Each CPU generation has his own sweat spot for memory frequency, from that spot, looking for the kit with the lower CAS will allow you to find the best memory for the specific CPU in question.

For people who are looking to get the maximum performance of everything you’ll choose from the highest frequency kit the smallest CAS you can find. Head to our webstore for a huge range of DDR4, and DDR5 with all sorts of timings and latencies.