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If you’re looking for a RAM upgrade that has an edge over its competitors, you’ll want to look at the key RAM specifications to see if they will deliver the performance and value for money you need. A key characteristic you may have come across if you are doing a PC build or gaming is a C16 or C18 RAM rating on the RAM module. 

Scratching your head? The “C” in C16 or C18 refers to Column Access Strobe (CAS) latency, which is known to affect memory access speed. And yes, it is another factor you may want to juggle when weighing up which RAM upgrade will give you the best performance and value for your money. 

For folks unfamiliar with memory timings and latency, this informative overview of C16 and C18 RAM will explain what these are and how they impact the performance of your computer.

A motherboard with four RAM slots and four RAM sticks installed
C18 RAM (and C16 RAM) are types of synchronous dynamic random-access memory (SDRAM)


What is C16 RAM?

C16 RAM is a type of synchronous DRAM that has a delay or latency of 16 clock cycles between the request for RAM data and the execution of the request. 

The “C” of C16 refers to CL or CAS, which is the Column Address Strobe latency of the module. CAS latency measures the delay between a read command and the availability of data. It’s easy to understand that this time interval should be kept as short as possible to provide better computer speed and performance. So, by comparing C16 and C18, you are comparing the difference in latency, measured in clock cycles. 

The memory timing of asynchronous RAM is measured in nanoseconds, but clock cycles for synchronous RAM have the operation of its external pin interface controlled by the CPU clock. This means that the clock frequency also contributes to just how fast a C16 RAM module is going to be. The same C16 RAM module can have vastly different overall speeds if placed in computers with different clock rates. 

C18 RAM is a Type of SDRAM

C18 RAM (and C16 RAM) are types of synchronous dynamic random-access memory (SDRAM). This modern RAM type was developed in the 1990s and is characterized by its use of synchronous interfaces, where input control signals are recognized after a rising edge of the clock input. 

The clock signal works by oscillating between high and low-frequency states to coordinate the various integrated circuits of the memory controller and memory. This metronome-like control coordinates the RAM’s processing of incoming commands. With each clock cycle, the RAM completes previously initiated operations and receives new commands.

SDRAM technology is standardized by JEDEC and is used widely because it achieves higher data transfer rates and can support multiple simultaneous computations on a synchronized basis. 

What is C18 RAM?

C18 RAM is synchronous DRAM that has a Column Address Strobe latency of 18 clock cycles. We can also express this as CAS18, CL18, or CAS 18 timings. This means that it takes 18 clock cycles between a memory request and the performance of the operation by the RAM. This latency is at least two clock cycles longer than C16 RAM, meaning it takes longer to execute the transfer of data from the RAM to the CPU following an instruction from the memory controller. 

C18 performs slower than C16 because of the increased number of clock cycles the RAM takes to perform a memory request. However, latency is not the only factor to consider when evaluating RAM performance. 

C16 RAM is Comprised of Multiple Memory Banks

This volatile memory of C16 SDRAM has a multi-memory bank architecture, each with rows and columns of storage units distributed across its chips. For most synchronous RAM, a single read/write operation accesses a single bank at a time. The memory controller and arrangement of the memory slots on the motherboard determine the format of the memory bank, with the size of the bank determined by the memory bus width.  

What’s the Difference Between C16 and C18 RAM?

The primary difference between C16 and C18 RAM is the two-clock cycle difference in the speed of execution of memory requests. This means that C16 RAM is faster and has lower latency than a C18 counterpart. 

The difference in latency is because of the properties and engineering of the integrated circuits that make up the memory chips in the module. As volatile microelectronic components, they require a certain amount of time to generate electrical signals, switch transitory, and charge their capacitors. These functions add latency to the RAM’s performance but are essential for the correct execution of commands and for preserving data integrity.

CAS Latency is Just One Aspect of the Memory Timing of C16 and C18 RAM

CAS latency is one of four parameters that specify the timing of SDRAM. The other parameters include: 

  • tRCD: row address to column delay
  • tRP: row pre-charge time
  • tRAS: row active time
  • Command rate 

The timings for each of these parameters, including CAS latency, are given in units of clock cycles. You’ll find the memory timings written as numbers on a RAM module in the format CAS-tRCD-tRP. The command rate may also be included.

Taken together, these parameters give an overview of the clock latency of the RAM. The lower the individual numbers, the better because it means that the RAM works faster. 

Close up of a detached Random access memory module
Your CAS latency affects your computer’s memory access speed, so figuring out whether you need C16 or C18 is crucial to your RAM upgrade.


The Memory Clock Frequency is Just as Important as CAS Latency

Though the CL rating gives you an idea of the latency of the RAM, absolute latency and the system performance you’ll experience are determined by the memory clock frequency as well as the memory timings.

The clock frequency is absolutely essential if you want to know if a C16 RAM module is really going to be faster than a C18 RAM module. The clock frequency determines the rate of the system clock and the actual length of the clock cycles. For double data rate SDRAM, the clock cycles are much faster than commonly quoted transfer rates.

C16 vs. C18 RAM: A Side-by-Side Comparison

What it isMemory moduleMemory module
Primary UseStorage of temporary system filesStorage of temporary system files with an additional registry
Initial Release1990s1990s
Influential DevelopersSamsung, Hyundai, SK Hynix, Micron Technology, Nanya TechnologySamsung, Hyundai, SK Hynix, Micron Technology, Nanya Technology
Technologies InfluencedDDR SDRAMDDR SDRAM

Similarities and Differences


  • C16 and C18 RAM are types of volatile memory.
  • The CPU uses both types of RAM for the storage of temporary files and working data storage for operating system applications.
  • C16 and C18 RAM are types of SDRAM.
  • A memory controller controls both C16 and C18 buffered RAM.
  • The CPU uses both forms of RAM.


  • C16 RAM responds to data requests at a speed that is two clock cycles faster than C18 RAM.
  • These two RAM types may have latency differences because of their microcircuitry. 

What is C16 RAM Used for?

You can use C16 RAM for a variety of memory-hungry applications like:

  • Gaming
  • Video editing
  • Digital audio workstations
  • Virtualization

Compared to C18 RAM, C16 reads/writes with lower latency, providing a noticeable increase in speed and performance. However, C16 RAM is not the lowest latency RAM available; the lowest latency RAM available is the recently released 3200 MHz C14 RAM module by Taiwanese manufacturer G.Skill. 

Does C18 RAM Need to Be Upgraded?

Opinions vary on whether upgrading C18 RAM will make an appreciable difference to the performance of your computer. Upgrading RAM size, rank, and memory channels will have a bigger effect. For everyday computing, there are unlikely to be any gains from upgrading RAM because of latency or memory timings alone. 

RAM Frequency is Also a Consideration

RAM frequency is the actual speed at which your RAM operates. This is the number of cycles or mega transfers it can go through in one second. RAM frequency is measured in megahertz (MHz) and is usually quoted in the specification of the RAM (e.g. DDR4-3200).

The highest RAM frequencies that are widely available are in the 4266 MHz zone. This means that the RAM completes its cycles in less time and therefore can process more data.

An additional complication is that, at certain RAM speeds, the CPU becomes a rate-limiting factor and simply cannot process the data at the speed the RAM delivers. This means high-frequency RAM won’t benefit your system after a certain point.

C16 vs. C18 RAM: 6 Must-Know Facts

  • CAS latency measures the period between the supply of a column address and the receipt of the data. For DDR RAM, the CAS latency ranges between 10 and 15 nanoseconds.
  • C16 RAM has a delay or latency of 18 clock cycles between the request for RAM data and its execution. 
  • C18 RAM has a delay or latency of 16 clock cycles between the request for RAM data and its execution. 
  • C16 and C18 RAM are both types of SDRAM. 
  • CAS latency is only one of four factors, measured in clock cycles, that determine the absolute latency of RAM. 
  • The performance of a RAM module is equally dependent on RAM frequency and latency. 

Final Thoughts

CAS latency should certainly be considered when evaluating RAM modules for an upgrade, but this rating cannot be used in isolation. Alongside the CL, look at the RAM frequency and the clock frequency of your system. Other factors like RAM size and the number of channels available can deliver a significant uplift in system speed and performance. 


C16 vs. C18 RAM: Does CAS Latency Really Matter? FAQs (Frequently Asked Questions) 

What is clock frequency?

Clock frequency, clock speed, and clock rate refer to the frequency of the pulses generated by a clock generator of a processor (CPU). These pulses are used to synchronize the operation of processor components and the function of SDRAM. Clock frequency is measured in clock cycles per second (Hertz).

What is CAS latency?

Column Address Strobe (CAS) is latency that is created by a delay (measured in clock cycles) between a read command and the presentation of the requested data.

What is tWR?

tWR is Write Recovery time. This latency parameter measures the clock cycles between writing data and a pre-charge command. This is necessary to ensure that all buffered data is written to the memory core.

What is tRAS?

tRAS stands for Row Active time, which measures the clock cycles between a bank active command and the pre-charge command.

What is tRCD?

tRCD is another latency parameter called the Row Address to Column Address Delay. It measures the clock cycles between an active command and the read/write command.

What is tWTR?

Internal Write to Read Command Delay, or tWTR, is a delay between ending the last data from a write operation to the memory and the issue of a read command. 

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