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RAM Latency Calculator

Table of contents

Function of the random access memoryRAM Generations — DRAM to DDR5What is RAM latency?Reading the memory moduleHow to use the RAM latency calculator — ExampleWhy is RAM latency important?FAQs

Welcome to the RAM latency calculator! Are you comparing memory modules to improve your device memory and wondering how fast the system responds to a CPU request? Or is your device running slow, and you want to find the solution? Use this RAM latency calculator to learn how RAM works, how to calculate its latency, and the formula behind the calculator. Keep reading to find:

  • A table of the RAM latency for most common SRAM to DDR5-RAM technology configurations;
  • Explanations how to read the memory module;
  • Example calculations for RAM latency;
  • The difference between Megahertz (MHz) and Megatransfers per second (MT/s); and
  • much more...

Function of the random access memory

What is the RAM?

RAM (random access memory), the main memory in a computer, is the hardware that temporarily stores the data currently processing in the CPU to be quickly accessible. This can be, for example, the operating system (OS), application programs, and open documents in current use. It is much faster to read from and write to than other storage, such as a hard disk drive (HDD) or solid-state drive (SSD). Note that RAM is volatile. The data is kept in the RAM as long as the computer is on, but it is lost when you turn it off.

How does a random access memory work?

You can imagine RAM like a binary Excel table (if you want to know more about the binary code system, check our binary calculator). Each cell can hold a 0 or a 1 (i.e., an array of 0s and 1s).

Each cell has a unique address found by counting across the columns and down the rows. The RAM controller sends the column and row address down a thin electrical line to find a specific cell. Any data that is read flows back on a separate data line. This process is expressed as your memory module's data rate in MT/s (mega transfers per second). This value will influence your device's RAM latency. You can multiply the data rate (how many bits a module can transfer in a given time) by the bits in a byte (1 byte = 8 bits) to determine the speed or transfer rate in GB/s of your memory.

RAM is stored in microchips that are gathered into memory modules, which are then plugged into slots in the computer's motherboard. You can read the information from this memory module needed to calculate the RAM latency.

RAM Generations — DRAM to DDR5

RAM comes in 2 principal forms:

  1. DRAM — in Dynamic RAM, data is stored in capacitors. Because of the capacitor's charge and discharge properties, constant action (e.g., refreshing) of the data is required every few milliseconds (therefore, the name dynamic).

  2. SRAMStatic RAM saves data in a transistor that also needs constant power, but you don't need to refresh SRAM to make it remember the stored data (therefore, the name static).

    • SRAM is significantly faster and uses less power than DRAM, but it is also more complex and expensive and has lower memory capacity.

From there on, synchronous technology evolved with time. This means that, just like a clock, the memory controller knows the exact cycle of when the requested data will be ready. The CPU does not have to wait between memory accesses, which allows computers to execute tasks faster. The amount of data that can be transferred in one of these clock cycles is expressed as the CAS latency, which is necessary to determine the RAM latency.

  1. SDRAMsynchronous dynamic RAM can only read/write once per clock cycle — You will probably only find these in computers built before 2002, since after new technologies came up.

  2. DDR SDRAMdouble data rate synchronous dynamic RAM was introduced in 2000 as the next generation of SDRAM. It can move data multiple times in a single clock cycle.

Multiple generations of DDR-RAM have evolved to increase speed and frequency while decreasing power consumption continuously. Don't worry; our calculator determines the latency of all the mentioned technologies and generations.

🙋 Note: Motherboards are built to support only one type of memory. It means you cannot mix and match SDRAM or different DDR generations' memory with each other on the same motherboard.

What is RAM latency?

RAM latency is the delay before RAM can start processing CPU data, measured in nanoseconds. Ideally, you would want to choose a RAM with lower latency. Two factors affect RAM latency — the data rate and CAS latency (CL). You will find all the values in your device's memory module. In the section, reading the memory module, we explain how to read out all variables easily.

The data rate tells you how fast your memory controller can access or write data to the memory. We refer to it as the "speed" of the RAM and typically measure it in MT/s (mega transfers or million transfers per second). You might find the "speed" given in megahertz (MHz), which stands for million cycles per second.

In older RAM technologies, 1 MHz means 1×10⁶ data transfers per clock cycle because data was being transferred in synchronization with the clock speed of the RAM. However, modern DDR is a technology that doubles the data rate in regular SDRAM; it can transfer twice the data while running at the same clock speed. So if your value for the "speed" of a DDR is given in MHz, double it to get it in MT/s. For example, 3,600 MT/s is 1,800 MHz.

And what is the CAS latency? The Column Address Strobe latency is measured as the number of times data can be transferred in a clock cycle or, in other words, how many clock cycles it takes for the RAM module to access a specific dataset in one of its columns and make that data available. The lower the CAS latency, the better.

Use our calculator's formula to determine the RAM latency quickly:

RAM Latency=CAS Latency×2000Data Rate\scriptsize \!\text{RAM Latency}\! =\! \text{CAS Latency}\! \times\! \frac{2000}{\text{Data Rate}}

Or, if you are looking more for an overview, see the following table of RAM latency for standard configurations:

Technology

Data rate (MT/s)

CAS latency

RAM latency (ns)

SDR

100

3

24.00

SDR

133

3

22.50

DDR

333

2.5

15.00

DDR

400

3

15.00

DDR2

667

5

15.00

DDR2

800

6

15.00

DDR3

1333

9

13.50

DDR3

1600

11

13.75

DDR4

1866

13

13.93

DDR4

2133

15

14.06

DDR4

2400

17

14.17

DDR4

2666

19

14.25

DDR4

2933

21

14.32

DDR4

3200

22

13.75

DDR5

4800

40

16.67

Reading the memory module

On the sticker of your memory module or in the description of the RAM kit you are considering buying, you should find a code similar to the following:

8GB DDR4-3600 CL-16-19-19-36

where:

  • 8GB is the memory capacity of your RAM;

  • DDR4 is the RAM generation, in our example, the Double Data Rate generation 4;

  • 3600 is the data rate, here 3,600 MT/s; and

  • CL-xx-xx-xx-xx is the representation of the memory timing in the form CL-tCL-tRCD-tRT-tRAT, where:

    • tCL is the CAS latency of the memory module, in our example, 16 clock cycles;

    • tRCD is the row address to column address delay; in our example, it equals the first 19;

    • tRPT is the row precharge time; in our example, it equals the second 19; and

    • tRAT is the row active time. A row must stay open to access the data in these minimum clock cycles. In our example, it equals 36.

How to use the RAM latency calculator — Example

Let's look at an example to understand how to use this easy RAM latency calculator. If you are looking for an overview, you can also see this table for SDR to DDR5 RAM Kits.

We want to buy a RAM Kit, and from the options we found online, we already made a pre-selection based on our system's configuration and total capacity and are now left with three options. We want to compare their RAM latency to make the best decision with our buy. We struggle to decide between these three RAM kits with the following details about their memory module:

  1. 64GB DDR5-4800 CL-40-39-39;
  2. 64GB DDR5-4800 CL-38-38-38; and
  3. 64GB DDR5-5200 CL-36-36-36.

To compare their RAM Latency, we:

  1. Extract the data rate (in MT/s) from the three kits, in our case:

    1. 4800;
    2. 4800; and
    3. 5200.
  2. Extract the CAS latency — CL (in clock cycles) from the three Kit's, in our case:

    1. 40;
    2. 38; and
    3. 36.
  3. Type the values into the according calculator fields.

  4. The RAM latency calculator will perform the following calculations:

    1. 40×2000/4800 = 17 ns;
    2. 38×2000/4800 = 16 ns; and
    3. 36×2000/5200 = 13 ns.
  5. The RAM latency calculator will show you the result instantly. In our case, the third RAM-Kit with 13 nanoseconds RAM latency would be the fastest since its latency is the lowest.

Why is RAM latency important?

For standard tasks, such as office work, web browsing, and image- and video editing (take a look at our frames to timecode calculator or the bandwidth calculator to get help with these), etc., the standard device memory will be good enough.

But suppose your computer is very slow, or you work with more complex tasks requiring constant memory access. In that case, upgrading your memory is a great solution, also to increase speed in other connected components. Complex tasks include large database operations, games that push massive simulations, and ultra-high framerate targets where your system becomes more CPU-limited (waiting on the CPU to finish a task). With a lower RAM latency, your memory kit will respond and act on requests faster.

FAQs

Can I mix different memory sticks with variable timings?

Mixing memory with different timings might work, depending on your system. We do not recommend it, though. If you are running two memory sticks on dual channels, both will run, but on the lower of the two frequencies — the system will use the memory timing with the higher clock cycles. Mixing memory with different frequencies, timings, and capacities can cause system crashes and errors.

How much RAM do I need?

The amount of RAM you need depends on what you intend to do. For very basic usage, a RAM of 8 GB is sufficient. For office work and video editing, for example, it is recommended to have at least 16 GB RAM, and for more complex tasks, such as gaming or streaming, 32 GB will help to ensure that your computer does not slow down.

Is CL18 bad for RAM?

It depends on the module speed, but in general, for modern devices, it is acceptable for most tasks, while a smaller value is always desirable. For comparison:

A 2133CL18-RAM-Kit would have a RAM latency of 17 ns — 18 × 2000 / 2133 = 17 — which is ok, while a 4400CL18-RAM-Kit would have a much lower RAM-Latency of 8 ns — 18 × 2000 / 4400 = 8 — which is pretty good. The lower the RAM latency, the better.

What is CAS Latency in RAM?

A RAM module’s CAS (column address strobe) latency is how many clock cycles it takes for the RAM module to access a specific set of data in one of its columns. It tells you how fast it can make that data available, starting from when a memory controller tells it to. For example, a RAM kit with a CAS of 16 takes 16 RAM clock cycles to complete this task. The lower the CAS latency, the better.

What is the latency of 3200 RAM?

13.75 nanoseconds, if we assume a CAS latency of 22. You can verify this result with the equation for RAM latency:

  1. Find the CAS latency, e.g., 22.

  2. Use the RAM latency equation:

    RAM Latency = CAS Latency × 2000 / Transfer Rate

  3. Insert your values:

    RAM Latency = 22 × 2000 / 3200 = 13.75 ns

  4. Or use Omni Calculator's RAM latency calculator for even faster results.

What latency of RAM is good?

In general, the lower the RAM latency, the better. Nowadays, the RAM latency in most devices is around 8-20 ns, but modern technologies can go down to even 6 ns.

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