What Is CAS Latency (CL)? RAM Timings Explained

CAS latency (CL) is the number of clock cycles RAM takes to deliver data after the processor asks for it. A lower CL means a shorter delay. You will see it on every DDR4 RAM and DDR5 RAM label, for example, CL16 or CL40, and it is one of the key timings in computer RAM.

What does CL16 or CL40 mean on RAM?

CL16 means the RAM takes 16 clock cycles to respond to a request for data. CL40 means it takes 40 cycles. CL stands for CAS latency, and the number only counts cycles; it does not tell you the time in seconds yet. 

RAM works to a steady clock, like a metronome. When your processor asks for data, the RAM needs a few ticks of that clock before the data starts coming out. CL is the number of ticks in that wait. Fewer ticks mean a quicker response.

A typical RAM label reads like this: DDR4-3200 CL22. It has three parts:

• DDR4: The memory generation (DDR3, DDR4 or DDR5).
• 3200: The data rate in MT/s, often shown as MHz on product pages.
• CL22: The CAS latency, in clock cycles.

CL is the first of several RAM timings. You may also see a set like 16-18-18-38, which lists CL, tRCD, tRP and tRAS. Product pages sometimes write them out, for example, "CL46 (tRCD 46, tRP 46)". CL is the main one brands print on the label, so it is the one most buyers compare.

How do you calculate real CAS latency in nanoseconds?

Real CAS latency in nanoseconds = CL x 2000 / data rate (MT/s). This formula converts cycles into actual time, so you can compare RAM with different speeds fairly.

Example: DDR4-3200 CL16 gives 16 x 2000/3200 = 10 ns.

 
Notice that DDR3-1600 CL11 and DDR4-3200 CL22 both land at 13.75 ns, even though their CL numbers are very different. The same happens with DDR5-6000 CL30 and DDR4-3200 CL16, which both reach 10 ns.

Is lower CAS latency always better?

At the same speed and the same DDR generation, yes: a lower CL responds faster. But CL numbers cannot be compared directly across different speeds or DDR generations, because the clock runs at a different pace.

Take two DDR4 modules, both at 3200 MT/s. One is CL16 (10 ns) and the other is CL22 (13.75 ns). Here, the CL16 module is clearly quicker to respond. Now compare a CL40 DDR5 module with a CL16 DDR4 module. The CL40 module is not "slower" just because 40 is bigger than 16, so convert both to nanoseconds first.

Latency is also only one part of RAM performance. Two other things matter as much or more in daily use:

• Capacity: If your system runs out of RAM, it slows down far more than a few nanoseconds of latency ever would.
• Speed (bandwidth): A faster module can move more data per second, which can help in memory-heavy work even when its latency is slightly higher.

So a lower CL is a good tie-breaker when two modules have the same capacity, speed and DDR type. It should not be the first thing you choose.

CAS latency in DDR4 RAM vs DDR5 RAM

DDR5 RAM usually has a higher CL number than DDR4 RAM, but it also runs at a much higher data rate. This keeps real latency in nanoseconds close, while DDR5 delivers more bandwidth.

DDR5 modules run a much faster clock, so each cycle is shorter. The memory chips still need roughly the same amount of real time to fetch data, which means they need more cycles to do it. That is why DDR5 labels show numbers like CL40 or CL46 while DDR4 shows CL16 to CL22.

DDR4-3200 CL22 comes to 13.75 ns, while DDR5-5600 CL46 comes to about 16.4 ns. The DDR5 module responds a couple of nanoseconds later, but its theoretical peak bandwidth is much higher (44.8 GB/s against 25.6 GB/s).

DDR4 and DDR5 are not interchangeable: They use different slots, and your motherboard decides which one your system supports. Check your system's supported memory type first, then choose capacity and speed. If you are still deciding, read about The Complete DDR5 RAM Buying Guide (2026).

Does CAS latency matter for gaming and office work?

For most users, CAS latency has a small effect in daily use. Capacity and a compatible speed matter more: running out of RAM slows a computer far more than a few nanoseconds of latency.

If your system hangs or lags, check capacity first. Check 8GB vs 16GB RAM and How to Choose the Right RAM for Your PC. CL becomes a useful tie-breaker when two modules have the same capacity and speed.

Speed settings can matter more than CL. A 3200MHz DDR4 module on a board that defaults to 2666 or 2933MHz needs XMP/DOCP enabled in the BIOS to reach its rated speed.

Which RAM should you buy? (Foxin options)

Pick the form factor and DDR type your system supports first (UDIMM for desktops, SODIMM for laptops), then match capacity and speed to your work. Here are Foxin RAM models, with the real latency worked out from each CL:

 
The DDR3 and DDR4 modules above all sit at roughly 14 ns of real latency, so CL should not decide that purchase. The DDR5-5600 CL46 modules come in at about 16.4 ns, a couple of nanoseconds higher, but with much more bandwidth. Choose by compatibility first, then capacity (8GB for everyday use, 16GB for heavier multitasking), then speed.

Foxin DDR3 and DDR4 RAM modules go through 100% factory testing and carry a 10-Year Warranty, while Foxin DDR5 RAM modules carry a 5-Year Warranty. Browse everything here:

FAQs

What is a good CAS latency for DDR4?

CL16 at 3200 MT/s is a strong choice at about 10 ns. CL19 at 2666MHz and CL22 at 3200MHz, both about 14 ns, are normal for everyday DDR4 RAM.

Is CL40 bad for DDR5?

No. CL40 is common on DDR5-4800 and DDR5-5600. The higher data rate keeps real latency in a similar range to DDR4.

Can I mix RAM with different CAS latency?

Yes, but the system usually runs all modules at the slowest speed and highest CL. Matching modules is the safer choice.

What is the difference between CAS latency and RAM speed (MHz)?

RAM speed (MT/s or MHz) shows how fast data moves, while CAS latency shows how many clock cycles RAM waits before responding. You need both to judge a module, which is why real latency uses CL and data rate together.

Does lower CAS latency improve gaming FPS?

Usually only slightly, and mostly in games limited by the CPU. Enough capacity (16GB for modern games) and the right speed matter more.

How do I check my RAM's CAS latency?

Open the free CPU-Z tool and look at the Memory tab for current timings, or the SPD tab for each module's rated timings. The label on the module also shows the CL.

Should I choose lower CAS latency or more RAM capacity?

Capacity first. If your system lags or hangs while multitasking, upgrading from 8GB to 16GB helps far more than a lower CL, which works best as a tie-breaker between similar modules.