CAS Latency vs RAM Speed: Calculate First-Word Latency

Quick answer

CAS latency cannot be compared without memory speed. Calculate approximate first-word CAS latency with CL x 2000 / MT/s. DDR5-6000 CL30 and DDR4-3600 CL18 both calculate to 10 nanoseconds even though DDR5 has the larger CL number.

That formula isolates the CAS portion of an access. A memory benchmark measures the complete platform, so its reported latency will usually be higher than the calculated first-word value.

First-word latency formula

Use the effective DDR transfer rate printed on the memory kit:

First-word latency (ns) = CAS latency x 2000 / data rate (MT/s)

The factor 2000 converts the effective DDR transfer rate into the duration of one memory clock cycle in nanoseconds. Multiplying that cycle time by the CL value gives the approximate time represented by those CAS cycles.

Worked examples:

DDR4-3200 CL16: 16 x 2000 / 3200 = 10 ns
DDR4-3600 CL18: 18 x 2000 / 3600 = 10 ns
DDR5-6000 CL30: 30 x 2000 / 6000 = 10 ns
DDR5-6400 CL32: 32 x 2000 / 6400 = 10 ns

Memory configuration	CAS value	Calculated first-word latency
DDR4-3200 CL16	16	10.0 ns
DDR4-3600 CL16	16	8.9 ns
DDR4-3600 CL18	18	10.0 ns
DDR5-4800 CL40	40	16.7 ns
DDR5-5600 CL36	36	12.9 ns
DDR5-6000 CL30	30	10.0 ns
DDR5-6400 CL32	32	10.0 ns
DDR5-7200 CL34	34	9.4 ns

These values are useful for comparing specifications, but they are not complete measured memory latency.

Why DDR5 has larger CL numbers

Timing values count clock cycles, not a fixed amount of time. DDR5 uses higher clock rates, so each cycle is shorter. A greater number of shorter cycles can equal the same elapsed time as a smaller number of longer DDR4 cycles.

That is why "CL16 is always faster than CL30" is incorrect. DDR4-3200 CL16 and DDR5-6000 CL30 both represent an approximate 10 ns CAS interval, while DDR5-6000 offers a substantially higher theoretical transfer ceiling.

The comparison must include:

Effective transfer rate in MT/s
CAS latency value
Number of active memory channels
Capacity and rank layout
The CPU memory controller and its operating mode

CAS latency is only one memory timing

A kit may be labeled 30-38-38-96. These numbers describe several primary timings:

Timing	Common name	What it represents
tCL	CAS latency	Delay between a column read command and data availability
tRCD	Row-to-column delay	Delay after opening a row before accessing a column
tRP	Row precharge	Time needed to close a row before opening another
tRAS	Row active time	Minimum time a row must remain active

A row hit can avoid some operations that a row miss requires. This is one reason no single timing value describes every access. Secondary timings, refresh activity, command rate, rank behavior, and firmware training can also change measured performance.

Two kits with the same MT/s and CL can therefore produce different results when their remaining timings or platform settings differ.

Bandwidth and latency solve different problems

Bandwidth describes how much data can move over time. Latency describes how long an operation waits before useful data begins returning.

Large sequential transfers are often sensitive to bandwidth.
Small dependent accesses are often more sensitive to latency.
Games can respond to both through average FPS, 1% lows, and frame-time behavior.
Integrated graphics often benefit strongly from available memory bandwidth.
Insufficient capacity is usually worse than a modest timing difference because the system must use slower storage.

Use DDR5 theoretical vs real-world bandwidth to calculate the transfer ceiling separately.

Why benchmark latency is higher than the formula

The first-word formula calculates only the CL interval from the module specifications. A benchmark crosses a longer path that can include the CPU cores, cache hierarchy, integrated memory controller, interconnect, command scheduling, DIMMs, and test software.

Platform settings can add delay through memory-controller ratios or gear modes. Firmware may also select different secondary timings when a memory profile is enabled. It is therefore normal for a benchmark to report a value well above 10 ns for a kit whose calculated CAS interval is 10 ns.

Use this diagnostic table before comparing results:

Observation	What to verify
Calculated ns is unexpectedly high	Confirm the CL value and active MT/s
Benchmark latency is much higher than another system	Compare CPU architecture, controller mode, firmware, timings, and benchmark version
Rated profile lowers speed after reboot	Check training failure, CPU support, DIMM count, and stability
Same CL kit performs differently	Compare secondary timings, rank layout, channel mode, and active data rate
Lower timings cause crashes or errors	Restore stable settings and test one change at a time

Do not compare latency screenshots from different tools as if they use an identical method. Keep the benchmark version and configuration constant.

How to compare two RAM kits

Use this sequence instead of choosing the smallest CL number:

Confirm both kits provide enough capacity for the workload.
Verify that the CPU and motherboard support the intended memory type and profile.
Calculate first-word latency from CL and MT/s.
Calculate theoretical bandwidth at the same channel count.
Compare all primary timings, not only tCL.
Check whether the comparison uses two DIMMs, four DIMMs, or a different rank layout.
Prioritize a stable configuration over an unstable headline setting.
Compare repeatable results in the applications or games you actually use.

Priority	Prefer
Heavy multitasking or creation work	Sufficient capacity first
Large sequential transfers	More bandwidth, if the platform can use it
Latency-sensitive CPU workloads	Balanced MT/s and timing values
Everyday reliability	Validated profile and error-free stability
Future upgrade flexibility	A supported two-DIMM configuration with capacity headroom

Does lower CAS latency improve gaming?

It can, but a fixed FPS claim would be misleading. A CPU-limited game may respond to memory latency and bandwidth, while a GPU-limited test may show little change. The result also depends on the processor, game engine, scene, graphics settings, and channel configuration.

For a useful comparison, record average FPS, 1% low FPS, and frame times. Use the same save or benchmark scene, repeat each run, and report the median rather than the best result. Change only one memory variable at a time.

Frequently asked questions

Is DDR5 CL30 better than DDR4 CL16?

Not from CL alone. DDR5-6000 CL30 and DDR4-3200 CL16 both calculate to a 10 ns CAS interval. DDR5-6000 has more theoretical bandwidth, but the platform and workload determine final performance.

What is good first-word latency for RAM?

There is no universal cutoff. Compare configurations supported by the same platform and consider capacity, stability, bandwidth, and full application performance alongside the calculated value.

Is CL36 slow for DDR5?

The data rate is required to answer. DDR5-5600 CL36 calculates to about 12.9 ns, while DDR5-6000 CL36 calculates to 12 ns. The same CL value represents less time at a higher transfer rate.

Does lower CAS latency improve FPS?

It can help CPU-bound and latency-sensitive workloads, particularly frame-time lows, but the gain varies. Test average FPS, 1% lows, and frame times instead of predicting a fixed percentage from CL alone.

Why is measured RAM latency higher than first-word latency?

The formula covers only the CAS timing interval. A benchmark includes delays from the CPU, cache, memory controller, interconnect, command scheduling, other memory timings, and its own test method.

Can I manually lower the CL number?

Only if the complete system remains stable at the new timing and voltage. Change one setting at a time, stay within safe platform guidance, and run a proper memory stability test before trusting the result.

Sources and calculation method

The first-word values are independently calculated from the published CL and effective MT/s specifications. Timing definitions and DDR5 architecture are checked against the JEDEC DDR5 SDRAM standard and Micron DDR5 technical information. Profile behavior is cross-checked with Intel XMP and AMD EXPO guidance. The calculated CAS interval should not be presented as a complete platform latency measurement.