RAM Latency Calculator (CAS latency in nanoseconds for DDR4 & DDR5)

Understanding the RAM latency calculator — concept and principle

Memory kits are labelled with a data rate (such as DDR4-3200) and a CAS latency (such as CL16). A higher CL on a faster kit does not necessarily mean slower memory. This calculator converts CAS latency from clock cycles into nanoseconds so you can compare kits directly, and shows the clock cycle time and memory clock.

More cycles at a faster clock: CL36 at 6000 still takes 12 ns versus 10 ns.

How it is calculated

Memory clock (MHz) = data rate (MT/s) ÷ 2

Clock cycle time tCK (ns) = 2,000 ÷ data rate

First-word latency (ns) = CL × tCK = CL × 2,000 ÷ data rate

Difference = second kit latency − first kit latency

The principle

DDR memory transfers data on both the rising and falling edges of the clock, so the clock frequency is half the data rate. CAS latency counts how many clock cycles pass between a read command and the first data. Because a faster kit has shorter clock cycles, the same number of cycles takes less time. Multiplying CL by the cycle time gives a real time in nanoseconds that is comparable across speeds. Memory datasheets list this as tAA.

Worked example

Default inputs: kit 1 DDR4-3200 CL16; kit 2 6,000 MT/s CL36.

Memory clock = 3,200 ÷ 2 = 1,600 MHz; tCK = 2,000 ÷ 3,200 = 0.625 ns.
Kit 1 latency = 16 × 0.625 = 10 ns.
Kit 2 latency = 36 × 2,000 ÷ 6,000 = 12 ns.
Difference = 12 − 10 = 2 ns.

Each point is the calculator re-run with only “Data rate (MT/s)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Data rate (MT/s)	First-word latency (ns)
800	40
1,600	20
2,400	13.33
3,200	10
4,000	8
4,800	6.67
6,400	5

How to read the result

Lower first-word latency means the first data arrives sooner, but faster kits also deliver much more bandwidth, which often matters more for real performance. Here the DDR5-6000 kit has slightly higher latency but nearly twice the data rate. Whole-system memory latency measured by benchmarks is much longer than CAS latency alone, because it includes other timings and the memory controller.

Common mistakes

Comparing CL numbers across different speeds without converting to nanoseconds.
Treating MT/s as MHz; the clock is half the data rate.
Judging memory by CAS latency alone and ignoring bandwidth and other timings.
Assuming advertised speeds run automatically; XMP or EXPO profiles may need enabling.

Keep in mind: Real performance depends on the processor, motherboard and workload. Check your motherboard's supported memory list before buying.

Frequently asked questions

Is CL16 better than CL18?

Only at the same data rate; across speeds compare the nanosecond value.

Why is the memory clock half the data rate?

DDR transfers data twice per clock cycle.

What is tAA?

The datasheet name for the time from a read command to the first data, CL × tCK.

Is DDR5 slower than DDR4 because of higher CL?

Not necessarily; its higher data rate offsets the higher cycle count and adds bandwidth.

Sources: Micron Technology, "16Gb: x4, x8, x16 DDR4 SDRAM" datasheet, Rev. H (2021), Key Timing Parameters (DDR4-3200: tCK 0.625 ns)

Checked 9 Nov 2026. Micron Technology, “16Gb: x4, x8, x16 DDR4 SDRAM” datasheet, Rev. H (08/2021), Table 1 Key Timing Parameters (-062E: DDR4-3200, CL 22, tCK 0.625 ns, tAA 13.75 ns); double data rate: two transfers per clock.

Your numbers