How this instrument works
CAS latency (CL) is specified in clock cycles, not time — a DDR4-3200 CL16 module and a DDR4-2400 CL16 module share the same CL number but not the same real-world delay, because a clock cycle at 3200 MT/s is shorter than one at 2400 MT/s. Converting CL into an actual nanosecond figure requires factoring in the memory's data rate as well as its CL rating, which is exactly what this calculator does.
The formula, real latency (ns) = (CAS latency / data rate) x 2000, uses a factor of 2000 rather than 1000 because DDR memory transfers data on both the rising and falling edge of the clock — 'double data rate' is what the DDR in DDR4 and DDR5 stands for. That means the clock frequency itself runs at half the quoted MT/s data rate, so converting cycles to nanoseconds needs an extra factor of 2 folded into the usual cycles-to-time conversion.
This real latency figure, sometimes called 'true' or 'absolute' latency, is the number that actually determines how long a memory access takes in wall-clock time — a lower CL at a higher data rate can still mean a lower absolute latency than a higher CL at a lower data rate, which is why comparing CL numbers alone across different memory speeds is misleading. It's the same conversion behind the memory-timing calculators published by RAM manufacturers on their own kit spec sheets.
- Enter CAS latency (CL, clock cycles) — the CL number from the memory kit's spec, for example 16.
- Enter Memory data rate (MT/s, e.g. DDR4-3200 = 3200) — the data rate implied by the kit's model number; DDR4-3200 means 3200, DDR5-6000 means 6000.
- Read Real CAS latency (nanoseconds) directly — this is the actual time in nanoseconds the memory takes to respond, accounting for both CL and data rate together.
- Use this figure, not the raw CL number, when comparing memory kits running at different data rates — a lower CL at a slower data rate can still be a higher real latency than a higher CL at a faster data rate.
- CAS latency cannot be negative, and data rate must be greater than zero; the instrument will prompt you if either falls outside that range.
Worked example — DDR4-3200 CL16
Enter CAS latency (CL, clock cycles) = 16 and Memory data rate (MT/s, e.g. DDR4-3200 = 3200) = 3200 — a common consumer DDR4-3200 memory spec. Dividing gives 16 / 3200 = 0.005, and multiplying by 2000 gives 10. Real CAS latency (nanoseconds) reads 10.0 ns.
That 10 ns figure is the widely-published real latency for DDR4-3200 CL16, the exact number shown by memory manufacturers' own timing calculators for this common kit spec. For comparison, DDR4-3200 at the JEDEC-standard CL19 timing works out higher, at (19/3200) x 2000 = 11.875 ns — a higher CL rating at the same data rate translates directly into a longer real-world latency.
Questions
Why does the formula multiply by 2000 instead of 1000?
Because DDR ('double data rate') memory transfers data on both the rising and falling edge of its clock signal, so the clock actually runs at half the quoted MT/s data rate. Converting clock cycles to nanoseconds normally involves a factor of 1000 (for MHz to ns), but since the real clock frequency here is data rate ÷ 2, that extra halving flips into a doubling elsewhere in the formula, giving the combined factor of 2000.
Can a memory kit with a higher CL number actually be faster?
Yes — real latency depends on both CL and data rate together, not CL alone. A DDR5-6000 CL36 kit works out to (36/6000) x 2000 = 12.0 ns, while a DDR4-3200 CL16 kit works out to 10.0 ns; despite CL36 being more than double CL16, the much higher DDR5 data rate means the two kits' real-world latencies are in the same ballpark, and plenty of high-data-rate, high-CL kits beat lower-data-rate, low-CL ones on absolute latency.
What's the difference between CAS latency and real (absolute) latency?
CAS latency is a count of clock cycles — a unitless number that only means something once you know the clock speed it's measured against. Real (or absolute) latency converts that cycle count into actual nanoseconds using the memory's data rate, giving a wall-clock time that can be compared directly and fairly across memory kits running at different speeds, which the raw CL number alone cannot do.
Why does the DDR4-3200 CL16 result come out to a clean 10.0 ns?
Because 16 divided by 3200 gives exactly 0.005, and 0.005 x 2000 is exactly 10 with no rounding involved — DDR4-3200 CL16 happens to be a widely used, well-documented consumer spec where the arithmetic lands on a clean number, which is part of why it's frequently cited as a reference example in memory-timing discussions.
Does a lower real CAS latency always mean noticeably faster performance?
It means a shorter individual memory access time, but real-world system performance depends on many other factors too — memory bandwidth, the specific workload, CPU architecture, and other timing parameters beyond CAS latency alone. Real latency is a useful, fair way to compare CL ratings across different data rates, but it's one input among several that determine overall memory performance, not the whole picture by itself.