DDR Memory Speed and CAS Latency Explained: MT/s, MHz, CL, and Nanoseconds

Memory specifications commonly show a DDR data rate in megatransfers per second, written MT/s. Kingston describes RAM speed or data rate in MT/s, while Micron publishes DDR5 parts with separate speed-frequency and MT/s fields. Those are related values, but they are not interchangeable labels for the same quantity.

For the timing relationship used here, MHz describes the underlying memory-bus clock frequency and MT/s describes the effective DDR transfer rate. Keeping those units separate avoids the common mistake of treating a 6000 MT/s DDR data rate as a literal 6000 MHz memory clock.

Kingston technical support states that DDR data rate is twice the memory-bus frequency. That is why a 6000 MT/s data rate corresponds to a 3000 MHz underlying memory clock for this CAS timing conversion, and why a 5600 MT/s data rate corresponds to 2800 MHz.

Micron current DDR5 module and component listings independently show the same relationship in real specifications. For example, its 5600 MT/s DDR5 listings pair that data rate with a 2800 MHz speed field, rather than a 5600 MHz clock field.

CAS latency, commonly written CL or tCL, is a timing expressed as a number of clock cycles. Kingston describes CL as the number of cycles between the read command and the point at which the requested data becomes available, and its product guidance pairs CL values with the memory data rate.

A CL number therefore does not have one fixed duration in nanoseconds on its own. The duration of each clock cycle changes with the underlying clock frequency, so the same CL value can represent a different amount of time at a different DDR data rate.

Start with the DDR data rate in MT/s and divide by two to recover the underlying clock frequency in MHz. A clock running at that many megahertz has a period of 1000 divided by the clock frequency in nanoseconds. Multiply that period by the CL cycle count to get the time represented by the CAS timing.

Combining those steps gives a compact relationship: CAS timing in nanoseconds = 2000 × CL ÷ MT/s. Kingston publishes the same conversion relationship. For example, 6000 MT/s with CL30 corresponds to 10 ns of CAS timing because the underlying clock is 3000 MHz, one cycle is about 0.333 ns, and 30 cycles take about 10 ns.

Because CL counts cycles rather than nanoseconds, keeping CL constant while increasing the DDR data rate makes each cycle shorter. A hypothetical CL30 timing at 6000 MT/s represents 10 ns, while CL30 at a lower data rate would represent a longer interval because the underlying clock period is longer.

That is why comparing CL numbers alone can be misleading. A lower CL number is fewer cycles, but the duration of those cycles still depends on the clock associated with the selected DDR data rate.

Micron current DDR5 listings show that higher data-rate parts can also use higher CL cycle counts. Converting both parts of the specification into time is what makes the relationship comparable. Kingston likewise gives examples where different MT/s and CL combinations produce similar nanosecond timing.

This does not mean two memory configurations with similar CAS timing will perform identically. It only means the specific tCL portion of the timing can occupy a similar amount of time after the cycle count and clock period are combined.

A complete memory access can involve other DRAM timings such as row-to-column delay and precharge timing, along with memory-controller scheduling, rank and channel topology, command behavior, access patterns, CPU caches, and platform-specific effects. Kingston itself lists multiple primary timings in addition to CL.

For that reason, the nanosecond value derived from CL should be called CAS or tCL timing, not complete RAM latency, CPU latency, bandwidth, FPS, or application responsiveness. Core Tech Tips uses the DDR Memory Latency Calculator only to convert the CL portion of the specification into time on a consistent scale.