Ram Latency - CAS Cycles to Nanoseconds

A ram latency calculator turns the two numbers printed on a memory module — the CAS latency in cycles and the transfer rate in MT/s — into the one number that actually describes how long the DRAM keeps the CPU waiting: true latency in nanoseconds. Use it when comparing a DDR4-3200 CL16 kit against a DDR5-6000 CL30 kit, when reading a motherboard QVL sheet, or when an XMP/EXPO profile lists a CAS value you want to sanity-check against a benchmark.

• • Compare DDR4 and DDR5 by true latency: Enter a DDR4-3200 CL16 module and a DDR5-6000 CL30 module to see whether the higher CAS number on DDR5 hides a smaller nanosecond delay.
• • Read an XMP or EXPO profile: Type the speed grade and CAS from your motherboard's XMP/EXPO table and read the resulting true latency in nanoseconds.
• • Plan an overclock without a benchmark: Enter your current CAS and MT/s, then bump the transfer rate to model a faster kit and see how many nanoseconds you save.
• • Check a manufacturer's spec sheet: Verify whether a kit advertised as 'low latency' actually delivers fewer nanoseconds than a cheaper CL18 alternative at the same transfer rate.

Module labels lead with two figures: a transfer rate like 3200 or 6000, and a CAS latency like 16 or 30. The first tells you how much data the module moves per second; the second tells you how many clock cycles pass between a read command and the first byte on the data pins. Neither alone tells you which module is faster for latency-sensitive workloads.

The fix is to collapse both numbers into the same unit. Multiplying the CAS count by the time of one clock cycle (about 2000 nanoseconds divided by the transfer rate) gives a true latency in nanoseconds any DDR module can be ranked by.

If you also work on serial interfaces and need to convert baud to throughput, the baud rate calculator follows the same cycle-to-time pattern.

The calculator implements one equation used across DRAM datasheets and review sites: true latency in nanoseconds equals CAS latency in cycles multiplied by 2000 and divided by the transfer rate in MT/s. Memory clock and clock period fall out of the same transfer rate and add enough secondary output to explain the result.

• CAS latency (cycles): DRAM clock cycles between a column-read command and the first data word on the bus. Printed on the module as CL16, CL18, CL30, CL36.
• Transfer rate (MT/s): Data transfers per second in millions. For DDR memory the transfer rate is twice the underlying memory clock, so DDR4-3200 is 1600 MHz clock with 3200 MT/s.
• 2000: Converts clock period into nanoseconds. The factor is 2 (DDR transfers per clock) times 1000 (ms to ns).
• Memory clock (MHz): Underlying DRAM clock in MHz. Equal to transfer rate divided by 2 for DDR; used for motherboard QVL compatibility.

The two thousand is not arbitrary. The DRAM clock period in nanoseconds equals 2000 divided by the transfer rate because one transfer takes half a clock cycle on DDR and there are 1000 nanoseconds in a microsecond.

According to Wikipedia: CAS latency, true latency in nanoseconds equals CAS latency in cycles times 2000 divided by the memory transfer rate in MT/s.

According to Crucial: What are memory timings, CAS latency in cycles is the time it takes for a memory module to have data ready at the request of the memory controller, while true latency in nanoseconds accounts for clock cycle time and is the practical speed comparison across DDR generations.

Once you know the per-read latency, sizing the working set with the data storage converter helps decide how much capacity you actually need in DDR5.

Four ideas come up every time RAM latency is discussed. Read these once and the formula and worked examples make sense.

When debugging RAM timings by inspecting module SPD bytes, the ASCII converter makes the hex dump human-readable.

Pick a DDR generation, pick a speed grade, and read true latency. The form defaults to DDR4-3200 CL16 so the result is meaningful before you change anything.

1. 1 Pick the DDR generation: Select DDR3, DDR4, or DDR5 to load the matching list of presets. Choose Custom to type your own CAS and MT/s.
2. 2 Pick the speed-grade preset: Pick a JEDEC, XMP, or EXPO entry such as DDR4-3200 CL16 or DDR5-6000 CL30. The CAS and MT/s fields fill in automatically.
3. 3 Adjust CAS and MT/s if needed: Edit either number to model an overclock or a different bin. The preset is for convenience; the formula runs on the two numbers below.
4. 4 Read true latency in nanoseconds: The big number on the right is the true CAS latency in nanoseconds. Lower is better for random-access workloads.
5. 5 Compare against another module: Change the CAS or transfer rate to model a second kit and read its true latency next to the first.

Comparing a DDR4-3200 CL16 kit with a DDR5-6000 CL30 kit: pick DDR4 / DDR4-3200 CL16 to get 10.00 ns, then change to DDR5 / DDR5-6000 CL30 to get 10.00 ns again. The CAS number doubled, but the transfer rate nearly doubled too, so the wall-clock delay is identical.

The ram latency calculator collapses the two numbers on a memory module into the one number that decides latency-sensitive performance.

• • Skip the CAS to nanosecond division: Enter the cycle count and the transfer rate and read nanoseconds directly instead of dividing 2000 by the transfer rate by hand and multiplying by CAS.
• • Compare DDR4 and DDR5 on the same scale: Compare a DDR4-3200 CL16 kit (10.00 ns) and a DDR5-6000 CL30 kit (10.00 ns) without juggling cycles versus MT/s.
• • Validate an XMP or EXPO profile: Type the speed grade and CAS from a motherboard QVL sheet and confirm the resulting true latency matches a review site's number.
• • Plan an overclock budget: Bump the transfer rate by 200 MT/s to see how many nanoseconds you save, or hold the rate constant and lower the CAS to model a tighter bin.
• • Show the DDR generation tradeoff: Toggle the preset between DDR3-1600 CL11, DDR4-3200 CL16, and DDR5-6000 CL30 to see true latency move from 13.75 ns to 10.00 ns across generations.
• • Translate a CAS label into a wall-clock delay: Read the 'CL18' or 'CL36' label on the box as a real nanosecond figure without leaning on marketing copy.

Pair the per-read latency from this calculator with the upload time calculator to estimate how long a large dataset takes to stream out of system memory.

True latency depends on the cycle count and the transfer rate. A few real-world factors change how meaningful that number is once the module is installed.

• • The formula assumes standard DDR SDRAM and does not model HBM, GDDR, or LPDDR, which use channel parallelism and different timing conventions.
• • True latency covers only the first 64-bit word. Steady-state burst reads are faster because the DRAM pipeline keeps the data pins full after the first word.
• • Validation rejects transfer rates below 100 MT/s and CAS values below 1 cycle. Real DRAM modules run well above these floors.

According to Wikipedia: DDR5 SDRAM, JEDEC DDR5 modules start at DDR5-4800 (CL42) and scale to DDR5-8400 (CL50), with consumer sweet spots at DDR5-6000 CL30 and DDR5-5600 CL36.

Benchmark payloads that get encoded for transport should be measured with the Base64 encoder decoder so the encoding overhead is part of the latency budget.