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DDR5-4800 CL40 first-word latency is 16.7 ns

DDR5-4800 CL40 is the JEDEC baseline spec every Intel 12th-gen, 13th-gen, and early AMD AM5 platform validates against. Its 16.67 ns first-word latency is actually slightly worse than a tight DDR4-3600 kit on paper, which is why early DDR5 adopters often didn't see an immediate performance jump in latency-sensitive workloads.

First-word latency
16.67 ns
CL40 @ 2400 MHz bus
Row cycle time (tRC)
48.8 ns
tRP + tRAS
Peak bandwidth
38.4 GB/s
Per channel

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RAM Latency Calculator

MT/s
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clk
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Timings are in memory clock cycles. True latency in nanoseconds is derived from the data rate.

True Latency

First-word latency (CL)

16.67 ns

Time from a read command to the first bit of data arriving.

I/O bus clock

2400 MHz

Cycle time

0.417 ns

tRCD delay

16.67 ns

tRP delay

16.67 ns

tRAS

32.08 ns

tRC (tRP + tRAS)

48.75 ns

Peak bandwidth per channel38.4 GB/s

Assumes one 64-bit channel. Dual channel doubles this, quad channel quadruples it.

How this is calculated

At a 2400 MHz bus clock, each cycle is about 0.417 ns — so 40 cycles of CAS latency lands on 16.67 ns. Row cycle time (tRC = tRP + tRAS) comes in around 48.75 ns. Bandwidth is the real headline: 38.4 GB/s per channel means a dual-channel kit delivers 76.8 GB/s of peak memory bandwidth, roughly 50% more than a DDR4-3200 dual-channel setup.

Verdict

DDR5-4800 CL40 is DDR5's entry point — higher bandwidth than DDR4 but worse raw latency. Useful as a price-performance floor on a new build and an easy XMP target to overclock past.

More DDR5 scenarios

DDR5-5600 CL36
CL36 • 5600 MT/s
View details ➜
DDR5-6000 CL30
CL30 • 6000 MT/s
View details ➜
DDR5-6400 CL32
CL32 • 6400 MT/s
View details ➜
DDR5-7200 CL34
CL34 • 7200 MT/s
View details ➜
DDR5-8000 CL38
CL38 • 8000 MT/s
View details ➜

Frequently asked questions

What's the true latency of DDR5-6000 CL30?
True latency = (CL × 2000) ÷ speed = (30 × 2000) ÷ 6000 = 10 ns. That's one of the fastest mainstream kits currently shipping; most DDR5-6000 kits at stock JEDEC timings sit closer to 13-14 ns.
How do CL and MHz affect RAM speed?
Data rate (MHz) determines how much bandwidth you get; CL determines how long you wait for the first bit. Higher MHz + lower CL = more throughput plus faster access. A kit with twice the MHz and equal CL halves the true latency in nanoseconds.
Is DDR5-6000 CL30 better than DDR4-3600 CL16?
DDR5-6000 CL30 is 10 ns true latency; DDR4-3600 CL16 is 8.9 ns — DDR4 is actually slightly lower latency on that comparison. But DDR5-6000 has nearly 67% more bandwidth, which matters far more for modern CPUs, especially AMD Ryzen 7000/9000 and Intel 12th-gen+.
What are tRCD, tRP, and tRAS?
Secondary timings. tRCD is the delay to activate a row before accessing a column; tRP is the time to close a row before opening another; tRAS is the minimum time a row must stay open. All three are measured in cycles; converting to ns uses the same formula as CL.
Does faster RAM help gaming?
On AMD Ryzen and modern Intel platforms, yes — especially in CPU-bound 1080p titles where the gap between DDR5-5200 CL40 and DDR5-6400 CL32 can be 10-20 fps. At higher resolutions the GPU becomes the bottleneck and RAM speed matters less.
What is JEDEC vs XMP/EXPO?
JEDEC is the default SPD profile every DIMM boots with — conservative timings guaranteed to work anywhere. XMP (Intel) and EXPO (AMD) are overclocking profiles stored on the DIMM that you enable in BIOS to unlock the speeds printed on the heatspreader.

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