RAM latency calculator
First-word access latency, in nanoseconds, for any DDR memory configuration. Enter a data rate in MT/s and a CAS latency in clock cycles; the page recomputes both kits live as you type.
Kit A
Data rate in megatransfers per second. For DDR4-3200, enter 3200.
CAS latency in clock cycles, an integer.
Kit B
Data rate in megatransfers per second. For DDR4-3200, enter 3200.
CAS latency in clock cycles, an integer.
First-word latency
- Δ (B − A)
- +0 ns
- Δ %
- +0%
- Result
- Identical
How this is calculated
First-word latency is the time between the CAS command and the first beat of data on the bus. It is derived from two values published on every kit:
latency_ns = (CL / data_rate_MTps) * 2000Worked example: a DDR5-6000 CL30 kit transfers data at 6000 MT/s with a CAS latency of 30 cycles. Plugging in:(30 / 6000) × 2000 = 10.00 ns. That figure is the first-word access latency only — sustained bandwidth, row-cycle time, and rank/bank topology also affect real-world performance and are not modelled here.
Two kits with very different data rates can land at nearly the same first-word latency. This is why comparing memory by MT/s alone is misleading. For the broader picture, see our guides on RAM timings and RAM latency.
JEDEC reference bins
Standard speed grades from JESD79-4D (DDR4) and JESD79-5C (DDR5). Latencies below are computed with the same formula and rounded to two decimals.
| Speed bin | Data rate (MT/s) | CL | First-word latency (ns) |
|---|---|---|---|
| DDR4-1600 CL11 | 1600 | 11 | 13.75 |
| DDR4-1866 CL13 | 1866 | 13 | 13.93 |
| DDR4-2133 CL15 | 2133 | 15 | 14.06 |
| DDR4-2400 CL17 | 2400 | 17 | 14.17 |
| DDR4-2666 CL19 | 2666 | 19 | 14.25 |
| DDR4-2933 CL21 | 2933 | 21 | 14.32 |
| DDR4-3200 CL22 | 3200 | 22 | 13.75 |
| Speed bin | Data rate (MT/s) | CL | First-word latency (ns) |
|---|---|---|---|
| DDR5-3200 CL22 | 3200 | 22 | 13.75 |
| DDR5-3600 CL26 | 3600 | 26 | 14.44 |
| DDR5-4000 CL28 | 4000 | 28 | 14 |
| DDR5-4400 CL32 | 4400 | 32 | 14.55 |
| DDR5-4800 CL34 | 4800 | 34 | 14.17 |
| DDR5-5200 CL38 | 5200 | 38 | 14.62 |
| DDR5-5600 CL40 | 5600 | 40 | 14.29 |
| DDR5-6000 CL42 | 6000 | 42 | 14 |
| DDR5-6400 CL46 | 6400 | 46 | 14.38 |
| DDR5-6800 CL50 | 6800 | 50 | 14.71 |
| DDR5-7200 CL52 | 7200 | 52 | 14.44 |
| DDR5-7600 CL56 | 7600 | 56 | 14.74 |
| DDR5-8000 CL58 | 8000 | 58 | 14.5 |
| DDR5-8400 CL62 | 8400 | 62 | 14.76 |
| DDR5-8800 CL64 | 8800 | 64 | 14.55 |