RAM latency calculator: CAS to nanoseconds
A bigger CAS number does not always mean slower memory. Enter the speed and CAS latency of your kit, and see the true latency in nanoseconds, then compare it with a second kit.
Why CAS latency alone is misleading
CAS latency is the number of memory clock cycles the memory takes to answer a request. A lower number sounds better, but the length of a cycle depends on the memory speed. Faster memory has shorter cycles, so a higher CAS number at a higher speed can answer just as quickly as a lower number at a lower speed. To compare fairly, convert the cycles to time.
The conversion is simple. Memory transfers data twice per clock cycle, so the real clock is half the data rate. The latency in nanoseconds is the CAS number divided by the real clock in gigahertz, which works out as CAS times 2,000 divided by the data rate in MT/s. A DDR5-6000 kit at CL30 gives 10 nanoseconds, the same as a DDR4-3200 kit at CL16.
CAS latency versus true latency
The same CAS number means different times at different speeds. These examples apply the formula.
| Kit | CAS | True latency |
|---|---|---|
| DDR4-3200 | CL16 | 10.00 ns |
| DDR4-3600 | CL16 | 8.89 ns |
| DDR5-5600 | CL40 | 14.29 ns |
| DDR5-6000 | CL30 | 10.00 ns |
| DDR5-6400 | CL32 | 10.00 ns |
Latency, bandwidth and what games notice
Memory performance has two sides: how quickly it responds, which is latency, and how much data it moves per second, which is bandwidth. Newer memory generations raise bandwidth a great deal while keeping latency in a similar range of ten to fifteen nanoseconds. For gaming, both can matter, but most games are limited by the graphics card, so memory changes usually produce small gains.
The biggest differences show up in processor-limited games at high frame rates, where fast low-latency memory can add a few percent. Use the bottleneck checker to see if your processor limits you, and the RAM bandwidth calculator for the other half.
Choosing between memory kits
If two kits cost about the same, pick the one with the lower true latency at the same capacity, as long as the platform supports the speed. Stability matters more than a number on a box, so check your motherboard and processor memory support list, and enable the memory profile in the firmware so the kit actually runs at its rated speed. Many systems run memory slower than it is rated until a profile is enabled.
Capacity and channel count matter at least as much as small timing differences. See how much you need with the RAM for gaming calculator, and read our processor and memory bottleneck guide.
How to use the rAM latency calculator
- Find the data rate on the kit, for example 6000 for DDR5-6000.
- Find the CAS latency, usually shown as CL30 or as the first of four numbers in the timings.
- Enter both for Kit A and read the true latency.
- Add a second kit to compare two options.
- Choose the kit with the lower latency and enough bandwidth for your needs.
Common mistakes to avoid
- Comparing CAS numbers across different speeds. Compare nanoseconds instead.
- Entering the memory clock in MHz instead of the data rate in MT/s.
- Ignoring bandwidth. Latency is only half of memory performance.
Related tools and guides
Keep going with these: RAM bandwidth calculator, RAM for gaming calculator, CPU and GPU bottleneck checker, PC build cost and wattage summary, processor and memory bottleneck guide.
Frequently asked questions
How do you convert CAS latency to nanoseconds?
Multiply the CAS number by 2,000 and divide by the data rate in MT/s. For DDR5-6000 at CL30 that is 30 times 2,000 divided by 6,000, which equals 10 nanoseconds. The same formula works for any DDR generation as long as you use the data rate.
Is lower CAS latency always better?
Not on its own. A lower number at a lower speed can be slower in real time than a higher number at a higher speed. Compare true latency in nanoseconds, and also consider bandwidth, capacity and whether your system supports the kit at its rated speed.
Does RAM latency matter for gaming?
A little. Most games are limited by the graphics card, so memory changes help only slightly. In processor-limited games at high frame rates, faster memory can add a few percent. Spending on a better graphics card or processor usually gives more.
What is the difference between MT/s and MHz?
DDR memory transfers data twice per clock cycle, so the data rate in MT/s is double the real clock in MHz. A DDR5-6000 kit has a 3,000 MHz clock. Marketing often quotes MHz loosely, so check which figure a listing means.
Do I need to enable XMP or EXPO?
Usually yes. Memory kits run at a safe default speed until you enable their rated profile in the firmware. Enabling XMP or EXPO makes the kit run at its advertised speed and timings, so you actually get the latency and bandwidth you paid for.
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