RAM Latency Calculator
BY TWEAKFA
What does the RAM latency calculator do?
When you’re shopping for RAM, it’s hard to tell what matters more: a higher speed or a lower CL. The TweakFa RAM latency calculator turns both numbers into one figure, the kit’s true latency in nanoseconds, so you can compare kits directly.
Higher speed or lower latency?
When two kits have the same true latency, the faster one is always the better pick. For example, DDR4-3200 CL16 and DDR4-3600 CL18 both work out to 10 ns, so the 3600 kit wins: same response time, more bandwidth. It gets trickier when one kit is faster but also has higher latency. Then you’re trading bandwidth for responsiveness, and which one helps more depends on what you run. Some workloads scale with bandwidth, others with latency.
What is RAM speed, and how does it affect performance?
RAM speed is how many data transfers the memory can make per second. It’s measured in megatransfers per second (MT/s), though many stores and spec sheets still list it as MHz. The higher it is, the faster data moves between the RAM and the CPU, which shows up most in heavy workloads like games and engineering software. Faster RAM doesn’t always mean a faster PC, though: past a certain point your CPU can’t make use of the extra speed, and going higher stops paying off.
How does CL affect RAM latency?
CL, short for CAS latency, is one of the main timings behind RAM latency. It’s the number of clock cycles between the CPU requesting a piece of data and the first bit of that data coming back from the RAM. The lower the CL, the fewer cycles the memory needs to respond. Still, overall performance comes from speed and CL together, and neither one on its own is a complete measure of how fast a kit is.
Does CL alone tell you a kit’s latency?
No. CL is counted in clock cycles, and how long each cycle lasts depends on the speed. True latency, the actual time the RAM takes to respond, depends on both. A kit with a lower CL but a lower speed can take longer to respond than a kit with a higher CL and a higher speed. So always weigh CL and speed together, or let the calculator above do it for you.
What’s the formula for RAM latency?
To calculate the first-word latency of DDR memory, use this formula:
Here’s where it comes from. Dividing one second by the speed gives you the length of a single cycle, 1 s / speed. The top of that fraction is just 1, so it doesn’t show up in the final formula. That accounts for the speed at the bottom; what’s left to explain is the CL and the 2000.
Think of the speed as the number of cycles the RAM completes each second (we’ll correct for DDR in a moment), so one second divided by that number is the length of one cycle. CL is always given as a number of cycles, so multiplying CL by the length of one cycle turns it into a time. That’s why CL sits on top of the fraction. The 2000 is really 2 × 1000. The 1000 is a unit conversion: the speed is given in millions per second (MHz or MT/s), but we want the answer in nanoseconds. The 2 is there because the real clock of DDR memory is half the number on the box. A 3200 kit actually runs at 1600 MHz, and because it’s double data rate (DDR), it moves data twice per cycle. The 2 on top cancels out that doubling.
And that’s all there is to the RAM latency formula.
