RAM Latency Calculator

A memory kit’s two numbers — its speed in MT/s and its CAS latency in cycles — speak different languages, and the translation is one multiplication. Enter both and this calculator returns the only latency figure that compares across kits: the true first-word latency in nanoseconds. The results routinely embarrass faster-labelled kits, which is rather the point.

Two Numbers, One Truth in Nanoseconds

The conversion hides in the units. A kit's speed in MT/s counts transfers per second, and DDR transfers twice per clock cycle — so the underlying clock runs at MT/s ÷ 2 megahertz, each cycle lasts 2000 ÷ MT/s nanoseconds, and CAS latency, which is counted in cycles, becomes CL × 2000 ÷ MT/s nanoseconds of actual waiting for the first word of data. That single multiplication is the Rosetta stone of memory marketing: it lets a DDR4-3200 CL16 kit (10.00 ns), a DDR4-3600 CL18 kit (10.00 ns) and a DDR5-6000 CL30 kit (10.00 ns) stand revealed as the same latency at three prices and two generations — while their bandwidth (the speed figure, honestly reported) genuinely doubles across the row. Latency and bandwidth are different goods: latency is how long the first byte takes to arrive; bandwidth is how fast the rest follow. This page prices the first; most kits' boxes advertise the second and whisper the cycles that conceal the first.

Worked Example: The Upgrade That Wasn't (and the One That Is)

The defaults run the shelf's most common pairing: 3200 MT/s at CL16 → cycle time 2000 ÷ 3200 = 0.625 ns; × 16 = 10.00 ns. Now audition the tempting upgrade beside it on the shelf: 3600 CL18 → 2000 ÷ 3600 = 0.556 ns; × 18 = 10.00 ns — identical first-word latency, higher bandwidth, a real but different improvement than the box's bigger numbers imply. And the genuinely faster-latency kit hides at an unglamorous address: 3200 CL14 → 8.75 ns, a 12.5% latency cut at the same speed, sold quietly as a premium bin. The calculator's didactic cruelty is deliberate: run any two kits through it before purchase and the marketing ordering (bigger MT/s wins) is replaced by the physical one — nanoseconds for responsiveness, MT/s for throughput, and no single number entitled to speak for both.

What Latency Actually Governs (and What It Doesn't)

First-word latency matters where work is serial — pointer-chasing code, game engines waiting on one asset before computing the next step, latency-bound database lookups — and modern CPUs fight it with caches precisely because 10 ns, repeated, is an eternity at 5 GHz (roughly 50 wasted cycles per uncached access, before the rest of the timing chain — tRCD, tRP — adds its own verses to the wait). Bandwidth matters where work is broad — streaming media, integrated graphics feeding from system RAM, large model and texture flows. The timings string printed on every kit (CL-tRCD-tRP-tRAS, as in 16-18-18-38) is a small poem of these waits, of which this page translates the first and most quoted line; the others convert by the identical formula with their own cycle counts, and the reader who has run one conversion owns them all.

The Latency That Dare Not Speak Its Name: The Full Access

CAS latency is only the final act of a memory access: before the column can be read, the row must open (tRCD), and if the wrong row stands open, it must first close (tRP) — so a random access to a closed row costs roughly tRCD + CL in nanoseconds (the same conversion, cycle counts added), and a row-conflict access adds tRP before either. At the defaults' 0.625 ns cycle, a typical 16-18-18 string prices a closed-row access near 21 ns — more than double the headline CAS figure — which is why memory controllers keep rows open speculatively and why access patterns (sequential versus scattered) swing real memory performance further than kit timings do. The calculator's single number is the honest headline; this section is the fine print it stands for, and the reader who converts the whole timing string has the full anatomy.

The Controller's Vote: Ratios and Sweet Spots

A kit's nanoseconds reach the CPU through a memory controller with opinions: many platforms run the controller at the memory clock (a 1:1 ratio) only up to a threshold, beyond which a 1:2 mode engages and quietly taxes latency by several nanoseconds — erasing, at a stroke, the advantage of a nominally faster kit whose CL conversion looked superior on paper. This is the origin of "sweet spot" folklore (DDR4-3600 for one generation of Ryzen, DDR5-6000 for the next): the fastest configuration the controller sustains at 1:1, not the fastest kit the shop stocks. The calculator computes the kit's side of the bargain exactly; the platform's side is a motherboard manual away. Buying memory by nanoseconds without checking the ratio is pricing a flight without checking the airport — this page does the first half impeccably and points, here, at the second. The folklore, examined, is unusually respectable: sweet spots are simply the highest MT/s whose nanosecond figure the controller can deliver at 1:1, and they expire with each platform generation precisely because the ratio ceiling, not the DRAM, is what moved.

Related Tools

More from the Other Calculators hub: the Bandwidth Delay Product Calculator — the same latency-versus-bandwidth sermon preached on networks — and the eDPI Calculator for the other hardware number gamers argue about.

Frequently Asked Questions

Is lower CL or higher speed better?

Neither, in isolation — the nanosecond figure this page computes is the arbiter between them, and bandwidth is the separate prize the speed also buys. For latency-sensitive work, sort candidate kits by true latency (CL × 2000 ÷ MT/s); for throughput-sensitive work, by MT/s; for the mixed reality of a gaming or workstation build, weigh both, remembering that platform sweet spots (a memory controller's favoured 1:1 ratio on many AMD systems, for instance) can tax an eccentric kit more than its paper numbers promise. The calculator settles the latency half of every such argument in one line; the bandwidth half is printed on the box honestly, for once.

Why do DDR5 kits have such high CL numbers?

Because their cycles are much shorter: DDR5-6000's cycle lasts 0.333 ns against DDR4-3200's 0.625, so a CL of 30 spends 10.00 ns — the same real wait as DDR4's CL16, counted in twice as many, half-sized ticks. The CL inflation across generations is a units illusion, and it has sold a great deal of unnecessary nostalgia for DDR4's "tighter timings." Compare nanoseconds, always: the calculator exists because the industry's chosen units make the honest comparison just annoying enough that most buyers skip it.

What do the other timing numbers mean?

The string CL-tRCD-tRP-tRAS counts cycles for four operations: CAS latency (column access, this page's number), row-to-column delay (opening the row before the column can be read), row precharge (closing a row), and the minimum time a row must stay open. Each converts to nanoseconds by this page's cycle-time multiplication, and their practical weights differ — tRCD rivals CL in real access patterns, tRAS mostly bounds exotic cases. Kits that tighten only CL while loosening the rest are marketing in the timings' small print; the conversion habit this page teaches reads the whole string.

Does RAM latency affect gaming FPS?

Measurably, in the places games are CPU-bound: open-world streaming, simulation titles and high-refresh competitive play can show single-digit to low-double-digit percentage differences between a loose kit and a tuned one at the same MT/s — with integrated graphics (which borrow the system RAM as video memory) at the far end of sensitivity, where bandwidth joins latency as a co-defendant. The effects are real but second-order next to the GPU and CPU themselves; the calculator's service is proportion — it tells you whether the kit you are eyeing is 10.00 ns or 8.75, so the premium can be weighed in nanoseconds instead of in hope.

Can I just overclock my kit to better latency?

The arithmetic certainly moves: raising MT/s at fixed CL lowers the nanoseconds, and tightening CL at fixed speed does the same — which is the whole playground of memory overclocking, conducted in the BIOS with stability testing as its tax. The calculator prices any target configuration before you attempt it (6400 CL30 would be 9.38 ns — worth knowing before the evening of failed boots begins), and its honesty clause applies: a configuration that computes beautifully and corrupts silently is worse than a slower kit that tells the truth, so every computed improvement is provisional until the memory tests sign it.

What is the fastest true latency I can buy?

The enthusiast shelf bottoms out around 8–9 ns in the DDR4 era's finest bins (3200 CL14 = 8.75 ns; 3800 CL14 ≈ 7.37 ns for the very tightest tuned kits) and around 9–10 ns in DDR5's current bins (7200 CL34 ≈ 9.44 ns) — a remarkably flat frontier across a decade of "faster" memory, which is the calculator note's punchline restated as history: DRAM cell physics improves grudgingly, interfaces improve lavishly, and the nanosecond has been the industry's most conserved quantity. Spend accordingly: on latency where your work is serial, on bandwidth where it is broad, and on neither where the box's largest number is doing the persuading — the nanosecond conversion takes ten seconds and has ended more bad purchases than any benchmark chart ever published.