Memory Bandwidth Explained - What It Does (and Why MHz Numbers Mislead People)
Memory Bandwidth Explained - What It Does (and Why MHz Numbers Mislead People)
Quick summary if you're in a hurry
It is the rate at which data can be transferred, usually reported as GB/s.
MHz alone is not bandwidth because it only describes a clock, not the whole data path.
In double data rate designs, transfers happen on both clock edges, so the transfer rate can be about 2x the clock.
The same clock can produce different GB/s if the memory interface (bus width in bits) is wider or narrower.
What does memory bandwidth do?
If you have ever stared at a spec sheet and thought, "Higher MHz must mean faster memory," you are not alone.
Official performance guides define bandwidth in a very plain way: it is data-per-second. In other words, it is about how much can flow, not just how fast a signal ticks.
Think of it like a highway. Clock rate is closer to "how often cars are allowed to enter," but bandwidth is the actual throughput after you account for lanes, merges, and the rules of the road.
The biggest misconception: MHz is the whole story
The myth sounds simple: "Memory runs at 2000 MHz, so it must have huge bandwidth." The catch is that MHz is a clock unit, and different docs use it in different ways.
Micron's DDR FAQ makes the split explicit: MHz is the actual clock speed, while MT/s is the transfer rate. Because DDR moves data on the rise and fall of the clock, the transfer rate is commonly about 2x the clock (for example, 3200 MT/s and 1600 MHz refer to the same DDR behavior).
So, when you see only MHz, you still have to ask: is this the clock, or did someone already convert it into an effective transfer rate elsewhere?
How bandwidth is actually calculated
Official CUDA documentation shows a straightforward way to compute peak theoretical memory bandwidth from hardware specs.
At a high level, the idea is: clock (in Hz) times how many bytes fit in one "beat" of the interface, times a factor for double data rate, then convert to GB/s.
In that calculation, you use the memory interface width in bits and divide by 8 to convert bits to bytes. If the memory is double data rate, you multiply by 2 because transfers occur on both clock edges.
A clean mental model
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| From MHz and bus width to GB/s |
Use this as a sanity check:
Bandwidth (GB/s) roughly follows clock_hz x (bus_width_bits/8) x DDR_factor, then divided by 1e9 for GB/s.
If two devices show the same MHz, the one with a wider bus can still have higher GB/s. If two devices show the same bus width, the one with a higher transfer rate can still win. You need both.
AI Image Concept
Prompt: Create a horizontal, concept-focused infographic that explains how memory bandwidth is computed from three parts: clock (MHz), bus width (bits), and DDR factor (2x transfers). Use a left-to-right flow with three labeled blocks feeding into a final "GB/s" output block. Add a small note bubble that clarifies "MHz is clock" and "MT/s is transfer rate" for DDR, with a simple 2x relationship shown as an arrow. Use clean line art, semi-technical diagram feel, soft layered shading, muted desaturated colors, large high-contrast labels, generous margins so nothing touches edges, no logos, no real product shapes, no photos, no 3D, minimal background with subtle grain. Keep all text short and readable on mobile.
alt: Horizontal diagram showing clock, bus width, and DDR factor combining into a final bandwidth value in GB per second.
title: From MHz and bus width to GB/s
Why it matters when you read spec sheets
Here is the practical payoff: if you want to compare memory throughput, you want a number expressed as bytes-per-second, like GB/s, and you want it computed consistently.
CUDA documentation even calls out a common trap: some calculations use 1e9 for "GB," while others use 1024^3 (GiB). If you mix those unit systems, the comparison stops being valid.
So the most honest comparison is boring, but reliable: normalize the unit choice, and compare apples-to-apples GB/s (or GiB/s), not raw MHz.
Limitations and common gotchas
Peak theoretical bandwidth is a ceiling derived from specs. Effective bandwidth is what you get in a real program when you measure bytes moved and divide by time.
Official guidance is direct: when effective bandwidth is much lower than theoretical, design or implementation details are likely reducing bandwidth. That gap is not "mystery physics" - it is a signal that something in the real data movement is falling short of the spec limit.
Another real-world caveat mentioned in official CUDA documentation is ECC overhead on some memory types: enabling ECC can reduce available memory for data and can reduce effective bandwidth, with the impact depending on access patterns. The point is not the exact percentage. The point is that "peak" is not always "what you see."
Jargon decoder
The clock speed; a timing signal, not the full data-per-second outcome.
The transfer rate; for DDR it can be about 2x the clock in MHz.
How many bits move per transfer across the memory interface.
The bandwidth unit; bytes per second after you combine rate and width.
A spec-derived ceiling computed from clock, width, and DDR factor.
What you measure: (bytes read + bytes written) divided by elapsed time.
Wrap-up
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| The highway analogy for bandwidth |
If you remember one thing, make it this: bandwidth is a combined result of transfer rate and interface width, expressed as bytes per second.
MHz can still be useful, but only when you know what it represents (clock vs transfer rate) and you pair it with the bus width and consistent unit math.
That is why two devices can advertise similar MHz and still land in different GB/s ranges. You are not being picky. You are just reading the right number.
Always double-check the latest official documentation before relying on this article for real-world decisions.
Specs, availability, and policies may change.
Please verify details with the most recent official documentation.
For any real hardware or services, follow the official manuals and manufacturer guidelines for safety and durability.
