MT/s vs MHz Explained

Random Access Memory (RAM) is an essential part of your PC. RAM is a short-term memory that holds the data your processor works on in real time. A computer's processor processes data at a frequency of gigahertz (GHz), so it needs the fastest memory possible to provide it with the data it needs to perform its tasks optimally.

RAM speed used to be measured and expressed in megahertz (MHz). However, this measurement is now being reconsidered and could be expressed in megatransfers per second (MT/s).

Megahertz (MHz) is a measure of frequency. Mega means one million, while Hertz is a unit of frequency equal to one cycle per second. In other words, a million cycles per second.

MHz is used to measure the frequency of digital signals in the form of square waves. The peaks indicate the presence of voltage, while the lines at the base indicate the absence of voltage. Computers use these rising and falling voltages to create square waves, which are then converted into binary signals (the famous 0 and 1).

A megatransfer is a unit of data rate in megabytes. One megatransfer is equal to one megabyte. When measured in seconds, it is more natural way to measure the speed of RAM in terms of the amount of data that can be transferred per second.

RAM modules are made up of transistors that act like switches. They work like the ordinary switches you use to turn lights on and off. In electronics, a switch on means 1 and a switch off means 0. These make up all the data that runs on your system.

When we measure RAM speed by frequency, we're measuring how fast those transistors can switch cumulatively per second.

So if your RAM has a speed of 3,600 MHz, you can logically expect its transistors to switch a total of 3,600,000,000 times per second.

When measuring RAM speed using data rate metrics such as MT/s, the frequency or clock speed of the RAM does not necessarily determine how much data it can transfer per second. When we measure RAM speed using data rate, we are measuring the total bandwidth that RAM can transfer in and out of its memory modules. So if your RAM is rated at 3,600 MT/s, you can expect it to transfer a maximum of 28.8 gigabytes of data per second.

As we saw earlier, MHz is a measure of frequency, while MT/s is a measure of data rate. If you look at today's leading RAM manufacturers, such as Samsung, Micron and SK Hynix, you will see that their RAM is all measured in MHz (frequency). While most PC manufacturers agree with this way of advertising RAM speed, it doesn't mean it's the most useful.

When synchronous dynamic random access memory (SDRAM) was first introduced in the early 1990s, the use of frequency measurements such as MHz was the correct way to indicate RAM speed. This was because data was transferred synchronously with the clock speed of the RAM. So if RAM operates at 333MHz, its data rate is the same as its clock speed, i.e. 333MHz.

However, with the introduction of Dual Data Rate (DDR) for RAM, the data rate (MT/s) and frequency (MHz) are no longer synchronized 1:1. DDR is a technology that doubles the data rate of standard SDRAM. By shifting data on the rising and falling edges of a square wave, DDR RAM can transfer twice as much data at the same clock speed.

This does not mean that measuring RAM speed in MHz is wrong, any more than measuring it in MT/s is. Data rate and frequency are both good measures of RAM speed.

It has therefore become necessary to advertise RAM at twice the clock speed in order to make comparisons with SDRAM.

For example, modules advertised as running at 3600 MHz actually refer to the amount of data that DDR RAM can transfer per second (MT/s), not its clock speed (MHz). So instead of 3600 MHz, you would expect 3600 MT/s or 1800 MHz.

The use of MT/s instead of the usual MHz could have also led to further confusion. Therefore, in order to properly communicate the speed and efficiency of the new DDR RAM products, manufacturers should have displayed the figures in MT/s, while continuing to mention the usual MHz to make it easier for the public to understand. Of course, this practice has continued to this day, with MHz still being used instead of MT/s.

Frequency (MHz) and data rate (MT/s) are therefore both good parameters for measuring RAM speed. MHz measures RAM speed in terms of the number of times it can change voltage per second, while MT/s measures RAM speed in terms of the amount of data it can transfer per second.

Although RAM speed is still usually quoted in MHz, the numbers are actually MT/s, so this shouldn't be a problem when comparing modules.