RPM, what are they and what are they for in fans and hard drives?

Have you seen talk about RPM in computers and don't understand the concept? Even if you know about it outside of computing, which is very likely, you may not be clear about its use in hardware and in what components we can find it. If that is your case, or if you don't know the unit directly, this article is for you.

Table of contents

Introduction: Defining RPM

RPM stands for revolutions per minute. It is a unit of measurement with which, as its name suggests, we measure the number of revolutions at which something rotates, generally an engine, per minute. At a technical level we are talking about a unit that is used to express frequency and angular velocity. On a practical level, we simply talk about how many complete laps are done in each minute.

We can find this unit everywhere, be it the engine of a car, or a washing machine. As you will see, we practically talk about this unit when we are working with motors. In this article we focus on fans and mechanical hard drives, with a special mention to liquid cooling pumps, which are basically the 3 places where we will see this unit in a computer.

Hard Drives: What Does RPM Mean There?

In a mechanical hard drive, there is a motor that spins the platters at a certain speed. Thus, then each head can access the appropriate region to obtain the necessary information. At an intuitive level we can think that this speed has a lot to do with disk performance, and it is.

The speed of the platter or disk cannot be determined directly from the RPM, but we can make an association between this value and the effective speed of the entire unit. In particular, in the consumer market we can find units of 5400rpm or 7200rpm, the latter will be faster. Based on our tests with two Seagate drives, they deliver 88% faster reads and 59% faster writes.

For this reason, when HDDs were recommended for desktops (I speak in the plural because they are already being relegated to mass storage) it was always asked to prioritize 7200rpm models. There, a game will load much faster, within the slowness that characterizes HDDs.

But be careful, since the 5400rpm models also have their market: due to their better sound, lower temperatures, lower energy consumption and greater reliability they are the best option which is usually chosen if the idea is to do storage without more, that is, to have files stored without the need to run complex programs on the HDD that requires decent performance.

It should be noted that, obviously, the SSDs that are the great successors of HDDs and that we already use almost all of us in our laptops and desktop computers do not have any type of RPM since there is no mechanical part inside.

And what do they mean in fans?

In a fan, the meaning of RPM is radically simple, we are not talking about an HDD in which there are several pieces and we have to correctly define what is that turns In fans it is nothing more than the speed of its rotation. It is the same as in an HDD, but obviously we could be wrong and say that it is wrongly the speed of rotation of the head, while in a fan... well, there is no possible doubt.

That being said, how many RPMs do we usually see on fans? What is a lot and what is little? To what extent does it have to do with loudness?

Some people make a direct association between fan RPM and loudness. It is obvious that the same fan will make more noise at higher revs, but I am referring to making comparisons between different fans, or general evaluations. And the truth is that somehow it can be done.

We can't jump right in and say that any 800rpm fan will be quieter than a 1000rpm fan, but there really are statements like “a fan below 400rpm is barely audible”, and they generally hold true.

There are 'other cases' where low revs produce annoying noises until you reach a certain RPM level. We saw this behavior in sources with cheap fans like the Deepcool DQ850-M-V2L that we reviewed on the web.

Of course, we must clarify that the interpretation of the RPM is closely linked to the diameter of the fan. 800rpm is nothing on an 80mm fan, while on a 140mm one it can already be easily audible.

What regulates the RPM of a fan?

Let's continue talking about how the RPM of a fan is regulated. Since, unlike a normal HDD, the rotation rate can vary in many ways.

First let's see what to change to increase or vary the RPM:

And let's continue to look at ways to define how they're going to vary. This is done using voltage curves in the case of DC fans, or duty cycle curves in PWM fans. Basically, a variable is sought, such as the temperature of a CPU fan, and in that curve each temperature value will have a certain voltage or % PWM associated with it:

In this context, automatic curves defined by the motherboard manufacturer can be used, or the user can create their own manually. The same is true for GPU and case fans, while for power supply fans. Just as the speed is regulated, it is clear that the voltage or % PWM can also be set to levels at which the fan will be off.

We give you a good example of this by showing you how the RPM varies according to the % PWM of a NZXT E650 power supply with a PWM fan (photo 1) and according to the voltage applied to the DC fan of a Fractal Design Ion+:

The case of liquid cooling

We are not forgetting about liquid cooling either, where RPM is also a very important metric and not only because of the band of its fans : these systems have a pump that circulates the liquid and keeps the circuit in proper working order.

Obviously, these pumps are very small, and from what we explained to you before it is perfectly normal to see them working at a lot of RPM, usually over 2000.

How to measure the RPM of a PC fan

The most widely used and easiest method to measure the RPM of a fan is to simply check it in a monitoring software.

When software consultation is not available, “physical” methods are often used. The star instrument to use is one of the so-called laser tachometers. A tachometer is designed to measure RPM, in fact the value you can see in a car is from a tachometer.

In the case of PCs, we resorted to those that work by laser. Its working method is very simple: we place a reflective sticker on the fan, and when we point the laser at it, its light bounces, being received by the tachometer itself. Of course, if we think about it, we conclude that light will be received once for each turn of the fan, since the reflective sticker is on a blade. This is how RPM is measured!

The upside of this method is that it's remarkably cheap and works well enough for light usage. For 10-20 euros we can find a fully functional tachometer: