Do you have an unidentified fan that you don’t know the size of? Or maybe you have a case that needs fans, and you don’t know what fan sizes it takes? If so, then this guide on how to measure PC fan size is for you. It’s a simple task and one that you can master in no time at all. Let’s get started.
How to Measure a Case Fan
There are two approaches to measuring a case fan, depending on the situation you find yourself in. If you simply need to determine how large a case fan is, measure the case fan edge to edge along either side. The measurement in millimeters is the size of the fan.
A 140 mm fan measures 14 cm edge-to-edge. Source: Voltcave
This is as straightforward as it comes. Once you have that measurement, you can use it to look for identically-sized case fans to complement the one(s) you already have.
However, that won’t work if you need to buy fans and all you have to go on is the mounting holes on a case. In that situation, you want to measure the distance between the fan mounting screw holes to determine the case fan size you need. This measurement is taken horizontally or vertically along the edges, not diagonally between the corners.
140 mm fan mounts measure roughly 12.5 cm hole-to-hole. Source: Voltcave
Note that the screw hole spacing is slightly smaller than the fan size, and you’ll need to know what spacing matches up to what fan size. Thankfully, screw hole spacing is standardized, so you can figure that out quickly.
This table should help you figure out what size computer fan you need based on the hole spacing:
| Fan Size | Screw Hole Spacing |
| 40 mm | 32 mm |
| 50 mm | 40 mm |
| 60 mm | 50 mm |
| 70 mm | 60 mm |
| 80 mm | 71.5 mm |
| 92 mm | 82.5 mm |
| 120 mm | 105 mm |
| 140 mm | 124.5 mm |
| 200 mm | 154 mm |
| 220 mm | 170 mm |
If you can’t get hold of a spec sheet for your case, then this is the next best way of figuring out what case fans you need.
Of course, this screw holes method also works for measuring PC fans themselves. You likely won’t use it often, but it can be helpful if you have a fan with a round frame like the Noctua NF-A15 PWM. These fans sometimes feature smaller mounting hole spacing than their size might suggest, making edge-to-edge measurements less useful. The Noctua fan, for example, has 120 mm screw spacing despite being a 140 mm fan.
So, now that you know how to measure PC fan size, let’s quickly run through a few key specs that will help you find a suitable fan for your needs.
Airflow and Static Pressure
There are two metrics by which manufacturers measure a case fan’s performance: airflow and static pressure.
Airflow, measured in CFM (and occasionally in m³/h), refers to how much air a case fan can move over a given period. The best airflow fans will tend to boast more than 70 CFM. Static pressure, generally measured in mm-H2O, is how much air pressure the fan can produce. A fan with anything above two mm-H2O is a good static pressure fan.
Most fans excel at one or the other, although a few boast good numbers for both. Generally, airflow fans work best in situations without any airflow obstructions. So they’re great as exhaust fans, for example.
On the other hand, static pressure fans do their best work when pulling (or pushing) air through radiators, dust filters, and other such restrictions. They also tend to be more versatile, as high static pressure requires a relatively high CFM as well.
Noctua’s NF-A14 PWM, for example, has high airflow (82.5 CFM) and good static pressure (2.08 mm-H2O). This makes it, and other high-quality static pressure fans, a solid all-around choice.
However, note that manufacturers measure both of these metrics differently, so the numbers aren’t directly comparable. Treat them as general indicators of performance and not absolute measurements. Unfortunately, airflow and static pressure can’t really be measured at home, so you’re generally stuck with the manufacturer’s numbers here.
Noise
Airflow and pressure aren’t the only important aspects of a case fan’s performance. Its noise output is also crucial, especially if you’re concerned with how loud your rig is. Fan noise is usually measured in dBA, which is a way of evaluating loudness that takes into account how the human ear perceives noise and loudness.
Fan manufacturers will list their fans’ noise output at 100% fan speed, with most fans outputting between 30 to 40 dBA of noise at full tilt. However, you can find silence-focused PC fans that measure 25 dBA or less, which will be perfect for a quiet rig.
Source: American Academy of Audiology
According to the American Academy of Audiology, 30 dBA is the same volume as a whisper. So look for fans that produce less than 30 dBA of noise if you’re after a quiet fan. If you’re crazy about silence, you can opt for fans that make less than 15 dBA at maximum fan speed, like the Noctua NF-P14S redux-900.
However, that low-noise running comes at the cost of significantly reduced cooling performance. As with so many PC components, fan choice comes down to a balancing act between noise and performance.
Measuring Noise
If you’re interested in measuring your fans’ noise level, you’ll first want a decent decibel meter such as this Protmex unit. Then you just turn your PC on and point your decibel meter at your fans, usually at a distance of one to two feet.
Note that ambient noise and your room’s noise floor will affect the decibel meter’s readings. In fact, you may find that your fans just aren’t loud enough to be heard over the noise coming in through your window or the hum of your A/C unit. Most decibel meters also can’t measure below 30 dB(A), so they won’t help you if you have ultra-quiet fans that you want to measure.
So while it’s possible to measure your fans’ noise, it’s not something you should ever need to do. Unless you’re trying to start a hardware review channel, that is!
Closing Thoughts
So there you have it: how to measure PC fan size. Between both of these methods, you should be able to resolve any case fan-related issues swiftly. It’s not the most glamorous PC building tip you’ll ever learn, but it’s definitely worth knowing and may come in handy once in a while.
Curious to learn about the different fan size options available? Check out our guide explaining all of the common case fan sizes.
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