Short answer: air speed (m/s) measures how fast the air leaves the fan at one point. What a wearer feels is how much air actually arrives, and that is speed multiplied by the area the air passes through. On our own bench, two fans measured within about 2% of each other on air speed — and differed by 41% in the air they moved. One of them moved nearly twice as much air as the third unit on the same bench. The m/s figure is not wrong. It is simply incomplete, and it is the number that ends up on the label because it is the cheapest one to produce.
What the m/s figure actually measures
Air speed is read with an anemometer held in the airstream — on our bench, centred about 5 cm in front of the outlet, on each speed setting in turn. It is a point measurement: the probe reads the speed of the air passing through one small area, in one direction.
That has two consequences, and both are worth understanding before you compare two fans:
- Distance and position change the number. Move the probe closer, further away, or off to the side of the airstream, and the reading changes — sometimes dramatically towards the edge, where the airflow falls away. A speed figure quoted without its distance and position cannot be compared with anything, including your own earlier measurements.
- It describes reach, not quantity. A high point speed is a reasonable proxy for how far the air will carry before it slows down. It says nothing about how much air is being moved.
What air volume adds
Air volume is the same measurement multiplied by the cross-sectional area the air passes through. For a round outlet:
air volume = measured air speed × πr²
with the radius in metres, which gives a result in m³/s — multiply by 60 for m³/min. The outlet diameter is not a number you have to take on trust: it can be measured on the unit with a rule.
Here is the same three-unit bench test, with the diameters measured and the air volume computed from each unit's own maximum speed:
| Unit | Outlet diameter | Max air speed | Max air volume |
|---|---|---|---|
| Unit A | 47 mm | 7.76 m/s | 0.808 m³/min |
| Unit B | 37 mm | 6.92 m/s | 0.446 m³/min |
| Unit C | 40 mm | 7.60 m/s | 0.573 m³/min |
Read the two right-hand columns together. Units A and C are within about 2% of each other on air speed — a difference no buyer would ever notice on a spec sheet, and well inside measurement variation. On air volume they are 41% apart. Unit A moves close to twice the air of unit B despite a speed advantage of only about 12%.
Air speed sorts these three units one way. Air volume sorts them another. Only one of those orderings corresponds to what a person wearing the device experiences.
What the calculation assumes — and where it is honest about its limits
It is worth being explicit, because this method is a comparison tool rather than a calibration:
- It treats the outlet as a uniform face. Air speed is not constant across a real outlet — it is highest near the centre and lower towards the rim. Multiplying the centre reading by the full area therefore overstates the absolute air volume. Because the same bias applies to every unit measured the same way, the comparison between units remains valid even though the absolute figure is nominal.
- A partial obstruction shrinks the effective area. On units fitted with a semiconductor cooling plate, the cold plate occupies part of the outlet face, so the area the air can actually pass through is smaller than πr² of the full circle. If you are comparing a cooled unit with a non-cooled one, measure or estimate the free area rather than the full diameter.
- It says nothing about where the air goes. Volume delivered into an open room and volume delivered under a shirt are not the same number. For a wearable device, that difference is the whole product.
None of these limitations make m/s better. They make the point that neither number is sufficient on its own — which is an argument for quoting both, with the method attached, rather than for quoting the convenient one.
Why the label uses m/s anyway
This is not a conspiracy. Air speed is genuinely easier to produce as a specification:
- It needs one instrument and one reading, with no area measurement and no geometry.
- It produces a number in the range a buyer recognises, and a larger number sounds better.
- Air volume requires a diameter — which a buyer can check — and a method — which a buyer can challenge. The convenient figure is the one that cannot be audited.
So the practical rule is not to distrust m/s. It is to treat it as half of a measurement and ask for the other half.
How to compare two fans properly
- Ask for air speed at every setting, not just the maximum. A single maximum tells you what the unit can do at its top speed. It says nothing about the settings a wearer actually uses all day.
- Ask for the outlet diameter, or measure it. It takes a rule and ten seconds, and it is the figure that turns a speed into a quantity.
- Compute the air volume for each setting and compare those, not the speeds.
- Compare the settings that get used, not the one at the top of the range.
- Ask for noise at the same settings, at a stated distance. A volume figure without a corresponding noise figure is a claim about output only — and output is easy to buy.
For buyers sourcing this category, we ask for measured air speed and the computed air volume at each setting, noise at a stated distance, sustained output over a continuous run rather than a spot reading, and the measured battery capacity behind any capacity claim. That list is deliberate: every item on it can be checked by the buyer, which is the only kind of specification worth writing into a purchase order.
What this means when you are the one specifying the product
The same arithmetic runs in the other direction. Air volume depends on two things: how fast the blade spins, and how much area it sweeps. Only one of those is easy, and it is the wrong one.
- Chasing speed on a small blade buys you a number, and costs you noise and heat. Spinning a smaller impeller faster raises m/s and simultaneously raises noise, motor temperature and bearing wear. The m/s figure improves; the product gets worse.
- Blade diameter is a decision you make once, at the start. It is set by the housing, the duct and the position of the cold plate. Changing it later means changing the tooling. Changing the speed of an existing impeller means changing one line of firmware — which is exactly why speed is the knob that gets turned, and exactly why the cheap number wins.
- The two metrics can disagree in your favour. A larger blade with a modest top speed can deliver substantially more air at lower noise than a small high-speed unit. That is a genuinely better product, and it will look worse on a spec sheet that quotes m/s — which is an argument for how you write your own specification, not just for how you read someone else's.
Frequently asked questions
Is a higher air speed always better?
No. Higher speed with the same blade area means more air volume, so it is better in that respect — but it usually costs noise, motor heat and battery life, and it tells you nothing on its own about how much air the unit moves. Compare air volume, then compare noise at the same setting.
How do I measure air volume without a wind tunnel?
Measure air speed at a stated distance in front of the outlet with an anemometer, measure the outlet diameter, and multiply: speed × πr². It gives a nominal figure rather than a calibrated one, but applied the same way to several units it produces a valid comparison — which is usually what the decision needs.
Why do two fans with the same m/s feel different?
Because m/s is a point measurement of speed, not a quantity. Two units can read the same speed at the centre of the outlet while moving very different amounts of air, if their outlets differ in size. Aim, distance and duct design add further differences on top.
Does a bigger blade always move more air?
For the same air speed, yes — volume scales with area, and area scales with the square of diameter. In practice a larger blade usually turns more slowly at the same motor power, so the gain is smaller than the area calculation suggests, and it may lose at the very top of the range. It is still the more efficient direction to design in.
What should a fan spec sheet include?
Air speed at every setting, with the measurement distance and position stated; the outlet diameter; the resulting air volume; noise at a stated distance at the same settings; and the battery capacity behind any capacity claim. A sheet that lists only a maximum air speed has told you the one number that is cheapest to produce.
If you are sourcing in this category and want the airflow requirements written as testable figures, they are part of our OEM/ODM programme. For the same argument applied to a hair dryer — where a 27.7% gap at the lowest fan setting sat behind an identical maximum-airflow claim — see why maximum airflow is the wrong buying metric. Request a quotation with your target market, volume and target airflow window.
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