Short answer: air volume is not air speed. A fan can read faster at the outlet and still move less air, because air volume is speed multiplied by the area the air passes through - and area grows with the square of the outlet diameter. We measured two wearable fans with the same method, and the calculation changes the picture more than the readings do.
The two units we tested sit about 50% apart in price. They are not the same product at two price points, and the test does not show one of them "winning". What it shows is which dimension each unit is built around - and how much of the comparison our own method can actually carry.
This article gives the method, the raw readings, the calculation, and the three places where the method is weak. It does not publish a verdict on either unit, and it does not publish part costs.
The method we used
Both units were measured the same way, on a bench, with a handheld anemometer held above the centre of the outlet at roughly 50 mm. One reading was taken at each of five settings. The outlet diameter was measured with a steel rule across the outlet face.
Air volume was then calculated, not measured directly:
air volume = air speed × outlet area, where outlet area = π × radius². Both are converted to metres so the result lands in cubic metres per second, then multiplied by 60 for cubic metres per minute.
Stating the method matters here, because the numbers below are only meaningful with it. The rest of this article returns to what the method can and cannot support.
The raw readings
Air speed, in metres per second, at each setting:
| Setting | Lower-priced unit (47 mm outlet) | Higher-priced unit (37 mm outlet) |
|---|---|---|
| 1 | 4.43 | 3.30 |
| 2 | 5.26 | 4.12 |
| 3 | 6.07 | 4.67 |
| 4 | 7.09 | 5.56 |
| 5 | 7.76 | 6.92 |
Two things are worth noting. First, the lower-priced unit reads higher at every setting. Second, the gap narrows towards the top: the ratio falls from about 1.34 at setting one to about 1.12 at setting five. The top setting is where the two units are closest, which is consistent with what we have argued before about the top setting being the least useful number on the box.
From air speed to air volume
The outlet faces were measured with a steel rule: 47 mm on the lower-priced unit and 37 mm on the higher-priced one. Entering those, plus each unit's highest air speed, into the formula above:
| Unit | Outlet diameter | Outlet area | Max air speed | Max air volume |
|---|---|---|---|---|
| Lower-priced | 47 mm | 0.001735 m² | 7.76 m/s | ≈ 0.81 m³/min |
| Higher-priced | 37 mm | 0.001075 m² | 6.92 m/s | ≈ 0.45 m³/min |
This is the part of the test that changes the conclusion. Air speed differs by about 12% at the top setting. Outlet area differs by about 61%, because the diameter is about 27% larger and area scales with the square of the diameter. Multiply the two and the air volume ratio is about 1.8.
A 12% difference in the reading becomes an 81% difference in the delivered air. That is why a specification sheet quoting outlet air speed tells you very little on its own - a point we made in more detail in air speed is not air volume.
Where the method is weak
Three limitations are worth stating before any of the numbers above are used for a decision.
- One point, multiplied by the whole area. We read the air speed at the centre of the outlet and multiplied it by the full outlet area. The centre is the fastest point of the profile, so this combination overstates the true air volume. The schematic above shows where the overstatement comes from.
- The overstatement is not the same on both units. One outlet is bladed and the other is not, so the two velocity profiles do not have the same shape. The size of the overstatement therefore differs between them - which means the ratio between the two units is the least reliable number in this article, not the most.
- The diameter carries a tolerance. Reading a steel rule across an outlet gives you about ±1 mm. On a 47 mm outlet that is roughly ±4% on the area; on a 37 mm outlet, roughly ±5%. Quoting the results to three significant figures would claim more precision than the method has, which is why the air volumes above are rounded to two.
One further wording point. The printed cell rating on the higher-priced unit is about 10% higher than on the lower-priced one. We did not run a discharge test, so we call these rated capacities, not measured ones. The distinction matters: a rated figure is what the cell is sold as, and the only way to turn it into a measured figure is a discharge test at a stated rate and cut-off voltage.
What this comparison supports - and what it does not
Supported by the data above:
- The lower-priced unit moves substantially more air under the same measurement conditions - roughly 1.8 times as much at the top setting.
- The higher-priced unit carries a higher printed cell rating, by roughly a tenth.
- Price and airflow are not linked in either direction across these two samples.
Not supported, and we are not claiming it:
- That either unit is simply "better". Each leads on a different dimension, and the two units are not built around the same priority.
- Anything about what either unit costs to make. We did not compare part costs, and we do not publish them.
- Any claim that a specification on either product is overstated. That claim would need a discharge test at a stated rate and cut-off, plus a stated sample size. We have not run one.
- Any generalisation beyond these two units. Two samples is two samples.
What we require before approving a fan
- Airflow measured with a stated method. A stated distance, a stated tolerance on that distance, and either a multi-point traverse across the outlet or a flow hood - not a single centre reading multiplied by the full area.
- Outlet geometry documented. The diameter, the instrument used, and its tolerance, so the area calculation can be reproduced by someone else.
- Capacity as a measured value. A discharge test at a stated rate and cut-off voltage, reported per unit, rather than the printed rating.
- Protection circuit verified. Present, and tested - not merely visible on the board.
- Lot traceability. The cell lot recorded against the production batch, so a field failure can be traced back to a delivery.
If you are sourcing this category, these are the items we ask a supplier to document before a production run is approved, and they are the same items we will check on the shipment. You can see how we structure that on our OEM and ODM process page, or send us a specification to quote against.
Frequently asked questions
Is air volume the same as air speed?
No. Air speed is how fast the air leaves the outlet, in metres per second. Air volume is how much air leaves per unit of time, which is air speed multiplied by the area the air passes through. A narrow outlet can read a high air speed and still move less air than a wider one running slower.
How far from the outlet should the anemometer be held?
Whatever distance you choose has to be stated, and held to a tolerance. We used roughly 50 mm above the centre, which is convenient on a bench but has no stated tolerance and reads the fastest point of the outlet. A larger distance with a multi-point traverse, or a flow hood, gives a figure that is easier to defend and easier to reproduce.
Why does the calculation matter so much?
Because area scales with the square of the diameter. A 27% larger diameter is about 61% more area, so a small difference in the outlet changes the air volume far more than a small difference in the reading. This is why the same air speed figure can describe two very different fans.
Can I compare two fans from their rated battery capacity?
Not reliably. A rated capacity is what the cell is sold as. To compare two products you need a discharge test at the same rate and cut-off voltage, on the same number of samples. Without that, a printed number is a marketing figure rather than a measured one - the same principle we apply to cooling claims on this category.
Does a higher price mean better airflow?
Not in these two samples. Price and airflow are not linked in either direction here, and the more expensive unit leads on a different dimension. Price tells you what a product is positioned as; it does not tell you how it performs. Only a stated test does that. The motor choice inside is a separate question, covered in brushless versus brushed motors.
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