Specific speed is a single number that tells you which family of fan suits a duty, before you look at any product.
It combines airflow, pressure rise and rotational speed into a dimensionless value, and each range of that value points at a different impeller type.
Specific speed combines rotational speed, airflow and pressure into one dimensionless number. Low values point to radial and high-pressure centrifugal wheels. Medium values suit backward-curved and airfoil wheels. High values suit mixed-flow and axial fans. Calculating it takes a minute and rules out whole families before you open a catalogue.
What the number represents
Think of it as the shape of the duty rather than its size.
A job asking for high pressure and little air has a low specific speed, so it needs a wheel that works like a pump. A job asking for lots of air at almost no pressure sits high on the scale, so it needs something closer to a propeller.
Because the number has no units, two fans of very different sizes doing similar work share the same value. So it sorts machines by how they behave rather than by how big they are.
Calculating it
The maths is short. You need speed in radians per second, flow in cubic metres per second, and specific energy in joules per kilogram.
- First, turn rpm into radians per second by multiplying by 0.1047.
- Then divide m³/h by 3,600 to get cubic metres per second.
- Divide pressure in pascals by air density to get specific energy.
- Take the square root of that flow figure.
- Raise the specific energy to the power of 0.75.
- Finally, divide speed times root flow by the result.
Several versions exist with different units, so always check which one a chart uses before you compare values.
What each range points to
| Specific speed | Duty character | Suitable fan type |
|---|---|---|
| Very low | High pressure, low flow | Radial, regenerative blower |
| Low | Medium-high pressure | Radial tipped, forward-curved |
| Medium | Balanced pressure and flow | Backward-curved, airfoil |
| High | High flow, modest pressure | Mixed flow |
| Very high | Very high flow, low pressure | Axial, propeller |
Notice that this order matches the rules of thumb people already use. Specific speed simply puts numbers on them.
Why it is useful in practice
The number catches a badly matched choice before anybody wastes time on model numbers.
- Whole families get ruled out at once, rather than one product at a time.
- Duties that will never be efficient on a given type show up immediately.
- A speed change that would move you into a better family becomes obvious.
- You can explain why a fan is inefficient even though it meets the duty.
- Two suppliers proposing different fan types become comparable.
- Finally, duties that really need two stages stop looking like single-stage jobs.
Our centrifugal against axial guide covers the same decision in practical terms.
The speed lever
Speed sits on top of the fraction, so changing it moves the duty between families.
A job that looks like radial territory at 900 rpm may sit happily in backward-curved territory at 2,900 rpm. Meanwhile the mechanical side of that speed jump, from bearing life to noise, still has to be checked on its own.
Calculate specific speed at two or three candidate speeds. It often shows that a smaller, faster fan is a much better aerodynamic fit than a large, slow one, or the reverse.
Limitations worth knowing
Specific speed narrows the field, although it never picks a product for you.
It says nothing about dust, space, noise limits or cost. Meanwhile it assumes a single stage, so multi-stage and regenerative designs need their own treatment.
For formal definitions and fan classifications, AMCA publications set out the framework used across the industry.
Specific speed FAQ
Do I need to calculate specific speed for every project?
Not for routine duties where the fan type is obvious. It earns its keep on unusual duties, on high-pressure work and when comparing very different proposals.
Why do different sources give different numbers?
Because several unit conventions exist. Some use imperial units and rpm directly, others use the dimensionless form. Always confirm which convention a chart assumes.
Does specific speed predict efficiency?
Indirectly. Each fan family has a specific speed range where it performs best, so a duty far from that range will be inefficient whatever the product quality.
What about mixed-flow fans?
They sit between axial and centrifugal on the scale, which is exactly why they bridge duties that neither family handles comfortably. See mixed-flow fans.
Specific speed answers which family before you argue about which model. One calculation removes most of the wrong options in a minute.
