Specific Speed Calculator — Which Fan Type Fits Your Duty Point?
Engineers do not guess between axial and centrifugal; they compute the specific speed. It collapses airflow, pressure and rpm into one number that says which impeller geometry will reach its best efficiency at your duty point. Enter the three values and get the fan type, an estimated impeller diameter and the expected peak efficiency.
No signup · Works on mobile · Formulas shown below · Updated 2026-08-23
Specific Speed & Fan Type
Ns = N × √Q ÷ (Δp/ρ)¾ (dimensionless form, ω in rad/s) — shown also in US customary units.
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
What specific speed is
Specific speed is a similarity parameter: every fan of a given geometry, whatever its size, has the same specific speed at its best-efficiency point. So if you compute σ from your duty point and intended rpm, you immediately know which geometry family is "at home" there.
Dimensionless σ = ω × √Q ÷ (Δp/ρ)¾// ω rad/s, Q m³/s, Δp Pa, ρ kg/m³ US customary Ns = N × √CFM ÷ (in. w.g.)¾// rpm, CFM, inches of water; Ns ≈ 52,900 × σ... (same idea, different scale) Specific diameter δ = D × (Δp/ρ)¼ ÷ √Q Cordier line the δ–σ combination that gives peak efficiency; used here to estimate impeller size
σ range
Ns (US) approx.
Fan type
Peak η
Typical use
< 0.15
< 8,000
Regenerative / multistage
—
Aeration, vacuum
0.15–0.4
8,000–20,000
Radial / narrow centrifugal
55–70 %
Burners, conveying
0.4–0.9
20,000–45,000
Backward-curved / airfoil
70–85 %
AHU, plug fans, cabinets
0.9–1.4
45,000–70,000
Forward-curved, mixed-flow
55–70 %
Fan-coils, hoods, duct fans
1.4–2.5
70,000–130,000
Vane-axial / diagonal
65–78 %
In-line, high-pressure axial
2.5–5
130,000–260,000
Tube-axial
60–75 %
Condensers, roof, wall fans
> 5
> 260,000
Propeller
35–55 %
Circulators, tunnel fans
Worked example: condenser vs AHU
Condenser: 8,000 m³/h (2.22 m³/s) at 60 Pa, 900 rpm → ω = 94.2 rad/s, Y = 50 J/kg → σ = 94.2 × 1.49 ÷ 18.8 = 7.5 → propeller/axial. Cordier diameter ≈ 630 mm — which is exactly what condenser makers use.
AHU: 10,000 m³/h (2.78 m³/s) at 800 Pa, 1,800 rpm → ω = 188, Y = 667 → σ = 188 × 1.67 ÷ 131 = 2.4 → mixed-flow / vane-axial; at 1,400 rpm σ = 1.9; at 1,000 rpm σ = 1.3 → backward-curved plug fan of ~560 mm. That is why EC plug fans for this duty run at 1,000–1,300 rpm.
Frequently Asked Questions
What is fan specific speed?
A dimensionless number combining airflow, pressure rise and rotational speed, σ = ω√Q/(Δp/ρ)^¾. Fans of similar geometry share the same specific speed at best efficiency, so the value tells you which impeller type suits a duty point: low σ = centrifugal, high σ = axial.
Axial or centrifugal — how do I decide?
Compute specific speed at the rpm you can use. Below ~1.4 (high pressure relative to flow) choose backward-curved centrifugal; above ~2.5 (high flow, low pressure) choose axial; in between, mixed-flow or forward-curved. If the result lands in a poor-efficiency zone, change the rpm (motor poles or EC speed) and recalculate.
What is the Cordier diagram?
A plot of specific diameter against specific speed on which well-designed fans and pumps cluster along one line. Given σ, the line gives the impeller diameter that reaches peak efficiency — the basis of the size estimate in this tool.
Why do forward-curved fans have poor efficiency?
They achieve pressure by discharging air at high absolute velocity that must then be slowed in the scroll; the kinetic-to-static conversion is lossy. Their strength is very high airflow from a small wheel at low rpm — ideal for compact, quiet fan-coils, range hoods and cross-flow units, not for energy efficiency.
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<a href="https://www.longwellfans.com/resources/fan-specific-speed-calculator/">Fan Specific Speed & Type Selector</a> by LONGWELL Fans
APA: LONGWELL Fans. (2026). Fan Specific Speed & Type Selector. https://www.longwellfans.com/resources/fan-specific-speed-calculator/
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