Fan Motor Calculator — Shaft Power, kW/HP, Amps to kW & Full Load Amps
Turn a duty point into an electrical specification: air power → shaft power → motor rating → full load amps, for AC induction, EC and DC motors. Also works backwards: enter measured amps to estimate the power a fan is actually drawing.
No signup · Works on mobile · Formulas shown below · Updated 2026-08-23
Motor Power & Amps from Duty Point
Wshaft = Q × pt ÷ ηfan; I = Pin ÷ (√3 × V × PF) for three-phase.
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.
From duty point to amps — the chain
Air power Wair = Q [m³/s] × ptotal [Pa] Shaft power Wshaft = Wair ÷ ηfan Motor rating ≥ Wshaft × (1 + margin) → next IEC/NEMA size Electrical input Pin = Wshaft ÷ ηmotor (÷ ηdrive if VSD) Current, 3-phase I = Pin ÷ (√3 × VL-L × PF) Current, 1-phase I = Pin ÷ (V × PF) Current, DC I = Pin ÷ V // Imperial: BHP = CFM × in.w.g. ÷ (6,356 × ηfan)
Two efficiencies matter. Fan efficiency is the aerodynamic conversion — it is highest near the middle of the curve and collapses at the extremes. Motor efficiency is the electrical conversion and falls quickly below 50 % load, which is why over-sizing a motor "to be safe" costs energy for the life of the installation. EC motors hold 80–90 % efficiency down to 20 % load; AC induction motors do not.
Reference: efficiencies, power factor and FLA
Fan type
Peak fan efficiency (static)
Power curve
Airfoil backward-curved centrifugal
75–85 %
Non-overloading
Backward-curved (sheet metal) centrifugal / plug fan
60–75 %
Non-overloading
Forward-curved centrifugal
45–60 %
Overloading — rises with airflow
Vane-axial / large axial
60–75 %
Overloading toward shut-off
Tube-axial, propeller
40–55 %
Overloading toward shut-off
Compact axial (≤ 200 mm), cross-flow
15–40 %
—
Motor
Efficiency
Power factor
Approx. FLA at 400 V 3-ph per kW
IE3 three-phase 0.75 kW
80 %
0.75
1.9 A/kW
IE3 three-phase 7.5 kW
90 %
0.85
1.9 A/kW (≈ 14.5 A)
IE3 three-phase 37 kW
94 %
0.88
1.75 A/kW (≈ 65 A)
Single-phase capacitor-run 0.37 kW @ 230 V
60–70 %
0.7
≈ 3.4 A
External-rotor AC 230 V (typ. 450 mm axial)
55–70 %
0.65–0.75
≈ 1.5 A per 200 W input
EC 230 V with active PFC
85–92 %
0.95–0.99
≈ 4.7 A/kW
Shaded-pole 230 V
15–30 %
0.5–0.6
—
Worked example: AHU plug fan
Given: 8,000 m³/h (2.22 m³/s) at 600 Pa total, backward-curved plug fan η 68 %, IE3 motor η 88 %, PF 0.85, 400 V three-phase.
Air power = 2.22 × 600 = 1,333 W
Shaft power = 1,333 ÷ 0.68 = 1,961 W → +15 % = 2,255 W → choose 3 kW (4 HP) IEC motor; 2.2 kW would run at 89 % load with no margin for filter loading.
Input = 1,961 ÷ 0.88 = 2,228 W
Current = 2,228 ÷ (1.732 × 400 × 0.85) = 3.8 A per phase at duty point; a 3 kW motor's FLA ≈ 6.2 A — set the overload at 6.2 A.
Frequently Asked Questions
How do I calculate fan motor HP from CFM and static pressure?
BHP = CFM × static pressure (in. w.g.) ÷ (6,356 × fan efficiency). Example: 10,000 CFM at 0.5 in. w.g. with 55 % efficiency = 10,000 × 0.5 ÷ (6,356 × 0.55) = 1.43 BHP → choose a 2 HP motor. In SI: kW = m³/s × Pa ÷ (1,000 × efficiency).
How do I calculate full load amps for a fan motor?
Three-phase: I = P ÷ (1.732 × V × PF × efficiency), with P the motor output in watts. Single-phase: I = P ÷ (V × PF × efficiency). A 3 kW, 400 V, 0.85 PF, 88 % efficient motor draws 3,000 ÷ (1.732 × 400 × 0.85 × 0.88) ≈ 5.8 A. The nameplate value always takes precedence.
What service factor or margin should a fan motor have?
10–15 % above the shaft power at the duty point for backward-curved (non-overloading) fans; 25 % or sizing for the maximum possible airflow for forward-curved and axial fans, whose power rises when the system resistance drops (filter removed, door open). EC fans limit their own power electronically and need no margin.
Why does my fan motor trip the overload?
Most often the fan is moving more air than designed because the system has less resistance than assumed — a forward-curved or axial fan then draws more power. Other causes: wrong rotation (draws more and moves less), low voltage, high ambient, or dense cold air at start-up in cold stores.
Can I estimate fan power from a clamp meter reading?
Yes: P = √3 × V × I × PF for three-phase (V × I × PF single-phase). Use a true-RMS meter, measure all three phases, and use the nameplate power factor. The calculator does this when you enter measured current, and flags if the fan is off its design point.
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<a href="https://www.longwellfans.com/resources/fan-motor-sizing-calculator/">Fan Motor Amps & Power Calculator (kW, HP, FLA)</a> by LONGWELL Fans
APA: LONGWELL Fans. (2026). Fan Motor Amps & Power Calculator (kW, HP, FLA). https://www.longwellfans.com/resources/fan-motor-sizing-calculator/
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