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Free Online Fan Power Calculator

Calculate fan brake power (BHP/kW) and specific fan power from airflow and static pressure. AMCA / ASHRAE 90.1 basis. Free online HVAC tool, no sign-up.

📐 Standard: AMCA / ASHRAE 90.1
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Fan Power & Selection Calculator
Reference: AMCA / ASHRAE 90.1
🌀 HVAC
Calculate fan brake power (BHP/kW) and specific fan power from airflow and static pressure. AMCA / ASHRAE 90.1 basis. Free online HVAC tool, no sign-up.
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ℹ About This Calculator

A fan's power depends on the airflow it moves and the pressure it works against. This calculator returns the fan brake power in kW and BHP, the motor input power and the specific fan power (SFP) from your airflow and total static pressure, following AMCA and ASHRAE 90.1 practice.

Fan power rises directly with both airflow and pressure, so cutting duct friction (lower static pressure) is the cheapest way to save fan energy. Specific Fan Power - watts per litre/second of air - is the key efficiency yardstick that codes like ASHRAE 90.1 and the ASHRAE 90.1 cap. High SFP usually means undersized ducts or a poorly matched fan.

Fan Power Formula

AMCA / ASHRAE 90.1

P_air = Q × ΔP
P_shaft = P_air / η_fan
SFP = P_motor(W) / Q(L/s) P_air = air power, W
Q = airflow, m³/s
ΔP = total static pressure, Pa
η_fan = fan total efficiency (0.5-0.8)
SFP = specific fan power, W per L/s

Worked Example

Example: A fan moves 2 m³/s (2000 L/s) against 400 Pa at 65% fan efficiency. Air power = 2 × 400 = 800 W; shaft power = 800 / 0.65 ≈ 1.23 kW. With a 90% motor, input ≈ 1.37 kW, giving SFP = 1370 / 2000 ≈ 0.68 W per L/s - a good, efficient result well within typical code limits.

Fan Efficiency & Specific Fan Power Limits

Specific Fan Power (SFP) is the headline efficiency metric. Indicative limits and fan efficiencies:

SystemTypical SFP (W per L/s)
Simple supply-only0.5 – 0.8
Central AHU (supply + return)1.0 – 1.5
AHU with heat recovery / filtration1.5 – 2.0
Fan typeTotal efficiency
Forward-curved centrifugal0.55 – 0.65
Backward-curved / airfoil0.70 – 0.82
Axial (ducted)0.60 – 0.75

To lower fan power, reduce system static pressure first (larger ducts, low-loss fittings, clean filters), then choose a fan running near its peak-efficiency point. Fan laws mean a 20% speed cut roughly halves the power.

Indicative values - confirm against the fan curve and the applicable ASHRAE 90.1 SFP limit.

Frequently Asked Questions

How do I calculate fan power from airflow and pressure? +
Air power (W) = airflow in m³/s × static pressure in Pa. Divide by the fan efficiency to get shaft power, then by the motor efficiency for input power. A fan moving 2 m³/s at 400 Pa and 65% efficiency needs about 1.23 kW at the shaft.
What is specific fan power (SFP)? +
SFP is the fan input power in watts divided by the airflow in litres per second - a direct measure of fan-system efficiency. Lower is better. Energy codes such as ASHRAE 90.1 and the ASHRAE 90.1 set maximum SFP values by system type.
What is a good SFP value? +
A simple supply-only fan should achieve about 0.5-0.8 W per L/s; a central AHU with supply and return around 1.0-1.5; systems with heat recovery and heavy filtration up to about 2.0. Values well above these point to undersized ducts or a poorly selected fan.
How does static pressure affect fan power? +
Fan power is directly proportional to static pressure, so halving duct resistance halves the air power. Reducing static pressure through larger ducts and low-loss fittings is usually the most cost-effective way to cut fan energy.
What fan efficiency should I use? +
Backward-curved and airfoil centrifugal fans reach 70-82% total efficiency; forward-curved fans 55-65%; ducted axial fans 60-75%. Use the manufacturer's curve at the actual duty point rather than a peak-catalogue figure.
How do the fan laws affect power? +
Fan power varies with the cube of speed, so a small speed reduction saves a large amount of energy - dropping speed 20% cuts power to about half. This is why variable-speed drives are so effective on variable-air-volume systems.
What is the difference between static and total pressure? +
Static pressure is the pressure the fan develops against duct resistance; total pressure adds the velocity pressure of the moving air. Fan selection usually uses total pressure, but duct sizing works to a static-pressure budget.
What standard governs fan power? +
AMCA standards cover fan performance and rating, while ASHRAE 90.1 and US ASHRAE 90.1 set maximum fan-power and SFP limits for energy efficiency. This calculator gives the power and SFP you then check against those limits.

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