Fan Power Calculator

Brake horsepower, motor sizing, and system efficiency
Inputs
Results
Air Horsepower
—
AHP (theoretical)
Brake Horsepower
—
BHP (shaft)
Motor Size
—
HP (next standard)
Input Power
—
kW
Full Load Amps
—
A (estimated)
System Efficiency
—
% overall (wire-to-air)
Specific Fan Power
—
W/CFM
Annual Energy
—
kWh/yr (8,760 hrs)
Inputs, results, and method — as an Excel workbook or plain CSV.
Book a Demo →
See this step inside a connected mechanical project — then a 14-day free trial. No credit card.
About This Calculator

This fan power calculator turns airflow and total static pressure into the numbers you need to size a fan and its motor. Enter the airflow in CFM, the total static pressure in inches of water column, and the fan, motor, and drive efficiencies, then pick the supply voltage. The tool returns air horsepower, brake horsepower, the next standard NEMA motor size, input kW, estimated full load amps, overall wire-to-air efficiency, specific fan power, and annual energy.

Fan power is often the largest single electrical load in an air system, so getting brake horsepower and motor selection right drives both first cost and operating cost. The specific fan power (W/CFM) result gives a fast efficiency check, and the calculator flags values that should be verified against · ASHRAE 90.1 fan power limits.

Formula & Method
Air horsepowerAHP = CFM × TSP ÷ 6,356
Brake horsepowerBHP = AHP ÷ ηfan
Input powerkW = BHP × 0.7457 ÷ (ηmotor · ηdrive)
Specific fan powerSFP = kW × 1,000 ÷ CFM  (W/CFM)
Full load amps (3Ø)FLA = kW × 1,000 ÷ (√3 · V · PF)

The constant 6,356 = 33,000 ft·lbf/min per HP ÷ 5.192 lbf/ft² per inch of water column, and it converts CFM × in.w.c. directly into horsepower. The factor 0.7457 converts horsepower to kilowatts. Motor size snaps up to the next standard NEMA rating, and the full-load-amp estimate uses an assumed 0.85 power factor (single-phase drops the √3 term and divides by motor efficiency). These relationships follow standard ASHRAE and NEMA fan and motor practice; verify specific fan power against ASHRAE 90.1 fan power limits.

Frequently Asked Questions
How do you calculate fan brake horsepower (BHP)?
First find air horsepower: AHP = CFM times total static pressure divided by 6,356. Then divide by the fan total efficiency to get brake horsepower: BHP = AHP divided by fan efficiency. For example, 10,000 CFM at 2.5 in.w.c. gives about 3.93 AHP, and at 65 percent fan efficiency that is about 6.05 BHP at the fan shaft.
What does the constant 6,356 mean in the fan power equation?
The constant 6,356 converts airflow in CFM and pressure in inches of water column into horsepower. It comes from 33,000 ft-lbf per minute per horsepower divided by 5.192 lbf per square foot per inch of water column. Dividing CFM times static pressure by 6,356 yields the theoretical air horsepower before any fan or drive losses.
Why is the selected motor larger than the calculated BHP?
Brake horsepower is the power the fan shaft actually needs, but motors are only made in standard NEMA sizes such as 5, 7.5, 10, and 15 HP. This tool selects the next standard size at or above the required shaft horsepower so the motor is not overloaded, which leaves a service margin and accounts for drive losses and real-world operating variation.
What is specific fan power (SFP) and why does it matter?
Specific fan power is the electrical input power per unit of airflow, in watts per CFM. It is a quick efficiency benchmark for a fan system: lower is better. Values above roughly 1.5 W/CFM are flagged here to verify against ASHRAE 90.1 fan power limits, since high SFP usually signals excess static pressure, an undersized duct, or a low-efficiency fan.
Related Tools
Fan Energy Cost Fan Affinity Laws Duct Static Pressure Static Pressure Budget Duct Sizer Duct Leakage Calculator Motor FLA Calculator
AIM Works · launches Oct 15, 2026

Connect the calculation to the rest of the project.

AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.

  • ✓  Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
  • ✓  Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
  • ✓  Design Review that cites the check behind every finding
  • ✓  An AI co-pilot that drafts changes — you approve each one
Book a Demo →
A walkthrough first, then a 14-day free trial. No credit card.
How the calculations work →

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.