Fan Power Calculator

Inputs

Airflow (CFM)

Total Static Pressure (in.w.c.)

Fan Total Efficiency (%)

Motor Efficiency (%)

Drive Efficiency (%)

Voltage (V)

Results

Air Horsepower

—

AHP (theoretical)

Brake Horsepower

—

BHP (shaft)

Motor Size

—

HP (next standard)

Input Power

—

Running Current

—

A at duty point (est., PF 0.85)

Motor FLA (NEC)

—

A, NEC Table 430.250

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.

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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 and its NEC table full-load current, input kW, the estimated running current, overall wire-to-air efficiency, specific fan power, and annual energy at continuous operation.

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 tool screens the fan’s brake horsepower against the ASHRAE 90.1-2022 §6.5.3.1.1 fan-system base allowances so you know early whether the system will need pressure-drop credits.

Formula & Method

Air horsepower	AHP = CFM × TSP ÷ 6,356
Brake horsepower	BHP = AHP ÷ η_fan
Input power	kW = BHP × 0.7457 ÷ (η_motor · η_drive)
Specific fan power	SFP = kW × 1,000 ÷ CFM (W/CFM)
Motor size	next NEMA rating ≥ BHP ÷ η_drive
Running current (3Ø)	I = kW × 1,000 ÷ (√3 · V · PF)
90.1 screen	bhp_allow = CFM × 0.00094 (CV) or × 0.0013 (VAV), + A

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. Because in.w.c. is a true pressure and CFM is actual flow, this BHP is already correct at any elevation — no air-density correction applies. Motor size snaps up to the next standard NEMA rating at or above the motor output the fan and belt need; the “Motor FLA” is the NEC Table 430.250 (three-phase) or 430.248 (single-phase, 115 V column) full-load current for that rating, which NEC 430.6(A)(1) requires for conductor and protection sizing. The running current is an estimate at the duty point from the input power with an assumed 0.85 power factor (single-phase drops the √3). The 90.1 screen applies ASHRAE 90.1-2022 §6.5.3.1.1, Table 6.5.3.1-1 Option 2 base allowances without the pressure-drop credits A of Table 6.5.3.1-2; the limit covers every supply, return/relief and exhaust fan in the system, so treat a single-fan result as a screen.

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 needs; with a belt drive the motor must also supply the belt losses, so the required motor output is BHP divided by the drive efficiency. Motors come only in standard NEMA ratings such as 5, 7.5, 10, and 15 HP, so the tool selects the next standard rating at or above that output, and the step up is the margin. If the motor would run above about 90 percent of nameplate the tool says so, since many designers then take the next size rather than lean on the service factor. For wiring and protection it shows the NEC Table 430.250 full-load current of the selected motor, which is the value NEC 430.6(A)(1) requires, not the running current.

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: a quick efficiency benchmark where lower is better, and high values usually signal excess static pressure, undersized ductwork, or a low-efficiency fan. It is not a code limit. ASHRAE 90.1-2022 section 6.5.3.1.1 limits the brake horsepower of the whole fan system instead, to CFM times 0.00094 plus pressure-drop credits for constant volume or CFM times 0.0013 plus credits for VAV (Option 2); the tool screens this fan against those base allowances and reminds you to add the system’s other fans.

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.

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