Fan HP & Energy Cost Calculator

Fan brake horsepower, motor size, annual energy cost, and upper-bound VFD savings.

Fan Inputs
Results
Air HP
—
horsepower
Brake HP (BHP)
—
horsepower
Motor Size
—
HP NEMA
Input kW
—
kilowatts
Annual Energy
—
kWh/yr
Annual Cost
—
$/year
VFD Savings Estimate — ideal cube law (upper bound)
VFD kW at Speed
—
kilowatts
VFD Annual Cost
—
$/year
Annual Savings
—
$/year
Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This calculator turns a fan's airflow and pressure into the numbers that drive equipment selection and operating cost. Enter the airflow in CFM, the total static pressure the fan must develop, and the fan and motor efficiencies, and it returns air horsepower, brake horsepower, the next standard NEMA motor size, electrical input in kilowatts, and the annual energy and cost based on your run hours and energy rate.

It also estimates the savings from a variable frequency drive. Because ideal fan power falls with the cube of speed, slowing the fan cuts energy far faster than it cuts airflow; the tool compares full-speed cost with the cube-law draw at the speed you enter and reports the difference as an upper-bound savings estimate. Use it to sanity-check fan submittals, compare constant-speed and VFD operation, and put a first dollar figure on static-pressure reductions.

Formula & Method
Air HPAHP = CFM × TSP ÷ 6356
Brake HPBHP = AHP ÷ ηfan
Input powerkW = BHP × 0.7457 ÷ ηmotor
Annual costCost = kW × hours × rate
Motor sizenext NEMA rating ≥ BHP
VFD powerkWvfd = kW × (speed)³  (ideal, upper bound)

The constant 6356 = 33,000 ft·lbf/min per hp ÷ 5.192 lbf/ft² per in. w.c. — a unit conversion with no air density in it, so the BHP needs no altitude correction — and 0.7457 is kilowatts per horsepower. The motor is the next standard NEMA horsepower rating at or above the BHP (direct drive; belt losses are not modelled). The VFD estimate applies the ideal fan affinity law, power ∝ speed³, at the speed you enter for all run hours; it is an upper bound, because a duct static-pressure setpoint, drive losses, and part-load motor efficiency all reduce real savings. The tool also reports specific fan power, SFP = kW × 1000 ÷ CFM in W/CFM, as a benchmark only — ASHRAE 90.1 limits fan power by system brake horsepower, not by a W/CFM figure; see the Fan Power Calculator for that screen.

Frequently Asked Questions
How is fan brake horsepower calculated from CFM and static pressure?
Air horsepower is airflow times total static pressure divided by 6,356: AHP = CFM times in.w.c. divided by 6356. Brake horsepower (BHP) is the air horsepower divided by the fan's mechanical efficiency, so a less efficient fan needs more shaft power for the same airflow and pressure. The constant 6,356 is a pure unit conversion: 33,000 ft-lbf per minute per horsepower divided by 5.192 lbf per square foot per inch of water column. It contains no air density, so because inches of water is a true pressure and CFM is the actual airflow, the result is already correct at any elevation.
How does the calculator convert horsepower to kilowatts?
Input electrical power in kilowatts equals brake horsepower times 0.7457 divided by motor efficiency. The 0.7457 is the exact kilowatts per horsepower conversion, and dividing by motor efficiency accounts for losses between the wires and the shaft. Annual energy is that kilowatt draw times the operating hours per year, and annual cost is the energy times your dollar-per-kWh rate.
How much can a VFD save on fan energy?
On a system curve that passes through zero, fan power follows the affinity law and falls with the cube of speed: at 80 percent speed a fan draws about 0.8 cubed, or roughly 51 percent, of full-speed power, a savings near 49 percent. The calculator applies that ideal cube law to the speed you enter for all of the run hours, so treat the result as an upper bound. Real VAV savings are smaller because the duct static-pressure setpoint keeps the system curve from passing through zero, the drive itself loses a few percent, and motor efficiency drops at part load. If the speed varies through the year, average the cube of the speed, not the speed.
What motor size does the tool recommend?
A motor nameplate rates its shaft output, so the tool selects the next standard NEMA horsepower rating at or above the fan brake horsepower, the same rule the AIM Works app uses; the step up to the next rating is the margin. It reports the motor load and notes when it is above about 90 percent, where many designers take the next size up rather than rely on the service factor. Belt-drive losses are not modelled here; use the Fan Power Calculator for a belt drive. Specific fan power in watts per CFM is reported as an efficiency benchmark, not a code limit.
Related Tools
Fan Power Calculator Fan Affinity Law Duct Static Pressure Static Pressure Budget Duct Sizer Motor FLA Calculator Simple Payback & ROI
AIM Works · launches Oct 15, 2026

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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.