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Fan Brake Horsepower Calculator

Find the shaft power a fan needs from its airflow, pressure and efficiency — and the electricity its motor draws and the heat it adds to the airstream.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Total (or static) pressure across the fan, from the fan curve or measured.

From the fan curve. Must match the pressure you entered (total or static efficiency).

Result

Fan brake horsepower
0.42hp
Motor electrical input
0.368kW
Heat added to the air
1,256BTU/h

0.42 hp at the fan shaft, 0.37 kW from the supply, 1,256 BTU/h into the air.

Air horsepower
0.252hp
Show the working
  1. Brake horsepower = CFM × pressure ÷ (6,356 × fan efficiency)
    2,000 × 0.8 ÷ (6,356 × 0.6) = 0.42 hp
  2. Motor input = bhp × 0.7457 ÷ motor efficiency
    0.368 kW
  3. Heat to air = motor input × 3,412 BTU/h per kW
    1,256 BTU/h

How it is calculated

Brake horsepower is the power delivered to the fan shaft: CFM × pressure ÷ (6,356 × fan efficiency), with pressure in inches of water. The 6,356 converts cubic feet per minute at one inch of water into horsepower. The motor’s electrical input is brake horsepower × 0.7457 kW per hp ÷ motor efficiency.

Fan efficiency comes from the manufacturer’s fan curve at your operating point, and it must match the pressure you use — total efficiency with total pressure, static efficiency with static pressure.

Fan heat

All the energy a fan uses ends up as heat. If the motor sits in the airstream, as it does in most air handlers, the whole electrical input heats the air; if it is outside, only the shaft power does. That heat adds to the cooling load and can be enough to matter in duct design.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Airflow
2,000 CFM
Fan total pressure
0.8 in. w.c.
Fan efficiency
60 %
Motor (and drive) efficiency
85 %
Is the motor in the airstream?
Yes (all motor input becomes heat in the air)

Result

Fan brake horsepower
0.42 hp
Motor electrical input
0.368 kW
Heat added to the air
1,256 BTU/h
Air horsepower
0.252 hp

0.42 hp at the fan shaft, 0.37 kW from the supply, 1,256 BTU/h into the air.

Assumptions

  • 6,356 converts CFM × in. w.c. to horsepower (33,000 ft·lbf/min per hp ÷ 5.19 lbf/ft² per in. w.c.).
  • 1 hp = 745.7 W; 1 W = 3.412 BTU/h (NIST).

Limitations

  • Fan efficiency must be read at the operating point from the manufacturer’s curve; it varies widely along the curve.

Frequently asked questions

What is the difference between air horsepower and brake horsepower?

Air horsepower is the useful work done on the air (CFM × pressure ÷ 6,356). Brake horsepower is what the shaft must supply, which is higher because the fan is not 100 % efficient.

What fan efficiency should I use?

The value from the fan’s performance curve at the airflow and pressure you are working with. It varies a lot along the curve, so a single catalogue figure can mislead.

How do I turn fan heat into a temperature rise?

Divide the heat (BTU/h) by 1.08 × CFM to get the approximate temperature rise of the air in °F.

Sources

  1. Fan System Cheat Sheet (Better Plants) — U.S. DOE Better Buildings Solution Center. Fan brake horsepower = CFM × pressure ÷ (6,356 × fan efficiency); fan affinity laws (flow ∝ speed, head ∝ speed², power ∝ speed³); cites DOE’s Improving Fan System Performance sourcebook.
  2. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

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