Calculation sequence
Step-by-step method
- Calculate useful air power from airflow and fan pressure.
- Convert entered fan efficiency from percent to a decimal.
- Divide useful air power by fan efficiency.
MechanicalIntermediateFans
Fan Brake Horsepower Calculator
Estimate required fan shaft power from airflow, fan pressure, and entered fan efficiency.
Why it matters: Required shaft power exceeds useful air power and must be separated from motor nameplate or electrical input power.
Calculated result
Engineering evaluation
Estimated fan shaft power (Wshaft)7.261 bhp
- Useful air power (Wair)
- 4.72 air hp
- Entered fan efficiency (ηf)
- 65 %
Engineering interpretation
At 10000 cfm and 3 in. w.g., an entered 65% fan efficiency requires approximately 7.261 bhp at the fan shaft.
Important limitations
- This result excludes belt, coupling, motor, and VFD losses unless those effects are embedded in the entered efficiency.
Motor-driven equipment
Estimate electrical demand and energy use
Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.Estimated result
Total motor input power (Pin,total)6.21 kW
- Current per motor (I)
- 9.18 A
- Total coincident current (Itotal)
- 9.18 A
- Daily energy (Ed)
- 50 kWh/day
- Annual operating hours (Hy)
- 2,080 h/year
- Annual energy (Ey)
- 12,925 kWh/year
- Annual energy cost (Cy)
- $1,551/year
Equations and assumptions
Nameplate-based screening method
Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.
Technical basis: U.S. Department of Energy — Determining Electric Motor Load and EfficiencyEngineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Wair = QP/6356
Wshaft = Wair/(ηf/100)
Current substitution
Wair = 4.72 air hp; Wshaft = 4.72/(65/100) = 7.261 bhp.
Equation legend
Inputs and calculated quantities
- Q
- Fan airflow — cfm or m³/s
- P
- Fan pressure — in. w.g. or Pa
- ηf
- Fan efficiency — %
- Wshaft
- Estimated fan shaft power — bhp
- Wair
- Useful air power — air hp
- ηf
- Entered fan efficiency — %
Applications
How this calculation is used
Building design
Screen shaft-power demand before manufacturer selection.
Construction
Check whether measured airflow and pressure are consistent with the installed drive and motor.
Commissioning
Estimate shaft power at the tested condition using an explicitly entered efficiency.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe the same stable operating condition.
- Units, pressure definitions, fluid properties, and equipment boundaries are applied consistently.
Limitations
- This tool supports preliminary evaluation and field verification; it does not replace manufacturer curves or project-specific engineering analysis.
- Project criteria, applicable standards, measurement uncertainty, and authorized professional judgment remain controlling.
Field use
Verification procedure
- Confirm the approved reference condition, instrument range, and calculation boundary.
- Stabilize the system and record calibrated field measurements.
- Compare the result with current manufacturer data and project requirements before disposition.
Common engineering mistakes
- Mixing measurements from different operating conditions or equipment boundaries.
- Treating a calculated screening status as automatic acceptance.
Technical basis
References and source standards
- U.S. Department of Energy—Improving Fan System PerformanceFederal sourcebook covering fan-system relationships, fan laws, power, efficiency, system curves, and field performance considerations.
- ASHRAE Handbook—FundamentalsCurrent handbook chapters covering fluid flow, pressure relationships, system curves, and mechanical-system fundamentals; the licensed edition must be verified during professional review.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement