💻

Fan Power Calculator (Air Flow and Pressure)

Fan shaft power and motor size from airflow and total pressure (bhp = cfm × in.wg ÷ 6356η): air power, motor input, specific fan power, yearly energy and running cost.

Units

Air flow (m³/h or cfm)

Fan total pressure (Pa or inH₂O)

Fan efficiency

Motor and drive efficiency

Operating hours per year

Electricity price per kWh

Loading calculator…

How it works

Air power = Q × Δp_total · shaft power = air power ÷ fan efficiency (US: bhp = cfm × inH₂O ÷ (6,356 × η)) · motor input = shaft power ÷ motor efficiency · SFP = motor input ÷ Q · energy = input × hours

A fan delivers useful power equal to the airflow times the total pressure it adds. Dividing by the fan efficiency gives the brake (shaft) power the motor must supply, and dividing again by motor and drive efficiency gives the electricity drawn. Motors are normally chosen at the next standard size above the shaft power with some margin for system uncertainty and belt losses. Specific fan power (SFP), the electrical input per unit of airflow, is how energy codes such as EN 16798 and ASHRAE 90.1 cap ventilation fan energy.

Worked example

Moving 5,000 m³/h (1.389 m³/s) against 500 Pa takes 694 W of air power; at 65% fan efficiency the shaft power is 1.07 kW, and with a 90% motor the input is 1.19 kW, a specific fan power of 0.85 kW/(m³/s). A 1.5 kW motor covers it with margin. Running 4,000 hours a year uses 4,748 kWh, costing $569.80 at 0.12 per kWh.

Sources: Engineering ToolBox — Fan power consumption · AMCA — Fan application manual (fan power and efficiency)

Frequently asked questions

How do I calculate fan horsepower?

In US units, brake horsepower = cfm × total pressure (inches of water) ÷ (6,356 × fan efficiency). In metric, shaft power (W) = flow (m³/s) × pressure (Pa) ÷ efficiency.

Static or total pressure — which should I use?

Total pressure (static + velocity pressure) with the total efficiency, or static pressure with the static efficiency from the fan curve. Mixing the two underestimates the power.

How does fan power change with speed?

By the fan affinity laws, flow rises with speed, pressure with speed² and power with speed³ — cutting speed 20% saves about half the power.

What is a good specific fan power?

For a whole ventilation system, about 1–2 kW/(m³/s) is efficient and over 3 is poor; single exhaust fans can be well under 1. Lower duct pressure drop and efficient EC fans reduce SFP most.

Related calculators

Fan Laws Calculator (Airflow, Static Pressure & Horsepower)

Apply the fan laws: change fan speed and see the new CFM, static pressure and brake horsepower (∝ rpm, rpm², rpm³), find the rpm needed for a target airflow, the pressure and power that follow, the effect of air density, and pulley size to hit the new speed.

Duct Static Pressure Calculator

Add up duct friction and component pressure drops to estimate a system’s total external static pressure and compare it to the blower rating.

Room Ductwork CFM Calculator

Work out how much airflow (CFM) a room’s ducts should deliver from the system tonnage and the room’s share of the conditioned area.

VFD Energy Savings Calculator (Fans & Pumps)

Energy and cost saved by controlling a centrifugal fan or pump with a variable-frequency drive instead of a damper, valve or on/off cycling — affinity-law power at each speed, annual kWh from a duty-cycle profile, drive losses, payback and CO₂.

Duct Size Calculator (CFM & Velocity)

Find the round duct diameter — and equivalent square duct — needed to carry a given airflow at a chosen air velocity.

This calculator is for educational purposes. Double-check important results before acting on them.