Fan Calculations

How to Calculate Fan Airflow, Static Pressure Power, and Motor Size Step by Step

A worked walkthrough of the five formulas that turn a fan duty point into a buildable machine: airflow, static pressure power, motor horsepower, the fan laws, and balance tolerance.

Every fan calculation starts from a duty point: a required airflow at a given static pressure. Airflow is volumetric, in cubic feet per minute, and it is rarely the bare catalog number. Take a 12,000 CFM design requirement, apply a 1.15 correction multiplier for altitude, elevated temperature, and filter loading, and the Airflow CFM tool returns 13,800 CFM as your selection basis. If the requirement was given as standard CFM at sea level and 70 degrees F, bake the density ratio into that multiplier so the corrected figure reflects actual conditions. Size the fan to 13,800 CFM, not 12,000, or the first dirty filter pushes you off the curve.

The air horsepower behind that flow comes from AHP equals CFM times static pressure in inches of water column, divided by 6,356. At 13,800 CFM and 6 inches w.c., air horsepower is 13,800 times 6 divided by 6,356, which is 13.03 AHP. That is the ideal power in the airstream, not the shaft power. Divide by fan static efficiency to get brake horsepower: at 65 percent efficiency, BHP is 13.03 divided by 0.65, or 20.0 BHP. Static Pressure Power runs this chain. The 6,356 constant is fixed for inches of water and CFM, so guard your units; using pascals or cubic meters per second silently corrupts the answer.

Motor selection sits one step past brake horsepower. You never install a motor at its BHP, because filter loading, damper changes, and density swings raise shaft demand above the design point. Motor Sizing applies a service factor or margin, typically 1.10 to 1.15 for routine service and 1.25 where fouling is heavy. A 20.0 BHP duty at a 1.15 margin needs 23.0 hp of capability, so you round up to a standard 25 hp frame. Standard NEMA frames run 15, 20, 25, 30, and 40 hp, so intermediate results always round up. A motor with a 1.15 nameplate service factor gives extra headroom, but do not stack margins twice.

The fan laws govern what happens when speed changes, and they matter more than any single relationship here because power scales with the cube of RPM. Airflow tracks speed linearly, static pressure with the square, and brake horsepower with the cube. Push a fan from 1,750 to 1,925 RPM, a 10 percent increase, and airflow rises 10 percent, pressure rises 21 percent (1.1 squared), and power jumps 33 percent (1.1 cubed). That 20 BHP fan now demands 26.6 BHP. Belt Drive Ratio sets sheave diameters to land the target RPM: driver sheave diameter times motor speed equals driven sheave diameter times fan speed. Recheck Motor Sizing after any speed change.

Impeller balance closes the mechanical side. The allowable residual unbalance from ISO 21940 is U equals 9,549 times G times rotor mass in kg, divided by service RPM, where G is the balance grade in mm per second, commonly G6.3 for general industrial fans and G2.5 for higher-speed units. A 40 kg wheel at grade G6.3 spinning 1,750 RPM allows 9,549 times 6.3 times 40 divided by 1,750, which is 1,375 g-mm total, or 688 g-mm per correction plane on a two-plane rotor. Impeller Balance Tolerance then compares your measured residual against that allowable to report the margin left before the wheel trends out of spec.

Chain the tools in order and the numbers stay consistent. Airflow CFM fixes the corrected volume, Static Pressure Power converts flow and pressure into air and brake horsepower, and Motor Sizing turns BHP plus margin into a frame. The fan laws and Belt Drive Ratio let you retune speed without redoing the whole sequence from scratch, and Impeller Balance Tolerance qualifies the rotor against its grade. Carry units at every step: CFM with inches of water for the 6,356 constant, kW at 0.746 per hp when you cross into electrical work, and g-mm for unbalance. Most fan calculation errors are unit mismatches, not arithmetic.

One worked example ties it together. A dust collection fan needs 13,800 corrected CFM against 6 inches w.c. Air horsepower is 13.03, brake horsepower at 65 percent efficiency is 20.0, and a 1.15 margin points to a 25 hp motor. Converting 20.0 BHP to kilowatts for the electrical feed, multiply by 0.746 to get 14.9 kW at the shaft, then divide by a 0.93 motor efficiency for roughly 16.0 kW drawn from the line. Those two numbers, 25 hp mechanically and 16 kW electrically, are what you hand to the panel builder and the energy estimator, and both fall straight out of the same duty point.

Published 2026-07-01.