How do you calculate fan brake horsepower?

Brake horsepower equals CFM times total static pressure divided by 6356 times fan total efficiency. For 10,000 CFM at 2.5 in. w.g. and 68% efficiency, the airflow-times-pressure product is 25,000, which sets the shaft load before the service factor is applied.

What does the 6356 constant mean?

6356 is the conversion factor that turns CFM multiplied by inches of water gauge into horsepower (it derives from 33,000 ft-lb/min per HP and the 5.193 lb/ft2 per inch w.g. conversion). It lets you skip unit gymnastics and go straight from duty point to BHP.

Why apply a motor service factor?

A service factor such as 1.15 is the headroom the motor can carry continuously above nameplate. Sizing with it ensures the motor isn't run at 100% of rating at the design point, which preserves bearing life, insulation life, and tolerance for dirty filters that raise static pressure.

What is a good fan total efficiency to assume?

Backward-curved and airfoil plenum fans commonly run 65-80% total efficiency; forward-curved and many small direct-drive fans fall in the 50-65% range. The 68% default is a reasonable mid-range airfoil assumption, but always use the efficiency at your actual operating point from the curve.

Should I size the motor for the calculated BHP exactly?

No — round up to the next standard NEMA frame size (1, 1.5, 2, 3, 5, 7.5 HP, etc.) after reviewing the fan curve. The calculation tells you the minimum; the standard size and the fan's non-overloading characteristic determine the final pick.

What raises static pressure and forces a bigger motor?

Longer duct runs, more fittings and elbows, higher-MERV or loaded filters, tighter coils, and dampers throttled toward closed all add static pressure. Because BHP scales with pressure, even a half-inch w.g. increase can push you into the next motor size.

HVAC Ductwork, Air Handling & Mechanical Products calculator

Fan Motor Power Sizing Calculator

Fan Motor Sizing converts a fan's design duty point — airflow against total static pressure — into the brake horsepower the shaft actually needs, then applies a service factor to land on a safe standard motor. Air-handling and ductwork engineers, AHU builders, and TAB technicians use it during equipment selection and field troubleshooting. Undersize the motor and it overheats or trips on overload; oversize it and you waste capital, draw poor power factor, and run the fan off its efficient operating range. Getting the brake horsepower right against the real system curve is the difference between a unit that delivers rated CFM quietly and one that nuisance-trips on a hot day.

What this calculator does

Estimate the required fan motor shaft power or brake horsepower from design airflow, total static pressure, fan efficiency, and a motor service factor. Use for supply, return, and exhaust fan selection in air handling units and ductwork systems.
Use this when selecting a supply fan, return fan, or exhaust fan motor for an air handling unit or ductwork system. Enter the design CFM, total static pressure in inches water column, fan total efficiency, and a service factor to get estimated brake horsepower. Round up to the next available motor frame size and confirm with the fan curve.
It computes fan shaft brake horsepower from airflow, total static pressure and fan efficiency, then scales it by the motor service factor to indicate the standard motor size to select.

Formula used

Fan shaft power (BHP) = (CFM × total static pressure) ÷ (6356 × fan efficiency) × service factor
Round up to the next standard motor size after reviewing the fan curve

Inputs explained

Design airflow:
Total static pressure:
Fan total efficiency:
Motor service factor:

How to use the result

Use it when selecting or replacing an AHU or inline fan motor, validating a vendor selection, or diagnosing an overloaded fan after duct changes raised static pressure.
It uses a single design point and an assumed fan total efficiency; it does not read your actual fan curve, drive losses, or altitude/temperature air-density corrections, so always confirm against the published performance curve before ordering.

Current U.S. benchmarks

Industrial electricity averages 8.66 cents per kWh across the U.S. (EIA, Apr 2026), up 5.5% from a year earlier. Energy-intensive steps carry this directly into unit cost.
U.S. housing starts run at 1,177k per year (Census, May 2026), down 8.7% from a year earlier, the demand driver for building products.

Common questions

How do you calculate fan brake horsepower? Brake horsepower equals CFM times total static pressure divided by 6356 times fan total efficiency. For 10,000 CFM at 2.5 in. w.g. and 68% efficiency, the airflow-times-pressure product is 25,000, which sets the shaft load before the service factor is applied.
What does the 6356 constant mean? 6356 is the conversion factor that turns CFM multiplied by inches of water gauge into horsepower (it derives from 33,000 ft-lb/min per HP and the 5.193 lb/ft2 per inch w.g. conversion). It lets you skip unit gymnastics and go straight from duty point to BHP.
Why apply a motor service factor? A service factor such as 1.15 is the headroom the motor can carry continuously above nameplate. Sizing with it ensures the motor isn't run at 100% of rating at the design point, which preserves bearing life, insulation life, and tolerance for dirty filters that raise static pressure.
What is a good fan total efficiency to assume? Backward-curved and airfoil plenum fans commonly run 65-80% total efficiency; forward-curved and many small direct-drive fans fall in the 50-65% range. The 68% default is a reasonable mid-range airfoil assumption, but always use the efficiency at your actual operating point from the curve.
Should I size the motor for the calculated BHP exactly? No — round up to the next standard NEMA frame size (1, 1.5, 2, 3, 5, 7.5 HP, etc.) after reviewing the fan curve. The calculation tells you the minimum; the standard size and the fan's non-overloading characteristic determine the final pick.

Last reviewed 2026-05-12.