Fan Horsepower Calculator
Work out the brake horsepower a fan or blower needs from its airflow, pressure and efficiency, choose a motor, and see how flow, pressure and power change when you change fan speed.
A fan's power requirement comes from two things it does to the air: how much it moves and how hard it pushes. Multiply airflow by pressure and you have the useful power in the airstream. Divide by the fan's efficiency and you have the brake horsepower the motor must supply at the shaft. This calculator does both, suggests a motor size, and applies the fan laws for speed changes.
Fan Brake Horsepower Formula
Motor output = BHP ÷ drive efficiency
The constant 6,356 converts the units. One horsepower is 33,000 ft·lb per minute. One inch of water column is a pressure of about 5.19 lb per square foot at room temperature. Multiplying cubic feet per minute by pounds per square foot gives ft·lb per minute, and 33,000 ÷ 5.19 = 6,356.
Use matching figures. Fan makers publish both total and static efficiency. Total efficiency goes with total pressure, the static pressure plus the velocity pressure of the air leaving the fan. Static efficiency goes with static pressure. Mixing a static pressure with a total efficiency understates the power.
Static, Velocity and Total Pressure
Static pressure is the resistance the system puts up: ducts, filters, coils, hoods and dampers. Velocity pressure is the pressure equivalent of the air's speed. Total pressure is the two added together. For standard air, velocity pressure in inches of water is (velocity in feet per minute ÷ 4,005)². Air leaving a fan at 2,000 ft/min carries about 0.25 inches of velocity pressure, and at 3,000 ft/min about 0.56 inches.
That difference matters. If the example fan works against 2 inches of static pressure with 0.25 inches of velocity pressure, its total pressure is 2.25 inches. At a 65% total efficiency it needs 10,000 × 2.25 ÷ (6,356 × 0.65) = 5.45 brake horsepower, not 4.84, which pushes the motor up to the next size. Always use the pressure that matches the efficiency figure you have.
Worked Examples
Kitchen exhaust fan. 3,000 CFM against 1.0 inch of static pressure at 55% efficiency needs 3,000 × 1.0 ÷ (6,356 × 0.55) = 0.86 BHP, so a 1 HP motor.
Dust collector. Dust collection runs at much higher pressure. 1,200 CFM at 10 inches of water and 60% efficiency needs 1,200 × 10 ÷ (6,356 × 0.60) = 3.15 BHP, so the next standard motor is 5 HP. The airflow is less than half the kitchen fan's, yet the power is almost four times higher, because the pressure is ten times higher.
How Much Efficiency Changes the Answer
Efficiency is the input that moves the result most, and it varies widely with fan design, size and where on its curve the fan operates. The same 10,000 CFM at 2 inches of water needs:
| Fan efficiency | Brake horsepower | Next standard motor |
|---|---|---|
| 50% | 6.29 HP | 7.5 HP |
| 60% | 5.24 HP | 7.5 HP |
| 65% | 4.84 HP | 5 HP |
| 70% | 4.50 HP | 5 HP |
| 80% | 3.93 HP | 5 HP |
Take the efficiency from the manufacturer's fan curve at your actual airflow and pressure, not from a general figure. Engineering references commonly publish ranges of roughly 75 to 85% for backward-curved centrifugal fans, up to the high 80s for airfoil designs at their best point, and roughly 55 to 65% for forward-curved fans. Treat those as orientation only. A fan pushed far from its best efficiency point can lose a large part of its rated efficiency.
The Fan Laws
For a given fan and system, three relationships describe what happens when the speed changes, as long as the air density and the system stay the same:
The cube law for power is why variable-speed drives save so much energy on fans. Slowing the example fan from 1,000 to 800 RPM cuts airflow to 8,000 CFM and pressure to 1.28 inches, but power falls to 51.2% of the original, from 4.84 to 2.48 brake horsepower. Speeding a fan up works the other way: a 10% speed increase needs 33% more power, which can overload a motor that was sized close to the original duty.
Air Density, Temperature and Altitude
Fan performance figures are normally given for standard air of 0.075 lb per cubic foot. At the same speed a fan moves the same volume of air whatever its density, but the pressure it develops and the power it absorbs scale with density. In the standard atmosphere, air at 5,000 feet is about 86% as dense as at sea level, so a fan there develops about 86% of its sea-level pressure and needs about 86% of the power at the same RPM. Hot air works the same way. A fan selected for hot process air can overload its motor during a cold start, when the air is much denser.
One fan calculator we checked states that a fan moving 1,000 CFM at sea level delivers only about 820 CFM in Denver. That mixes up volume and mass. At the same speed the fan still moves about 1,000 CFM of air. What falls is the mass of that air and, with it, the pressure and the power. If the process needs a fixed mass of air, the fan must run faster at altitude.
Fan Power Limits in ASHRAE 90.1
HVAC designers in the US often work to ASHRAE Standard 90.1, which caps fan system power. Its brake-horsepower option allows roughly CFM × 0.00094 for constant-volume systems and CFM × 0.0013 for variable-volume systems, plus allowances for items such as filters and heat recovery. For a 10,000 CFM constant-volume system that base limit is about 9.4 BHP. The example fan at 4.84 BHP sits comfortably inside it.
Choosing a Motor
Motors come in standard NEMA ratings: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 HP and up. The calculator picks the next size above the required shaft power, after any belt-drive losses. Engineering references commonly allow about 3 to 5% for a belt drive. Two cautions apply. First, allow for the fan's maximum load, not just its design point, especially for forward-curved fans whose power rises as airflow increases. Second, service factor is a margin for occasional overload, not a rating to design to. Size the motor so it runs at or below its nameplate horsepower in normal operation.
For the electrical side, convert the motor rating with the HP to amps calculator or the HP to kVA calculator. For liquid instead of air, the pump horsepower calculator uses the same idea with head and flow.
Frequently Asked Questions
Brake horsepower = CFM × pressure in inches of water ÷ (6,356 × fan efficiency). A fan moving 10,000 CFM at 2 in. w.c. and 65% efficiency needs 4.84 BHP.
It converts units: 33,000 ft·lb per minute per horsepower divided by about 5.19 lb per square foot, the pressure of one inch of water.
Air horsepower is the useful power delivered to the air, CFM × pressure ÷ 6,356. Brake horsepower is the power the fan shaft needs, which is higher because no fan is 100% efficient.
Power varies with the cube of speed. Running a fan at 80% speed needs about 51% of the power, and at 50% speed only about 12.5%.
You also need the pressure the fan works against and its efficiency. CFM alone does not set the power: the same airflow at higher static pressure needs proportionally more horsepower.
Less, at the same speed. Thinner air means the fan develops less pressure and absorbs less power, roughly in proportion to air density. At 5,000 feet air is about 86% as dense as at sea level, so power falls to about 86%. The fan still moves the same volume per minute, but less mass of air.
Divide brake horsepower by drive efficiency for belt drives, then pick the next standard NEMA size above it. Check the fan's maximum-load point as well as its design point.