Calculate the fan Air Hp, brake Hp, or Motor Hp of the fan using the tabs below.
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Fan Horsepower Formula
Air Horsepower (ideal power delivered to the airstream):
AHP = (Q × SP) / 6356
where Q = airflow in CFM, SP = static pressure in inches of water gauge (in. wg).
Brake Horsepower (power required at the fan shaft):
BHP = (Q × TP) / (6356 × ηf)
where TP = total pressure (in. wg) and ηf = fan mechanical efficiency (0–1).
Motor Horsepower (size of the motor to specify):
Motor HP = (BHP / ηd) × SF
where ηd = drive efficiency (belt or direct) and SF = safety/service factor.
Interpretation
Air horsepower is the theoretical minimum — the work done on the air itself. It never matches what the motor actually draws because no fan is 100% efficient. Brake horsepower is what the fan shaft has to deliver and is the number you match against a fan's performance curve. Motor horsepower is what you buy: it accounts for drive losses and a margin for startup, fouling, and duty variation.
Typical fan efficiency benchmarks:
- Propeller fans: 45–55%
- Forward-curved centrifugal: 55–65%
- Backward-inclined / axial: 65–75%
- Airfoil centrifugal: 75–85%+
If your calculated BHP is within 10–15% of the fan's rated power at that operating point, the selection is reasonable. If BHP exceeds the fan's rating or motor nameplate, re-check pressure inputs or move to a larger fan.
Quick Reference Table
Approximate brake horsepower at 70% fan efficiency, in inches of water gauge:
- 1,000 CFM @ 1 in. wg: ~0.22 BHP
- 2,500 CFM @ 2 in. wg: ~1.12 BHP
- 5,000 CFM @ 3 in. wg: ~3.37 BHP
- 10,000 CFM @ 4 in. wg: ~8.99 BHP
- 20,000 CFM @ 6 in. wg: ~26.96 BHP
- 50,000 CFM @ 8 in. wg: ~89.87 BHP
Standard NEMA motor sizes (hp): 0.25, 0.33, 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100. Round up to the next standard size after applying drive losses and safety factor.
FAQ
What units should pressure be in?
The 6356 constant assumes inches of water gauge (in. wg) with CFM. If you have Pa, kPa, or in. Hg, convert first: 1 in. wg ≈ 249 Pa ≈ 0.0736 in. Hg.
Should I use static pressure or total pressure?
Use total pressure (static + velocity) for brake horsepower if you want the full shaft power. Static pressure alone gives you static air horsepower, which slightly understates BHP. For most HVAC ductwork, velocity pressure is small and the two are within a few percent.
What fan efficiency should I use if I don't know it?
Pull it from the manufacturer's fan curve at your operating point — efficiency varies with flow. If you're estimating, use 65–70% for a generic centrifugal fan and 75–80% for airfoil wheels.
Why is motor horsepower higher than brake horsepower?
The motor has to overcome belt or coupling losses (3–8% for V-belts, ~0% for direct drive) and needs headroom for startup, dirty filters, and duty swings. The service factor bakes in that margin before you round up to a stock motor size.
Does air density matter?
Yes, but the 6356 constant assumes standard air (0.075 lb/ft³ at sea level, 70°F). At high altitude or high temperature, the fan develops less pressure at the same RPM, so measured pressure already reflects actual density — no extra correction needed for BHP. Density correction matters mostly when translating between test conditions and field conditions.
