Fan/Blower Motor HP Calculator

About the Fan/Blower Motor HP Calculator

This fan and blower motor HP calculator determines required motor horsepower from airflow (CFM) and static pressure (inches of water column). Use it to size HVAC supply fans, exhaust fans, dust collectors, industrial blowers, and any air-moving equipment with known performance requirements.

The calculator outputs both brake HP at the fan shaft and motor HP required including motor losses. Round up to the next standard NEMA HP size for the motor.

How fan motor HP is calculated

BHP = (CFM × SP) ÷ (6356 × Fan Efficiency)
Motor HP = BHP ÷ Motor Efficiency

The constant 6356 comes from converting units: cubic feet/min × inches of water column / 33,000 ft·lb/HP. Fan efficiency varies widely by type (centrifugal 65–85%, axial 50–70%, propeller 40–60%). Motor efficiency for modern Premium motors is 90–96%.

Worked example

An HVAC supply fan moving 5,000 CFM at 1.5 in. WC static pressure, with 65% fan efficiency and 92% motor efficiency: BHP = (5000 × 1.5) ÷ (6356 × 0.65) = 1.81 HP. Motor HP = 1.81 ÷ 0.92 = 1.97 HP. Specify a 2 HP NEMA motor.

When to use this calculator

• Sizing motors for HVAC supply, return, and exhaust fans
• Specifying motors for dust collection systems
• Sizing combustion air blowers for boilers and furnaces
• Selecting industrial process ventilation fans
• Verifying existing fan motors during HVAC retrofits
• Sizing inducer fans for furnaces and water heaters

Fan efficiency by type

• Backward-curved centrifugal: 75–85% (highest efficiency, common in HVAC)
• Forward-curved centrifugal: 60–75% (compact, used in residential furnaces)
• Radial blade centrifugal: 65–75% (handles dust/particulates well)
• Axial flow (vane axial): 70–85% (high efficiency at high airflow)
• Tube axial: 55–70% (moderate pressure capability)
• Propeller (free-air): 40–60% (low pressure, high airflow)

Frequently Asked Questions

What is static pressure in fan terminology?

Static pressure (SP) is the resistance the fan must overcome to push air through ducts, filters, coils, and dampers. Measured in inches of water column (in. WC). Typical values: residential HVAC 0.5–1.0 in. WC, commercial 1.0–3.0, industrial 3.0–10+.

What is CFM?

Cubic Feet per Minute — the volume of air moved per minute. Standard reference is 70°F at sea level. For high-altitude or hot-air applications, you may need to apply density correction factors.

How do I determine fan efficiency?

From the fan manufacturer’s performance curve at your design CFM and SP. If unknown, use industry typical values: 65% for backward-curved centrifugal HVAC fans, 50% for propeller fans, 75% for vane-axial.

How does altitude affect fan motor sizing?

At higher altitude, air density drops, reducing the actual mass airflow at the same CFM. Fan power requirements scale with density — at 5,000 ft (~83% of sea level density), motor HP can be reduced by ~17% if airflow specification stays the same.

Should I oversize the fan motor?

Standard practice: round up to the next standard NEMA size, then consider 1.15 service factor as additional margin. Don’t oversize beyond that — oversized fan motors run at part-load with reduced efficiency.

What about variable-air-volume (VAV) systems?

For VAV systems with VFD control, size the motor for design (peak) airflow + 10% margin. The VFD modulates the motor speed down for partial loads, saving energy. Fan affinity laws: 50% airflow = 12.5% power.

Related calculators and Electracore products

Sizing a fan motor? Browse Fan & Blower motors, HVAC Blower motors, or HVAC motors from leading brands.

Related calculators: Pump Motor HP Calculator · Motor FLA Calculator · Motor Energy Cost Calculator · VFD Sizing Calculator.