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Fan Calculator

Calculate fan airflow CFM, static pressure, and fan power for HVAC and ventilation system sizing.

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Fan Sizing Basics

HVAC fans move air against static pressure in ductwork. The relationship between airflow (CFM), static pressure (inches of water), and brake horsepower is used to size fans for ventilation, cooling, and exhaust systems.

Fan Power Equation

$$\text{BHP} = \frac{\text{CFM} \times \text{SP}}{6356 \times \eta}$$

Where BHP is brake horsepower, SP is static pressure in inches H₂O, and η is fan efficiency. Rearrange to solve for CFM or static pressure when other values are known.

Related tools: CFM Calculator, Air Changes Per Hour Calculator, and Liters Per Minute Calculator.

Frequently Asked Questions

What is CFM?

CFM stands for cubic feet per minute. It measures the volume of air a fan moves each minute and is the standard airflow unit in HVAC in the United States.

What is static pressure?

Static pressure is the resistance air encounters in ducts, filters, and fittings. It is measured in inches of water gauge (in H₂O) and directly affects fan power requirements.

How does efficiency affect fan power?

Lower efficiency means more input power is needed for the same CFM and pressure. Typical commercial fans range from 50% to 75% efficiency.

When should I solve for CFM vs power?

Solve for CFM when you know the motor size and duct resistance. Solve for power when you know required airflow and system pressure drop.

What fan efficiency should I use?

Use manufacturer data when available. For preliminary sizing, 60% to 70% is a reasonable estimate for belt-driven commercial fans.

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