How to Use the FPM to CFM Calculator

A FPM to CFM Calculator converts airflow velocity measured in Feet Per Minute (FPM) into a volumetric flow rate measured in Cubic Feet per Minute (CFM), using the cross-sectional area of a duct.

What It Does:

In HVAC and fluid dynamics, airflow is often measured by how fast air moves through a duct (FPM). But for practical applications like system sizing or fan selection, we need to know how much air volume is being moved — that's where CFM comes in.

This calculator takes:

• FPM: the velocity of air
• Area (sq ft): the duct’s cross-sectional area

…and gives you:

• CFM = FPM × Area

How to Use It:

1. Enter the FPM value (feet per minute of air velocity).
2. Enter the duct’s cross-sectional area in square feet.
   • For rectangular ducts, Area = width × height
   • For circular ducts, Area = π × (radius²)
3. Click "Convert to CFM".
4. View the results in the display section.

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FPM to CFM Calculator FAQ

Q1: What’s the formula used in the calculator?
A: The formula is simple: CFM = FPM × Area (in sq ft). It multiplies air velocity by duct size.

Q2: What units do I use for area?
A: Always use square feet (ft²). If you have inches, convert first. Example:
12" × 12" = 1 sq ft.

Q3: What’s the difference between FPM and CFM?
A:

• FPM (Feet Per Minute): Speed of air through a duct.
• CFM (Cubic Feet per Minute): Volume of air moved.

Q4: Can I use this for round ducts?
A: Yes. Just calculate the area of the round duct using:
Area = π × (diameter/2)² / 144 (to convert from in² to ft²).

Q5: Is this useful in HVAC design?
A: Absolutely. HVAC engineers use CFM for equipment sizing, system balance, and compliance with building codes.