FPM to CFM Calculator

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The FPM to CFM calculator converts Feet Per Minute (FPM) to Cubic Feet Per Minute (CFM), helping professionals and hobbyists measure and plan air flow correctly.

What is an FPM to CFM Calculator?

An FPM to CFM calculator is a specialized tool that converts linear air velocity (FPM) into volumetric flow rate (CFM). This conversion is vital for:

  • HVAC system design and maintenance
  • Industrial ventilation planning
  • Clean room air flow calculations
  • Exhaust system specifications
  • Air quality control measurements

Understanding FPM and CFM

FPM (Feet Per Minute): Measures the linear velocity of air movement in feet per minute.

CFM (Cubic Feet Per Minute): Measures the volumetric flow rate of air in cubic feet per minute.

The Conversion Formula

CFM = FPM × Area (sq ft)

Where:

  • CFM = Cubic Feet per Minute
  • FPM = Feet per Minute
  • Area = Cross-sectional area in square feet

Practical Examples

Example 1: Round Duct

Given:

  • Air velocity: 1000 FPM
  • Duct diameter: 1 foot
  • Area = π × (diameter/2)² = 0.785 sq ft
  • CFM = 1000 × 0.785 = 785 CFM

Example 2: Rectangular Duct

Given:

  • Air velocity: 800 FPM
  • Duct dimensions: 2 feet × 1.5 feet
  • Area = 2 × 1.5 = 3 sq ft
  • CFM = 800 × 3 = 2400 CFM

How to Convert FPM to CPM?

To convert Feet Per Minute (FPM) to Cubic Feet Per Minute (CFM), you need to know the cross-sectional area of the duct or space where the air is moving.

Here’s how you do it:

  1. Find the area of the duct in square feet (ft²). For a rectangular duct, multiply the width by the height. For a circular duct, use the formula: Area=π×(Diameter/2)^2
  2. Multiply the FPM by the area: CFM= FPM×Area

Example:
If the duct is 2 ft wide and 1.5 ft tall, and the air velocity is 500 FPM: Area=2 ft×1.5 ft=3 ft^2

CFM= 500 FPM×3 ft^2= 1500 CFM

How to Calculate FPM?

To calculate Feet Per Minute (FPM), you need to know the CFM and the cross-sectional area of the duct or space.

The formula is: FPM= CFM/Area

Steps:

  1. Find the area of the duct in square feet (just like above).
  2. Divide the CFM by the area to get FPM.

Example:
If the airflow is 1500 CFM and the duct area is 3 ft²:

FPM=1500 CFM/3 ft^2=500 FPM​=500FPM

How to Calculate FPM in Duct?

To calculate FPM in a duct, you need to measure the air velocity or determine it using the airflow rate (CFM) and the duct size.

Here’s what to do:

  1. Measure the airflow (CFM): Use an anemometer or get the CFM value from the HVAC system specs.
  2. Calculate the duct’s area:
    • Rectangular duct: Multiply the width and height.
    • Circular duct: Use the formula: Area=π×(Diameter/2)^2
  3. Use the formula: FPM=CFM/Area

Example:
For a circular duct with a diameter of 2 ft and an airflow of 800 CFM:

Area=π×(2/2)^2=π×1= 3.14 ft^2

FPM= 800/3.14 ≈ 255 FPM

Frequently Asked Questions

Q: Why is converting FPM to CFM important?

A: This conversion helps ensure proper airflow design, system efficiency, and compliance with ventilation standards.

Q: How accurate are FPM to CFM calculations?

A: Calculations are highly accurate when using precise measurements of both velocity and cross-sectional area.

Q: Can I convert CFM back to FPM?

A: Yes, simply divide CFM by the cross-sectional area to obtain FPM.

Q: What tools are needed to measure FPM?

A: An anemometer or velometer can measure air velocity in FPM directly.

Conclusion

The FPM to CFM calculator is a must-have tool for anyone working with airflow systems. Knowing how to convert these measurements is important for designing, maintaining, and improving these systems. Whether you work in HVAC, industrial engineering, or facility management, being good at these calculations helps your ventilation systems work well and meet the needed standards.