Calculator Cfm

CFM Calculator

Use this calculator to determine the Cubic Feet per Minute (CFM) of airflow through a rectangular or circular opening/duct, given its dimensions and the air velocity.

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Calculated CFM:

Understanding CFM: Cubic Feet per Minute

CFM, or Cubic Feet per Minute, is a crucial measurement used to quantify the volume of air moving through a space or system over a specific period. It's a fundamental metric in various fields, including HVAC (Heating, Ventilation, and Air Conditioning), dust collection, range hood performance, and general ventilation design. Understanding CFM helps ensure proper air circulation, maintain indoor air quality, and optimize system efficiency.

Why is CFM Important?

• HVAC Systems: Proper CFM ensures that heating and cooling systems can effectively distribute conditioned air throughout a building, maintaining comfortable temperatures and humidity levels.
• Ventilation: Adequate CFM is essential for removing stale air, pollutants, and excess moisture from indoor environments, promoting healthy air quality and preventing issues like mold growth.
• Dust Collection: In workshops or industrial settings, dust collectors rely on specific CFM ratings to effectively capture airborne particles, protecting equipment and worker health.
• Range Hoods: Kitchen range hoods use CFM to exhaust cooking fumes, smoke, and grease, preventing them from spreading throughout the home.
• Air Purifiers: The effectiveness of an air purifier is often measured by its Clean Air Delivery Rate (CADR), which is essentially a CFM rating for purified air.

How to Calculate CFM

The most common way to calculate CFM, especially for airflow through ducts or openings, is by multiplying the cross-sectional area of the opening by the velocity of the air moving through it. The formula is:

CFM = Area (in square feet) × Air Velocity (in feet per minute)

To use this formula, you need to ensure your units are consistent. If your dimensions are in inches, you must convert them to feet before calculating the area.

For Rectangular Openings/Ducts:

First, calculate the area in square feet:

Area (sq ft) = (Width in inches / 12) × (Height in inches / 12)

Then, multiply by the air velocity.

For Circular Openings/Ducts:

First, calculate the area in square feet:

Area (sq ft) = π × (Radius in inches / 12)²

Since Radius = Diameter / 2, this can also be written as:

Area (sq ft) = π × ((Diameter in inches / 2) / 12)²

Then, multiply by the air velocity.

Examples of CFM Calculation

Example 1: Rectangular Duct

Imagine a rectangular HVAC duct that is 14 inches wide and 8 inches high, with air flowing through it at a velocity of 600 FPM.

• Width: 14 inches = 14 / 12 = 1.1667 feet
• Height: 8 inches = 8 / 12 = 0.6667 feet
• Area: 1.1667 ft × 0.6667 ft = 0.7778 sq ft
• Air Velocity: 600 FPM
• CFM: 0.7778 sq ft × 600 FPM = 466.68 CFM

Using the calculator above, input 14 for Width, 8 for Height, and 600 for Air Velocity to verify this result.

Example 2: Circular Exhaust Vent

Consider a circular exhaust vent with a diameter of 6 inches, where air is moving out at 800 FPM.

• Diameter: 6 inches
• Radius: 6 / 2 = 3 inches = 3 / 12 = 0.25 feet
• Area: π × (0.25 ft)² = 3.14159 × 0.0625 sq ft = 0.1963 sq ft
• Air Velocity: 800 FPM
• CFM: 0.1963 sq ft × 800 FPM = 157.04 CFM

Using the calculator, input 6 for Diameter and 800 for Air Velocity to confirm this calculation.

By accurately calculating CFM, you can make informed decisions about ventilation requirements, select appropriate equipment, and ensure optimal air movement for various applications.