Air Volume Flow Rate Calculator
Imperial (Inches, FPM, CFM)
Metric (mm, m/s, CMH)
Rectangular / Square
Round
Average air speed measured inside the duct.
Calculation Results
Duct Cross-Section Area:
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Volume Flow Rate (Primary):
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Alternative Units:
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How to Calculate Air Volume Flow Rate
Calculating the air volume flow rate, often expressed as CFM (Cubic Feet per Minute) in imperial units or CMH (Cubic Meters per Hour) in metric units, is a fundamental task in HVAC engineering, aerodynamics, and industrial ventilation.
The calculation determines how much air moves through a specific point in a duct or passageway within a given timeframe. This metric is crucial for ensuring proper ventilation, heating, and cooling efficiency in residential and commercial buildings.
The Flow Rate Formula
The basic physics behind air flow calculations relies on the continuity equation. The fundamental formula is:
Q = A × v
Where:
- Q = Volumetric Flow Rate (e.g., CFM or m³/s)
- A = Cross-sectional Area of the duct (e.g., ft² or m²)
- v = Velocity of the air (e.g., FPM or m/s)
Step-by-Step Calculation Guide
1. Determine the Duct Area (A)
First, you must calculate the area of the duct's cross-section. The math depends on the shape of the duct:
- Rectangular Ducts: Area = Width × Height.
Note: If measuring in inches, divide the result by 144 to get square feet. - Round Ducts: Area = π × (Diameter / 2)².
Note: If measuring in inches, divide the result by 144 to get square feet.
2. Measure Air Velocity (v)
Velocity is typically measured using an anemometer or pitot tube traversing the duct. In the imperial system, this is measured in Feet Per Minute (FPM). In the metric system, it is measured in meters per second (m/s).
3. Calculate the Flow Rate (Q)
Multiply the Area by the Velocity.
Example (Imperial):
A 12″ x 12″ duct has an area of 1 square foot.
If the air velocity is 500 FPM:
Q = 1 ft² × 500 FPM = 500 CFM.
A 12″ x 12″ duct has an area of 1 square foot.
If the air velocity is 500 FPM:
Q = 1 ft² × 500 FPM = 500 CFM.
Why Air Flow Rate Matters
Correct airflow ensures that HVAC systems operate within their design specifications. Low airflow can lead to frozen evaporator coils, overheated heat exchangers, and poor temperature control. Excessive airflow can cause noise issues and inefficient humidity removal.
Common Unit Conversions
- 1 CFM ≈ 1.699 m³/h (CMH)
- 1 m³/s = 3600 m³/h
- 1 m/s ≈ 196.85 FPM