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Air Flow Rate Calculator

Plan your hvac & plumbing project with our free air flow rate calculator. Get precise measurements, material lists, and budgets.

Reviewed by Abdullah, Technical Content Specialist

Reviewed by Abdullah, Technical Content Specialist

Formula

Q (CFM) = Velocity (FPM) x Duct Area (sq ft)

The volumetric flow rate Q in cubic feet per minute equals the air velocity in feet per minute multiplied by the cross-sectional area of the duct in square feet. For rectangular ducts, area equals width times height (converted from inches to feet). For round ducts, area equals pi times the radius squared.

Worked Examples

Example 1: Rectangular Duct Flow

Problem:A 12 x 12 inch rectangular duct has air moving at 800 FPM. What is the CFM?

Solution:Area = (12/12) x (12/12) = 1.0 sq ft\nCFM = Velocity x Area = 800 x 1.0 = 800

Result:800 CFM airflow rate

Example 2: Round Duct Flow

Problem:A 10-inch round duct has air velocity of 1,000 FPM. What is the CFM?

Solution:Radius = (10/12) / 2 = 0.4167 ft\nArea = 3.14159 x 0.4167^2 = 0.5454 sq ft\nCFM = 1,000 x 0.5454 = 545.4

Result:545.4 CFM airflow rate

Frequently Asked Questions

How is air flow rate calculated?

Air flow rate is calculated by multiplying the air velocity (in feet per minute) by the cross-sectional area of the duct (in square feet). The formula Q = V x A gives the volumetric flow rate in cubic feet per minute (CFM). Air velocity is typically measured with an anemometer or pitot tube at the duct opening. The duct area depends on the shape, using width times height for rectangular ducts or pi times radius squared for round ducts.

What is a normal air velocity in ductwork?

Main trunk lines in residential HVAC systems typically have air velocities of 700 to 900 feet per minute. Branch ducts run at 500 to 700 FPM. Supply registers deliver air at 300 to 500 FPM for comfort. Commercial systems may use higher velocities up to 2,500 FPM in main ducts. Velocities above 900 FPM in residential systems create noticeable noise, while velocities below 300 FPM may cause poor air distribution.

References

Reviewed by Abdullah, Technical Content Specialist · Editorial policy