Air Flow Rate Calculator
Plan your hvac & plumbing project with our free air flow rate calculator. Get precise measurements, material lists, and budgets.
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.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
How accurate are the results from Air Flow Rate Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
Is Air Flow Rate Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.