Ductwork Calculator
Calculate the required duct size for HVAC systems based on airflow and friction loss. Helps in designing efficient ductwork.
Published by:
isCalculator Editorial Team
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How to Use the Ductwork Calculator
This calculator determines the required duct size based on airflow and friction loss. All values must be in Imperial units (CFM and inches of water gauge per 100 feet).
- Enter Airflow (CFM): Input the desired airflow rate in cubic feet per minute (e.g., 1200).
- Enter Friction Loss: Input the allowable friction loss in inches of water gauge per 100 feet (e.g., 0.1).
- Select Duct Shape: Choose Round or Rectangular from the dropdown.
- If Rectangular: Enter the Aspect Ratio (width/height, minimum 1). This field appears only when Rectangular is selected.
- Click "Calculate Duct Size": The result shows the required diameter (for round) or width × height (for rectangular).
Note: All inputs must be positive numbers. The result uses the Darcy-Weisbach equation for round ducts and an equivalent diameter method for rectangular ducts.
Previous Results
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Frequently Asked Questions
It helps determine the appropriate duct diameter or rectangular dimensions for HVAC systems based on airflow (CFM) and friction loss.
You need airflow in CFM, friction loss in inches of water gauge per 100 feet, and duct shape (round or rectangular with aspect ratio).
It uses the Darcy-Weisbach equation adapted for HVAC duct sizing: D = ((0.109136 * Q^1.9) / (f^0.5))^(1/5.02).