Formula Explanation
Area = CFM / velocity; diameter = sqrt(4A/pi)
This calculator applies the formula Area = CFM / velocity; diameter = sqrt(4A/pi). Enter each value with the matching unit, and AZCalculate updates the result instantly.
Engineering
Duct Size Calculator
Use this duct size calculator to calculate estimate round duct diameter from airflow and target air velocity.
duct sizecfmair velocityhvac
Calculator
Calculate instantly
Calculator Inputs
Choose what you want to calculate, enter realistic values, adjust units where shown, and use the scenario section for repeated-use planning.
Input and Formula Breakdown
This calculator applies the formula Area = CFM / velocity; diameter = sqrt(4A/pi). Enter each value with the matching unit, and AZCalculate updates the result instantly.
Airflow
600 CFM
Air velocity
900 ft/min
Duct area
0.67 sq ft
Round duct diameter
11.06 in
Scenario and Comparison
Current
0.67 sq ft
Projected
8
Adjusted
0.73
Visual Comparison
Current result0.67
Projected usage8
Comparison scenario0.73
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Use Cases
- Size ducts
- Compare airflow
- Plan HVAC
Step-by-Step Explanation
- Step 1Enter the known values in the calculator fields.
- Step 2Keep the units consistent with the labels shown beside each input.
- Step 3Apply the formula: Area = CFM / velocity; diameter = sqrt(4A/pi).
- Step 4Review the primary result and supporting values.
Example calculation
Example duct size calculator
Given
- Airflow: 600 CFM
- Air velocity: 900 ft/min
Steps
- Use the values shown above.
- Apply Area = CFM / velocity; diameter = sqrt(4A/pi).
Result
The calculator above shows the computed answer instantly from these example values.
Calculator guide
About this Duct Size Calculator
Estimate round duct diameter from airflow and target air velocity. This page includes an interactive calculator, concise formula notes, worked examples, FAQs, related calculators, and practical guidance you can revisit whenever needed.
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Calculator usage
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Total saved calculations
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Most active date
2026-06-05
FAQ
Duct Size Calculator FAQs
How does the duct size calculator work?+
It uses Area = CFM / velocity; diameter = sqrt(4A/pi) and calculates the result from the values you enter.
Can I copy or print the result?+
Yes. AZCalculate calculator pages include copy, share, and print actions.
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