Duct CFM Calculator
Estimate airflow through a rectangular duct from duct dimensions and average air velocity. Enter project-specific values and review every assumption before using the estimate.
Fill in the required fields to see your result.
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The primary result shows the main estimated capacity, current, airflow, metric or business value. Supporting outputs expose intermediate calculations.
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How to Use This Calculator
Enter the requested values using the units shown. Confirm that all inputs describe the same period, system or project.
Formula & Methodology
The calculator applies the displayed arithmetic or engineering formula directly to the entered values without silently adding hidden assumptions.
Worked example
The estimate assumes the entered velocity represents average velocity across the duct. Field measurements and system effects can differ.
Results may omit equipment-specific starting current, conductor temperature, derating, local codes, climate design data, duct losses, static pressure, infiltration and building-envelope details.
Educational preliminary estimate only. Not an electrical design, HVAC load calculation, permit document or code-compliance determination. Work on electrical and mechanical systems should be verified by qualified professionals.
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Frequently Asked Questions
- What does the Duct CFM Calculator calculate?
- Estimate airflow through a rectangular duct from duct dimensions and average air velocity.
- Are the results guaranteed or code-compliant?
- No. It is a preliminary educational estimate and does not establish code compliance, equipment suitability, permit approval or installation safety.
- Can I change the assumptions?
- Yes. The calculator exposes the main design, operating or financial assumptions so you can replace examples with project-specific values.
- Should the result be independently verified?
- Yes. Verify the result with applicable codes, manufacturer data, approved load calculations and a qualified electrician, engineer or HVAC professional.
Last updated 7/21/2026