Enter values using your preferred unit cost and local pricing.
Rules used in this calculator
These rules are also included in the PDF summary.
Required duct area in square feet = CFM ÷ target velocity FPM.
Required duct area in square inches = square feet × 144.
Round duct diameter = sqrt(4 × area ÷ π).
Rectangular duct height = area ÷ selected width.
Equivalent diameter is estimated from rectangular duct sides using the common ASHRAE approximation.
Final duct sizing should be checked for friction loss, fittings, noise, and HVAC design code.
Result options
Print, copy, or download your result
Use these quick actions after calculating. PDF summary buttons inside the result box still work where available.
HVAC Duct Calculator guide
Formula, example, FAQs, and notes
Formula explanation in simple language
HVAC Duct Calculator follows the visible calculator rules on this page. In simple terms, it takes the values you enter, applies the main formula steps, then returns a result summary. Key steps: Required duct area in square feet = CFM ÷ target velocity FPM. Required duct area in square inches = square feet × 144. Round duct diameter = sqrt(4 × area ÷ π).
Required duct area in square feet = CFM ÷ target velocity FPM.
Required duct area in square inches = square feet × 144.
Round duct diameter = sqrt(4 × area ÷ π).
Rectangular duct height = area ÷ selected width.
Equivalent diameter is estimated from rectangular duct sides using the common ASHRAE approximation.
Final duct sizing should be checked for friction loss, fittings, noise, and HVAC design code.
Example calculation
Example: enter Airflow (CFM): 400, Target air velocity (FPM): 700, Rectangular duct width (inches): 12, Round diameter increment (inches): 1. Then press Calculate Result. The calculator uses those inputs to generate a result, interpretation notes, and a downloadable PDF summary.
After changing any input, recalculate and compare the new result with the original assumptions.
Common mistakes and interpretation notes
Do not leave default values unchanged if they do not match your real situation.
When changing currency, confirm that money fields converted to the correct selected currency before comparing results.
Check units carefully before calculating. Mixing inches, feet, pounds, kilograms, days, months, or years can change the result.
Treat the result as an estimate, not a guarantee, quote, diagnosis, settlement promise, or final professional decision.
Do not ignore waste allowance, permit requirements, soil/site conditions, labor differences, or local building code.
Disclaimer
HVAC Duct Calculator is an estimate based on the inputs you enter. Review the assumptions before relying on the result. Verify local code, permits, site conditions, and contractor pricing before construction decisions.
HVAC Duct Calculator FAQ
Helpful answers for long-tail searches
How accurate is the HVAC Duct Calculator?
The HVAC Duct Calculator is accurate to the formula and assumptions shown on the page. Real-world results can change when costs, rates, taxes, laws, health factors, or project conditions differ from the values you enter.
Can I download the HVAC Duct Calculator result?
Yes. Use the page result controls to download the PDF summary when available, or use the Print, Copy Result, and Download TXT buttons added below the calculator.
Does the HVAC Duct Calculator convert currencies using exchange rates?
Yes. Money fields use the selected currency display and the site exchange-rate endpoint where supported. The PDF or result note shows the currency/rate information when available.
What inputs matter most in the HVAC Duct Calculator?
The most important inputs are usually the largest cost, rate, quantity, time, and risk assumptions. Small changes in these fields can move the final result significantly.
Is the HVAC Duct Calculator engineering, permit, or contractor advice?
No. This calculator is an educational estimate and is not engineering, permit, or contractor advice. Use it for planning and discussion, then verify important decisions with a qualified professional.
