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Duct Size Calculator

Result
—duct diameter
◆ LIVE 3D PREVIEW
Actual Velocity
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Flex vs Rigid
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CalcBuild › Duct Size Calculator

How to Calculate Duct Size

Use this free duct size calculator to get instant, accurate results for your construction project. Simply enter your measurements above and the calculator will automatically compute the result using industry-standard formulas.

Pro Tip: Always add 10-15% extra material to account for waste, cuts, and job-site conditions. This is standard practice recommended by experienced contractors.

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💡 Pro Tips — Duct Size

❓ Frequently Asked Questions

+How many BTU per square foot?
20-25 BTU/sq ft for mild climates, 30-40 for moderate, 40-60 for extreme heat or cold.
+What size AC do I need?
Rough guide: 1 ton per 500-600 sq ft. Actual sizing requires Manual J load calculation for accuracy.
+How often to change filters?
1" filters: every 1-3 months. 4" filters: every 6-12 months. Homes with pets: change more frequently.
+What SEER rating should I get?
Minimum is 14-15 SEER (varies by region). 16-20 SEER offers good efficiency. 20+ SEER is premium with highest savings.