Duct Calculator: HVAC Sizing

The digital ductulator — equal-friction duct sizing in your pocket, no cardboard slide rule.

Enter airflow (CFM) and a friction rate, get an instant duct size:

ROUND DUCT

• Exact required diameter, plus the nearest standard size
• Air velocity, flagged if it's above safe residential limits

RECTANGULAR DUCT

• Fix one side, get the matching size for your target equivalent diameter
• Equivalent round diameter (Huebscher) for cross-checking against round runs

BUILT ON REAL PHYSICS

• Darcy–Weisbach + Colebrook friction factor — the same method behind the ASHRAE friction chart and professional ductulators — not a rough approximation.
• Galvanized sheet metal, standard air. Supply (0.10 in wg/100ft) and return (0.08) defaults built in, or set your own friction rate.

Free to download and try. Ductulator Pro unlocks unlimited calculations with a 7-day free trial, then renews yearly — cancel anytime in Settings.

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