square inches / equivalent diameter
Duct Size Calculator
Estimate duct area and equivalent round duct size from CFM and target velocity.
HVAC and energy planning note
HVAC and energy calculators on ToolsBing provide preliminary planning estimates only. Actual equipment selection, code compliance, indoor air quality, combustion safety, ventilation, refrigerant work, electrical connection, and energy-system design should be verified by qualified professionals using local codes, manufacturer data, Manual J/S/D methods where applicable, and site-specific measurements.
Enter HVAC or energy inputs
Estimated result
Use for early sizing, energy comparison, education, and rough project planning only.
Confirm with building envelope data, equipment specs, local weather, utility rates, codes, and qualified design review.
Formula and assumptions
Duct area = CFM ÷ target velocity; equivalent diameter estimated from area
This calculator uses simplified assumptions. Actual HVAC and energy performance can be affected by building envelope, orientation, air leakage, duct losses, occupancy, control settings, climate, utility tariffs, equipment condition, and local code requirements.