Duct Size Calculator
Round and square duct size from airflow (CFM) and target air velocity.
Inputs
CFM
ft/min
Residential supply 600–900 · trunk 700–1000 · returns 500–700
Results
- Round duct diameter
- 10.24 in
- Round duct diameter
- 260 mm
- Square duct side
- 9.07 in
- Cross-section area
- 0.571 ft²
Formula
Area (ft²) = CFM ÷ velocity (fpm)
Diameter (in) = √(4 × Area ÷ π) × 12
Assumptions & limits
- Velocity method only; friction loss (equal-friction method) is not included.
- Round up to the next available duct size.
Worked example
400 ÷ 700 = 0.571 ft² → Ø 10.24 in.