Formula & example
A=Q/V; D=sqrt(4A/π); De=1.30(ab)^0.625/(a+b)^0.25.
400 CFM at 800 fpm needs 0.5 ft² area and about 9.57 in round diameter before rounding to a stocked size.
HVAC engineering calculator · Updated Sep 17, 2026
Duct Size Calculator — CFM, Velocity, Round & Rectangular
Size round and rectangular duct cross-sections from airflow and target velocity, with actual area and equivalent round diameter.
Inputs
Result
Required area 0.5 ft² (72 in²)
Round diameter 9.5746147 in
Rectangular size 9 in × 8 in
Rectangular equivalent round diameter 9.2717974 in
Design velocity 800 fpm (4.064 m/s)
Assumptions & limits
- Velocity method is a first-pass sizing method, not a full equal-friction network design.
- Fittings, leakage and available static pressure are outside the geometry solve.
Frequently asked questions
What does this Duct Size Calculator calculate?
Size round and rectangular duct cross-sections from airflow and target velocity, with actual area and equivalent round diameter.
Can I use SI and US customary units?
Yes. The calculator exposes the unit basis in the inputs and reports useful cross-unit outputs where the engineering workflow benefits from them.
Is this a replacement for HVAC design software?
No. It is a transparent deterministic engineering calculator for sizing, diagnostics and cross-checks. Final equipment, duct and refrigeration decisions should use measured conditions, manufacturer data and applicable codes or standards.