Duct Size Calculator
Area = CFM / velocity → round diameter and a rectangular equivalent. Click any i for detail.
Airflow
Airflow (CFM)
i
CFM
Supply or return airflow. |
|
|---|---|
Design velocity (FPM)
i
FPM
Common supply ~700–900 FPM. |
|
Rect aspect (W:H)
i
Aspect
For rectangular equivalent; e.g. 2 means W=2H. |
How to use this calculator
- Enter CFM.
- Set design velocity.
- Choose a rectangular aspect ratio.
- Read round diameter and rect size sketch.
Results explained
Duct cross-section area in sq ft is CFM divided by velocity in FPM. Round diameter follows from that area; a rectangle with a chosen aspect ratio shares the same area. Friction, fittings, and noise limits need a real design.
Quick reference: duct sizing sketch
A = CFM/FPM; D = 12×√(4A/π).
| Item | Detail |
|---|---|
| 400 CFM @ 800 FPM | A = 0.5 sq ft |
| Round D | ≈ 9.6 in |
| Aspect 2 | ≈ 12 × 6 in |
| Returns | Often lower velocity |
Sizing sketch only — not a Manual D replacement.
How the estimate is built
A=CFM/vel; D_in=12√(4A/π); H=√(A/aspect), W=aspect×H.
Example scenario
400 CFM at 800 FPM → ~9.6 in round.
FAQ
Supply vs return?
Returns often use lower velocities / larger ducts.
Flex duct?
Effective diameter and friction differ.
See CFM page?
Yes — for airflow from area × velocity.
Static pressure?
Not modeled here.