Size round or rectangular ducts from CFM and target velocity. Converts between shapes using equivalent diameter. Pass/fail against SMACNA limits.
| Application | Recommended (FPM) | Max (FPM) |
|---|---|---|
| Supply air — main trunk | 600–1200 | 1500 |
| Supply air — branch duct | 400–800 | 1000 |
| Return air — main trunk | 500–800 | 1000 |
| Return air — branch duct | 300–600 | 800 |
| Exhaust / relief | 500–1000 | 1500 |
| Aspect ratio (max) | ≤4:1 preferred; 8:1 absolute max | |
This duct area and size calculator turns an airflow and a target velocity into a duct size. Work in three modes: enter CFM and velocity to get the required round diameter and area, enter CFM with a height limit to get a rectangular width × height plus equivalent-diameter alternatives, or enter a rectangular width and height to read off its equivalent diameter and aspect ratio. Each result is checked against SMACNA velocity ranges and the 4:1 aspect-ratio guideline.
Velocity sets both noise and the duct's footprint. Sizing inside the recommended band keeps air quiet at the diffuser while avoiding oversized, costly ductwork. The rectangular-alternatives table makes it easy to fit a duct into a tight ceiling plenum without pushing the aspect ratio — and therefore friction and cost — too high.
| Area | A (ft²) = Q (CFM) ÷ V (FPM) |
| Round diameter | D = √(4 · A ÷ π) (A in in²) |
| Rect. width | W = A ÷ H (A in in², H = height) |
| Equiv. diameter | De = 1.3 · (a · b)0.625 ÷ (a + b)0.25 |
| Aspect ratio | AR = W ÷ H |
The core relation is the continuity equation A = Q ÷ V, with 144 used to convert between square feet and square inches. The round diameter inverts the circle area A = π · (D ÷ 2)². The equivalent-diameter constant 1.3 and the 0.625 / 0.25 exponents follow the ASHRAE Fundamentals equation for the round duct with friction loss equal to a rectangular duct of sides a and b. Recommended velocity ranges and the ≤4:1 aspect-ratio preference follow standard SMACNA duct-design practice.
This calculator handles one step. AIM Works runs the complete MEP design workflow — thermal load calculations, duct & pipe networks, equipment selection, code compliance, and an AI design assistant — in one tool.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.