Duct area and size from airflow and target velocity, with round and rectangular equivalents.
| Application | Typical (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 | ≤4:1 preferred (common guideline) | |
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
Book a Demo →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 typical design velocity ranges and the common 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 = longer side ÷ shorter side |
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. The velocity ranges and the ≤4:1 aspect-ratio preference are common rules of thumb for low-pressure commercial ductwork, not code limits. The check is green inside the typical range, amber below it or between the typical top and the listed maximum, and red above the maximum. Standard round sizes are checked at the next larger size, whose velocity is the one the installed duct will see.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Duct and pipe distribution sizing is planned for V2 — at launch, keep using this calculator for that step.
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.