By Zencalcy Editorial TeamLast reviewed About us
CFM is simply the volume of air moving through a duct each minute: the cross-sectional area of the duct in square feet multiplied by the velocity in feet per minute. Duct designers balance the two — too fast and the duct roars, too slow and it never delivers.
Airflow in CFM from duct size and velocity.
area (sq ft) = width × height ÷ 144 CFM = velocity × area
Comfortable residential duct velocity is 600 to 900 FPM at the register and up to 1,200 FPM in trunk lines. Above 1,500 FPM the duct becomes audible; below 400 FPM it may not reach the far rooms.
| Symbol | Meaning |
|---|---|
| A | Duct cross-section in square feet. |
| v | Air velocity in feet per minute. |
| Q | Volumetric airflow in CFM. |
0.583 sq ft × 500 FPM = 292 CFM — a sensible trunk velocity for a small return.
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600 to 900 FPM near the living space, up to 1,200 FPM in main trunks. Higher is not better — noise and pressure drop both climb with the square of velocity.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026