Duct area formula?
Area (sq ft) = CFM ÷ velocity (FPM).
Enter required CFM and a target duct velocity in feet per minute (FPM). Default velocity is 700 FPM (allowed range about 400–1,200 FPM).
The tool computes round duct diameter from CFM and velocity, then recommends the next standard size from a common residential set (6–20 inches).
It sizes round duct only. It does not calculate rectangular equivalents, fittings, friction rate, or full Manual D layouts.
Concise answers for common searches — definitions, steps, and comparisons.
Area (sq ft) = CFM ÷ velocity (FPM).
700 FPM.
Cross-sectional area (sq ft) = CFM ÷ velocity (FPM). Round diameter is derived from that area, then rounded up to the next standard duct size.
Formula
areaSqFt = cfm ÷ velocityFpm; diameterIn = sqrt((4 × areaSqFt × 144) ÷ π). Nearest standard size = first of [6,7,8,9,10,12,14,16,18,20] that is ≥ diameterIn.Type the airflow you need (typical residential entries about 50–5,000 CFM). Get CFM from equipment specs or the Duct CFM Calculator.
Enter velocity in FPM (default 700). Lower velocity usually means a larger duct; higher velocity a smaller duct with more noise risk.
Use the primary round duct diameter, then note the suggested next standard size for ordering.
Verify friction rate, fittings, and blower static pressure with Manual D or a licensed HVAC technician before installation.
Input
cfm 1000 · velocity 700Output
~16.2" round · order 18"Area = 1000÷700 sq ft; diameter from circular area; next standard ≥ 16.2" is 18".
Input
cfm 800 · velocity 700Output
~14.5" round · order 16"Area = 800÷700 sq ft; next standard size at or above calculated diameter is 16".
Common real-world scenarios where this tool saves time.
Enter 1,000 CFM and 700 FPM → about 16.2" calculated diameter, next standard size 18".
From a 3-ton × 400 CFM plan, enter 1,200 CFM at 700 FPM to size the main round trunk.
Same CFM at 500 FPM increases diameter versus 700 FPM—useful when noise matters more than compact ducts.
Step-by-step chains that connect related tools for common tasks.
Convert cooling capacity to airflow, then size round duct.
| CFM | Approx. calculated Ø | Next standard |
|---|---|---|
| 800 | ~14.5" | 16" |
| 1,000 | ~16.2" | 18" |
| 1,200 | ~17.7" | 18" |
Size airflow from equipment capacity first, then convert CFM to duct diameter here.
The validator expects about 400–1,200 FPM; 600–900 FPM is a common planning starting point for many trunks.
Higher FPM shrinks duct size but can raise noise and pressure drop—stay in a practical residential range.
Use the suggested next standard diameter so the duct matches stock sizes.
Trunk and branch networks need friction and fitting calculations beyond a single round-diameter estimate.
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Required CFM and target air velocity in FPM. It returns calculated round duct diameter and the next standard size.
Cross-section area in sq ft equals CFM ÷ FPM. Diameter is solved from that circular area, then compared to standard inch sizes.
The default is 700 FPM. Lower values enlarge the duct; higher values shrink it. Stay within roughly 400–1,200 FPM unless a designer specifies otherwise.
No. It estimates round duct diameter only. Rectangular equivalents need a separate conversion or Manual D design.
No. It is a quick geometric estimate. Final duct design should follow Manual D and equipment static-pressure limits.
From equipment blower data, a cooling-tonnage × 400 CFM estimate, or the Duct CFM Calculator on EverydayTools.
Duct size inputs remain local — EverydayTools does not collect duct-size calculator entries.
Estimates only — not professional HVAC or energy audit advice. Verify with a licensed contractor before purchasing equipment.
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Reviewed on 2026-06-28.
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Residential round duct is often sized for 600–900 FPM