How do I calculate duct size from CFM?

Divide the airflow by the target velocity to get the required area: area (sq ft) = CFM / FPM. Then solve the round diameter with d = sqrt(4 x area / pi) x 12 and round up to the nearest standard duct size. For 800 CFM at 900 FPM you need 0.889 sq ft, about a 12.8 inch diameter, so a 14 inch round duct.

What velocity should air move through ducts?

For residential systems, common targets are about 900 FPM in main supply trunks, 700 FPM in main returns, and 600 FPM in branch supply and return runs. Lower velocities are quieter and add less friction; higher velocities let you use smaller ducts but increase noise and static pressure.

How do I convert a round duct to rectangular?

Use the ASHRAE equivalent diameter formula: De = 1.30 x (a x b)^0.625 / (a + b)^0.25, where a and b are the rectangular sides in inches. A rectangular duct is equivalent when its De matches the required round diameter. This calculator solves the missing side for you in half-inch steps.

Is the velocity method accurate enough for duct design?

The velocity method is a solid estimate for sizing a single run or checking an existing duct. Full residential duct design uses ACCA Manual D, which accounts for available static pressure, duct length, and fitting losses across the whole system. For complete system design, work with a qualified HVAC professional.

Duct Size Calculator

Size supply and return ducts with the velocity method. Get round duct diameter, nearest standard size, and rectangular equivalents from CFM.

Back to Calculators

What size duct do you need?

Enter the airflow and pick the duct application. We preset the target velocity for you and size the duct with the velocity method.

Airflow (CFM)

Not sure? Use our CFM calculator first.

Duct Application

Enter the airflow in CFM to see your duct size.

How this calculator works

Velocity method:duct area (sq ft) = CFM ÷ FPM. Divide the airflow by the target air velocity to get the required cross-sectional area. For 800 CFM at 900 FPM, the duct needs 800 ÷ 900 = 0.889 sq ft.

Round duct diameter:diameter (in) = √(4 × area ÷ π) × 12. For 0.889 sq ft that is about 12.77", so you round up to the next standard size: a 14" round duct.

Rectangular equivalent:a rectangular duct with sides a and b matches a round duct when its equivalent diameter De = 1.30 × (a × b)^0.625÷ (a + b)^0.25meets the required round diameter. We solve the missing side in 0.5" steps.

Target velocities: residential systems typically aim for about 900 FPM in main supply trunks, 700 FPM in main returns, and 600 FPM in branch runs. Lower velocities are quieter; higher velocities need less duct area but add friction and noise.

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