Duct size calculator

How do I find air velocity in a duct?

Velocity (feet per minute) = airflow (CFM) ÷ duct area (square feet). 400 CFM through an 8-inch round duct (0.349 sq ft) moves at about 1,146 FPM.

What velocity should I design for?

The DOE Building America Solution Center, citing ACCA Manual D’s noise-control table, lists maximums of 700 FPM for supply trunks, 600 FPM for rigid supply branches, 700 FPM for flexible branches and 500 FPM for rigid returns.

How do I size a round duct?

Enter a target velocity: diameter = √(4 × CFM ÷ (π × FPM)) × 12 inches. 400 CFM at 700 FPM needs about a 10.2-inch duct.

Is velocity the only thing that matters?

No — a full duct design (ACCA Manual D) also balances friction loss, fittings and the blower’s available pressure. The calculator covers the velocity check.

Can I use rectangular ducts?

Yes — choose rectangular and enter the inside width and height; the area is width × height.

What is the difference between duct velocity and register face velocity?

Duct velocity is airflow divided by the cross-section of the duct, which is what this calculator finds. Face velocity is airflow divided by the open area of a register or grille, and it has its own limit. The same Building America guide, citing ACCA Manual D Table A1-1, suggests 700 FPM for supply registers (or a manufacturer noise criterion of 30), 600 FPM for flexible return ducts and 300 FPM face velocity for return grilles.

How the numbers work

Velocity is flow divided by flow area, so FPM = CFM ÷ area in square feet. A round duct of d inches has an area of π ÷ 4 × (d ÷ 12)² square feet, which turns into the diameter formula above; a rectangular duct has width × height ÷ 144. The result is an average over the whole section. The calculator takes the airflow and the inside dimensions you type at face value and knows nothing about the duct material, length or fittings, so it checks velocity, not pressure loss.

Worked example

A supply trunk has to carry 400 CFM at no more than 700 FPM. The diameter for that velocity is 10.2 inches. Round duct is sold in set sizes, so the choices are a 10-inch duct at 733 FPM, slightly over the limit, an 11-inch at 606 FPM or a 12-inch at 509 FPM. A 12 × 8 inch rectangular duct has an area of 0.667 square feet and runs at 600 FPM. Under the 600 FPM limit for a rigid supply branch, the 11-inch round duct would be just over and the 12-inch would pass.

Common mistakes

Passing the velocity check is not a duct design. Manual D also works out the friction rate, the pressure lost per 100 feet of duct in inches of water column, along the longest run, and fittings count as extra length: the Building America guide says a 90° bend can add as much as 35 feet of effective length, depending on how the duct is built. Flexible duct installed compressed has a much higher friction rate than the same duct stretched out, according to the same guide. Use inside dimensions, after any liner, not nominal or outside sizes. And do not apply the trunk limit to a branch or the duct limit to a register, because each has its own number.

Sources: Building America Solution Center (U.S. Department of Energy, PNNL), “Compact Air Distribution,” which cites ACCA Manual D Table A1-1, Air Velocity for Noise Control; A. Burdick, IBACOS, “Advanced Strategy Guideline: Air Distribution Basics and Duct Design,” DOE Building America, December 2011. ACCA Manual D itself is a paid standard and was not read. Related: CFM calculator, BTU calculator, furnace size calculator. How we check our calculators.