How to Calculate Duct Size from CFM and Velocity (Step-by-Step Guide)

Sizing ductwork by CFM comes down to one equation and one design decision. This guide walks the continuity equation A = Q / V with worked supply trunk and branch examples, shows how to convert to standard round and rectangular sizes, and includes a CFM to duct size chart you can work from in the field.

Last updated September 2026

The duct sizing equation

Every duct sizing method starts with conservation of mass. For air at ordinary building conditions, the volume flow rate through a section equals the cross-sectional area times the average velocity:

Q (CFM) = A (ft²) × V (FPM) → A = Q / V

Solve for area, then convert that area to a duct size. For round duct, area in square feet becomes a diameter in inches with:

D (in) = √( 4 × A × 144 / π )

That is the whole duct sizing equation. The engineering judgment is in choosing V — the design velocity — because velocity sets noise, friction rate and fan energy for the life of the system.

Step 1 — Get the design airflow for the section

Use the CFM the section actually carries, not the unit's total. A trunk downstream of two branches carries only what remains. Work from the diffuser schedule backwards: total the CFM of every outlet downstream of the section you are sizing.

Step 2 — Choose a design velocity

Velocity is the lever between first cost and operating cost. Higher velocity means smaller duct, cheaper sheet metal and more shaft space — but more noise, more static pressure and a bigger fan. These are the bands most commercial designers work to:

Application	Design velocity	Typical use
Branch / runout	400–800 FPM	Diffuser drops, VAV inlets, occupied-space runouts
Low-noise commercial	800–1,200 FPM	Offices, classrooms, conference rooms, healthcare
General commercial	1,000–1,600 FPM	Main supply and return trunks in commercial buildings
Industrial	1,600–2,400 FPM	Warehouse, process and mechanical-room mains

Step 3 — Solve for area and convert to a duct size

Worked example — supply trunk. A rooftop unit discharges 4,000 CFM into a main trunk in an office building. Design velocity 1,200 FPM.

A = 4,000 / 1,200 = 3.33 ft² → D = √(4 × 3.33 × 144 / π) = 24.7 in

Round up to a 26 in standard round, which drops the actual velocity to about 1,085 FPM. If the shaft cannot take a 26 in round, a 2:1 rectangular section of roughly 34" × 17" gives the same equivalent diameter.

Worked example — room branch line. A single 250 CFM diffuser drop in the same office. Design velocity 700 FPM.

A = 250 / 700 = 0.357 ft² → D = 8.1 in → use 9 in round

Note the different velocity. Sizing that branch at trunk velocity would give a 6 in duct that whistles at the diffuser — the single most common cause of noise complaints on otherwise correctly sized systems.

Try it: CFM to duct size

Solves A = Q / V and rounds to the next standard size.

Airflow (CFM)

Application

Design velocity (FPM)

Rectangular aspect ratio

Required area: 1.667 ft²
Standard round: 18 in
Rectangular equivalent: 24" × 12"

CFM to duct size chart

Round duct diameters that satisfy each velocity target, rounded up to the next standard size. Use the branch column for runouts and the trunk column for mains.

Airflow	Branch @ 700 FPM	Trunk @ 1,200 FPM	High velocity @ 1,800 FPM
100 CFM	6 in	4 in	4 in
200 CFM	8 in	6 in	5 in
400 CFM	11 in	8 in	7 in
600 CFM	14 in	10 in	8 in
800 CFM	16 in	12 in	10 in
1,000 CFM	18 in	14 in	11 in
1,500 CFM	20 in	16 in	14 in
2,000 CFM	24 in	18 in	16 in
3,000 CFM	30 in	22 in	18 in
4,000 CFM	34 in	26 in	22 in
6,000 CFM	40 in	32 in	26 in
8,000 CFM	48 in	36 in	30 in
10,000 CFM	52 in	40 in	32 in

Next standard round size above the exact requirement, so actual velocity is slightly below target.

For a version you can set your own friction rate and material on, use the full duct size chart or the duct sizer.

Velocity sizing vs friction rate sizing

Velocity sizing gets you a duct that moves the air quietly. It does not tell you what the fan has to overcome. On a long run, the equal-friction method — holding a constant friction rate, commonly 0.08 to 0.10 in. w.g. per 100 ft for commercial low-pressure duct — controls total static better. In practice, size to velocity for branches and short runs, size to friction rate for trunks, then check the other one.

Frequently asked questions

What is the formula to calculate duct size from CFM?

Use the continuity equation A = Q / V, where A is duct area in square feet, Q is airflow in CFM and V is design velocity in feet per minute. Convert the area to a round diameter with D = sqrt(4A x 144 / pi).

What velocity should I design a supply trunk for?

Commercial supply trunks are typically designed for 1,000 to 1,600 FPM, low-noise commercial areas for 800 to 1,200 FPM, and branch runouts for 400 to 800 FPM.

Should I round duct sizes up or down?

Round up to the next standard size. Rounding up lowers velocity, noise and friction rate slightly; rounding down raises all three and can push the fan outside its selection point.