Duct Velocity Chart: Recommended FPM for Supply, Return, and Branch Ducts
Duct design · Velocity · Updated 2026-07-22
For a typical house, design supply trunks at 700 to 900 FPM, supply branches at 500 to 700 FPM, and keep return grille face velocity at 500 FPM or lower. Light commercial mains run higher, 1,000 to 1,300 FPM, and industrial mains 1,200 to 1,800 FPM, because those buildings already have more background noise to hide in.
Velocity is not a code number. It is a comfort number. This guide gives the recommended FPM by duct role and building type, shows the one-line arithmetic that lets you check any run you can put a tape measure on, and names which source each figure comes from, because the residential numbers and the commercial numbers do not come from the same place.
Recommended duct velocity chart (FPM)
| Duct role | Residential (quiet) | Residential (standard) | Light commercial | Industrial | Noise note |
|---|---|---|---|---|---|
| Main supply trunk | 400 - 700 | 700 - 900 | 1,000 - 1,300 | 1,200 - 1,800 | Above 900 FPM a residential trunk starts to be audible as a steady rush |
| Supply branch | 300 - 500 | 500 - 700 | 600 - 900 | 800 - 1,000 | Manual D recommends 600 FPM for branches; 900 FPM max rigid, 700 FPM max flex |
| Supply register / outlet | 300 - 500 | 500 - 700 | 500 - 800 | 800 - 1,200 | Throw and NC come from the register manufacturer's data, not from the duct |
| Return branch | 300 - 400 | 400 - 600 | 600 - 900 | 800 - 1,000 | Manual D recommends 400 FPM, allows 700 FPM max |
| Return trunk | 400 - 600 | 600 - 700 | 900 - 1,300 | 1,200 - 1,500 | Manual D recommends 600 FPM, 700 FPM max; returns sit closer to occupants |
| Return grille face | 250 - 300 | 400 - 500 | 500 - 600 | 600 - 800 | Filter grilles want 300 FPM or less; a plain return grille wants 500 or less |
| Outdoor air intake | 500 or less | 500 or less | 500 or less | 500 or less | Free-area velocity, limited by water carryover through the louver, not by noise |
Read the residential columns as the governing ones for a house. ACCA Manual D Table 3-1 is materially quieter than commercial practice: 700 FPM recommended and 900 FPM maximum for rigid supply trunks, 600 FPM recommended for supply branches, 600 FPM recommended and 700 FPM maximum on return trunks, and 400 FPM recommended on return branches. Flex is held tighter still, roughly 600 FPM recommended and 700 FPM maximum on both supply trunks and branches. SMACNA HVAC Duct Construction Standards does not set these numbers at all - it drives duct construction through pressure class (1/2, 1, 2, 3, 4, 6 and 10 in. w.g.), and where the designer designates no class the commonly cited default basis is 1 in. w.g., rising to 2 in. w.g. upstream of VAV boxes. SMACNA constrains gauge and reinforcement, not acoustics.
The five velocity bands
| Band | FPM range | What it means |
|---|---|---|
| Quiet | 0 - 400 | Bedroom-grade. Also the range where a supply branch may be too slow to throw air across a room |
| Residential | 400 - 900 | The Manual D comfort window for houses. Trunks live at the top of it, returns near the bottom |
| Transitional | 900 - 1,200 | Past residential guidance. Fine in an office, audible in a bedroom |
| Commercial | 1,200 - 1,800 | Normal for light commercial mains and industrial trunks with background noise to mask them |
| High velocity | 1,800+ | Purpose-built high-velocity or industrial systems only. In a house this is a callback |
How to check any duct with a tape measure
Velocity is airflow divided by area: V = CFM / A, where A is the cross-sectional area in square feet. For round duct, A = pi x (d/2)squared / 144 with d in inches. For rectangular, A = (w x h) / 144. That is the whole calculation - no friction chart, no fitting losses.
- Measure the duct: An 8 in. round branch. Area = pi x 16 / 144 = 0.349 sq ft.
- Get the airflow: Design airflow for that branch is 200 CFM.
- Divide: 200 / 0.349 = 573 FPM. Residential band, inside the Manual D branch window. This run is fine.
- Now crush the flex: A sagging bend that pulls the same run down to an effective 7 in. leaves 0.267 sq ft, so 200 / 0.267 = 748 FPM - past the 700 FPM Manual D flex maximum, from one bad support strap.
CFM and duct size to FPM, at a glance
| Round duct | 100 CFM | 200 CFM | 400 CFM | 600 CFM | 1,000 CFM |
|---|---|---|---|---|---|
| 6 in. | 509 | 1,019 | 2,037 | 3,056 | 5,093 |
| 8 in. | 286 | 573 | 1,146 | 1,719 | 2,865 |
| 10 in. | 183 | 367 | 733 | 1,100 | 1,834 |
| 12 in. | 127 | 255 | 509 | 764 | 1,273 |
| 14 in. | 94 | 187 | 374 | 561 | 935 |
| 16 in. | 72 | 143 | 286 | 430 | 716 |
The tells that mean you are over velocity
- Registers whistling or hissing. Usually the outlet, not the duct. Check the register's rated CFM and NC before you blame the trunk.
- Return grille roar on startup. Face velocity over 500 FPM, or a filter grille asked to do 500 when it wants 300. The fix is a bigger grille, not a stronger blower.
- Noise that starts at a reducer. Every size reduction multiplies velocity by the area ratio. Dropping 10 in. to 8 in. cuts area by 36%, so velocity climbs about 56%.
- Crushed or sagging flex. Effective diameter shrinks, velocity climbs, and the sag adds friction on top. Flex is held to lower velocity limits than rigid for exactly this reason.
- Only one room is loud. Look for a branch takeoff sized off the trunk schedule instead of off its own CFM.
See the band before the duct goes in the ceiling. Ductulator: Duct Pro HVAC Calc classifies the velocity at your optimal size into the same five bands - quiet, residential, transitional, commercial, high velocity - as a single-glance color bar, and flags any over-velocity or over-friction option in safety red so it never gets installed. Type a CFM, pick round, rectangular, or flex, and eight valid sizes render with velocity and friction per 100 ft alongside the SMACNA gauge. Learn more about Ductulator.
Velocity is one of three checks, not the only one
A run passes when velocity is in band, friction rate is inside your available static pressure budget, and the size actually delivers the CFM the room needs. Duct Pro runs all three at once: Huebscher equivalent diameter per ASHRAE Fundamentals Chapter 21, full Darcy-Weisbach friction with the Colebrook friction factor for galvanized sheet metal, and SMACNA Table 1-4 gauge selection - with the velocity band and the citation printed under each result. Save the run to a project and the inspector-ready PDF carries the SMACNA and ASHRAE citations on every page.
Frequently asked questions
How many FPM should a duct have?
In a house, 700 to 900 FPM in the supply trunk, 500 to 700 FPM in supply branches, 600 FPM or less in the return trunk, and 500 FPM or less across a return grille face. ACCA Manual D Table 3-1 recommends 700 FPM for rigid supply trunks with a 900 FPM ceiling, and holds flex duct to about 700 FPM maximum.
What is the maximum air velocity for residential duct?
ACCA Manual D Table 3-1 puts the maximum at 900 FPM for rigid supply ducts and 700 FPM for supply flex, with 700 FPM on returns. Those are noise limits, not code limits, so nothing fails inspection at 950 FPM - it just gets audible, and generated sound power rises with roughly the fifth to sixth power of velocity.
What duct velocity is too loud?
In a bedroom or a quiet office, anything past about 900 FPM in the branch serving it. Quiet-room targets are lower still, commonly cited around 700 FPM in the main duct for NC-25 spaces. In an industrial building 1,800 FPM can be unremarkable because the background noise is already higher.
How do you calculate duct velocity from CFM?
Divide airflow by cross-sectional area in square feet: V = CFM / A. For round duct, A = pi x (diameter in inches / 2)squared / 144. A 10 in. round duct is 0.545 sq ft, so 400 CFM through it is 400 / 0.545 = about 733 FPM.