Duct Size Calculator
Duct diameter for a target airflow.
Reviewed by James Whitfield · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Round duct diameter
- 11.1 in
- Duct area
- 96 in²
- Rectangular width
- 12 in
- Next standard size
- 12 in
Quick answer
Duct area equals airflow divided by air velocity: area (ft²) = CFM ÷ FPM. Multiply by 144 for square inches, then round diameter = 2 × √(area ÷ π). For 600 CFM at 900 ft/min the area is 0.667 ft² (96 in²), which needs an 11.1 in round duct, so you would use 12 in, or a rectangular duct of 8 × 12 in by area.
What the Duct Size calculator does
This calculator turns an airflow in cubic feet per minute (CFM) into the duct size that carries it at a chosen velocity. It gives the round diameter, the cross-sectional area, the width of a rectangular duct at a height you choose, and the next whole-inch round size. The same arithmetic sizes supply branches, trunk lines, return ducts, plenums, jumper ducts, register boots, takeoffs and zone dampers: only the airflow and the velocity you allow change.
Velocity is the design choice. Faster air means smaller, cheaper ducts but more noise and more static pressure for the blower to overcome. Residential systems usually run slower than commercial ones because people notice whistling registers at home.
The method sizes by velocity alone. It does not total friction along the run, fittings or flex duct, which is what ACCA Manual D or a friction chart does, so treat the result as the starting size and check long or awkward runs with the duct friction loss calculator. The airflow itself should come from a room-by-room HVAC load calculation.
The formula
A = Q ÷ v A(in²) = A(ft²) × 144 D = 2 × √(A(in²) ÷ π) W = A(in²) ÷ H
| Symbol | Meaning | Unit |
|---|---|---|
| Q | Airflow the duct must carry (1 CFM ≈ 0.472 L/s ≈ 1.70 m³/h) | CFM |
| v | Target air velocity in the duct (1 ft/min = 0.00508 m/s) | ft/min (FPM) |
| A | Required free cross-sectional area | ft² or in² |
| D | Round duct inside diameter | in |
| H, W | Rectangular duct height you choose and the width that gives the same area | in |
This is the continuity equation (flow = area × velocity) used throughout the ASHRAE Handbook – Fundamentals and ACCA Manual D. The rectangular width matches the round duct's area, not its friction: a rectangle has more wall per unit area, so for equal pressure drop it needs to be somewhat larger (see the equal-friction section below). Tonnage-based sizing, used for whole systems, converts tons to CFM first at roughly 350–450 CFM per ton, most often 400.
Worked examples
Supply branch to a bedroom
A bedroom needs 100 CFM and you want a quiet branch at 600 ft/min. The joist bay allows a 4 in deep rectangular duct.
Inputs
- Airflow100 CFM
- Target velocity600 ft/min
- Rectangular duct height4 in
Result
- Round duct diameter5.5 in
- Duct area24 in²
- Rectangular width6 in
- Next standard size6 in
A 5.5 in calculated diameter rounds up to a 6 in branch, or 4 × 6 in rectangular by area. Long branches or flex duct often justify the next size up again.
Return duct for a 3-ton system
A 3-ton system at 400 CFM per ton moves 1,200 CFM. The return should run slowly at 600 ft/min for quiet operation, using a 10 in deep rectangular duct in a wall cavity or soffit.
Inputs
- Airflow1200 CFM
- Target velocity600 ft/min
- Rectangular duct height10 in
Result
- Round duct diameter19.1 in
- Duct area288 in²
- Rectangular width28.8 in
- Next standard size20 in
About 288 in² is needed: a 20 in round return or roughly 10 × 29 in rectangular. Undersized returns are one of the most common causes of noisy, high-static systems, so returns are usually sized more generously than supplies.
Main supply trunk or plenum takeoff
The same 1,200 CFM leaves the air handler in a supply trunk designed for 900 ft/min, 8 in deep.
Inputs
- Airflow1200 CFM
- Target velocity900 ft/min
- Rectangular duct height8 in
Result
- Round duct diameter15.6 in
- Duct area192 in²
- Rectangular width24 in
- Next standard size16 in
A 16 in round trunk or 8 × 24 in rectangular. Compared with the return example, accepting 900 instead of 600 ft/min cut the area by a third, which is why supply trunks can be smaller than returns carrying the same air.
Target velocities by duct type
These are common residential and light-commercial rules of thumb used with velocity sizing; designers using Manual D size from friction rate instead and then check that velocity stays below limits like these. Commercial systems with lined or acoustically treated ducts often run higher.
| Duct section | Residential | Light commercial |
|---|---|---|
| Supply trunk / extended plenum | 700–900 | 1,000–1,300 |
| Supply branch, takeoff, register boot | 500–700 | 600–900 |
| Return trunk | 600–700 | 800–1,000 |
| Return branch / jumper duct | 400–600 | 600–800 |
| Return grille face | 300–500 | 400–500 |
| Flex duct runs | 600 or less | 700 or less |
Equal-friction size for rectangular ducts
A rectangular duct with the same area as a round one has more friction, because more of the air touches a wall. The ASHRAE equivalent-diameter (Huebscher) formula gives the round size with the same pressure drop: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25.
For an 8 × 12 in duct that works out to about 10.7 in, not the 11.1 in its area suggests. So if a run is long, size the rectangle a little wider than this calculator's width, or check it with the equivalent duct diameter calculator. Keep aspect ratios under about 4:1 where you can; flat, wide ducts are inefficient and harder to seal.
Plenums, takeoffs, jumpers and zone dampers
- Plenum or extended plenum: size for the full blower airflow at trunk velocity, then reduce the trunk after groups of takeoffs so velocity stays roughly constant. The trunk duct size calculator steps through the reductions.
- Takeoffs and register boots: size for the branch airflow; a boot should be at least as large in area as the branch feeding it.
- Jumper ducts and transfer grilles: move return air out of closed bedrooms. Size for the room's supply CFM at low velocity (about 400–500 ft/min) so they stay quiet.
- Zone dampers: match the duct the damper sits in. A damper smaller than the duct adds a restriction even when fully open.
- Spiral duct: use the round diameter directly; spiral seams add little friction compared with flex.
How to use the Duct Size Calculator
- 1Enter the airflow in CFM for the section you are sizing: a room's supply, a trunk's total or the system's total (about 400 CFM per ton).
- 2Enter a target velocity from the table: lower for branches, returns and quiet rooms, higher for main trunks.
- 3Enter the height available for a rectangular duct, in inches, to get the matching width.
- 4Round the diameter up to the next stocked size, and go up one more size for long or flex runs.
Frequently asked questions
What size duct do I need for 400 CFM?
At 700 ft/min, 400 CFM needs about 82 in², just over 10 in round, so use the next stocked size up. At 900 ft/min it needs 64 in², about 9 in round.
How many CFM can a 6 inch duct carry?
A 6 in round duct has about 28.3 in² (0.196 ft²) of area, so it carries roughly 120 CFM at 600 ft/min and about 140 CFM at 700 ft/min. Flex duct of the same size usually delivers less because of its higher friction.
What size duct for a 3 ton AC unit?
A 3-ton system usually moves around 1,200 CFM. At 900 ft/min the supply trunk needs about 192 in² (16 in round); a return at 600 ft/min needs about 288 in² (20 in round).
Should the return duct be bigger than the supply?
Usually yes. Returns are designed for lower velocity to keep noise and static pressure down, so for the same airflow they end up larger than the supply trunk.
What happens if ducts are undersized?
Static pressure rises, airflow falls below what the equipment needs, registers whistle and the blower works harder. In cooling mode low airflow can also freeze the evaporator coil.
References
- Manual D – Residential Duct Systems — Air Conditioning Contractors of America (ACCA)
- ASHRAE Handbook – Fundamentals (Duct Design chapter) — ASHRAE
- HVAC Systems Duct Design — SMACNA
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Last reviewed: October 24, 2026
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