SettingsSettings for this calculationUS
Air the duct must carry — switch the unit to whatever your design figures are in.
For a branch, this is the room's share of the total. A common rule is 400 CFM per ton of cooling, so a 3-ton system moves about 1200 CFM in total, divided between branches by each room's load.
Design air velocity.
600-900 fpm for residential branches, 900-1200 for trunks. Above roughly 1000 fpm in a branch serving a bedroom you will hear it, and noise is the complaint that gets a system torn out.
Round is more efficient; rectangular fits in a joist bay.
Round moves the same air in less metal and with less friction, so use it wherever the space allows. Rectangular exists because a joist bay does not, and the trade-off is real: for a given cross-section a flatter duct has more wetted perimeter, so it costs more friction and more sheet metal than the round duct it replaces.
Duct size (round diameter, or width at your depth)
10.2 in diameter
Velocity sizing only. A complete design also balances the friction rate across the whole system so every branch delivers its share — see Manual D.
- Duct area
- 82.29 in²
- Diameter (mm)
- 259.99 mm
- Airflow in L/s
- 188.78 L/s
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Duct Size Calculator: 10.24 in diameter — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ACCA Manual D — residential duct design; velocity and friction-rate method
- ASHRAE Handbook — Fundamentals, Duct Design: recommended residential velocities of 3-5 m/s (600-900 fpm) for branches, up to 6-9 m/s (1200-1800 fpm) for trunks
Inputs used
- Airflow
- 400 CFM
- Target velocity
- 700 ft/min
- Duct shape
- Round
- Available depth
- 8 in
Intermediate steps
- Duct area
- 82.29 in²
- Diameter (mm)
- 259.99 mm
- Airflow in L/s
- 188.78 L/s
Confidence note: Velocity sizing only. A complete design also balances the friction rate across the whole system so every branch delivers its share — see Manual D.
What this calculation does not cover
- Sizes one duct at a target velocity. It does not balance a system, and a system of individually correct ducts can still deliver the wrong air to each room.
- Fitting losses are excluded. Elbows, takeoffs and transitions often exceed the straight-duct loss in a residential run.
- Round the result up to the nearest available size, not down — undersizing raises velocity, noise and static pressure together.
Add the equipment this sizes
This result is a specification — 10.2 in diameter — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations2
- ACCA Manual D — residential duct design; velocity and friction-rate method
- ASHRAE Handbook — Fundamentals, Duct Design: recommended residential velocities of 3-5 m/s (600-900 fpm) for branches, up to 6-9 m/s (1200-1800 fpm) for trunks
Which documents these citations point at
Standards referenced: ACCA Manual D (Air Conditioning Contractors of America, United States).
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