How duct sizing works (the short version of Manual D)
A duct must move its design airflow without exceeding a target velocity (noise, pressure loss) — residential systems aim for roughly 600–900 feet per minute depending on the run. This calculator applies the velocity method: area = CFM ÷ FPM, converts to a round diameter, rounds up to the next standard size, and lists rectangular ducts with the same ASHRAE equivalent diameter. It matches the friction-rate charts (0.08–0.10 in.wg/100 ft) used by installers for typical residential runs.
Duct size chart (supply, ~700 FPM)
| CFM | Round duct | Rectangular options | Typical use |
|---|---|---|---|
| 50–90 | 4–5" | — | Single register branch |
| 100–140 | 6" | 8×6 | Bedroom branch |
| 150–220 | 7" | 10×6, 8×8 | Large room branch |
| 230–300 | 8" | 10×8, 12×6 | Two-register branch |
| 310–420 | 9–10" | 12×8, 14×8 | Small trunk / 1-ton |
| 430–650 | 10–12" | 14×10, 16×8 | Trunk / 1.5-ton |
| 700–1,000 | 14" | 18×10, 16×12 | 2–2.5-ton trunk |
| 1,100–1,500 | 16" | 20×12, 18×14 | 3–3.5-ton trunk |
| 1,600–2,000 | 18" | 24×12, 20×16 | 4–5-ton trunk |
The three duct mistakes that choke systems
- Sizing returns like supplies. Returns need lower velocity (~600 FPM) — i.e. bigger ducts. Most "my new AC is loud and weak" complaints trace to starved returns.
- Treating flex like sheet metal. Flex duct has far higher friction when compressed. Pull it tight, keep runs short, and go one size up when the calculation is marginal.
- Forgetting total airflow. The trunk must carry ~400 CFM per ton of cooling. A 3-ton heat pump on a 14" trunk designed for 2 tons will never deliver rated capacity.
Frequently asked questions
What size duct do I need for 400 CFM?
At supply velocity (~700 FPM), 400 CFM needs a 10-inch round duct, or rectangular equivalents like 12×8 or 14×8. As a return (~600 FPM), step up to 12-inch.
How many CFM can a 6 inch duct handle?
About 100–140 CFM at typical residential supply velocity. Pushing 200+ CFM through a 6-inch duct makes it loud and wastes blower energy.
What size duct for a 3-ton AC unit?
A 3-ton system moves ~1,200 CFM, which calls for a 16-inch round trunk (or 20×12 rectangular) plus properly sized branches. Returns should be at least as large as supplies.
Round or rectangular duct — which is better?
Round moves more air per material and leaks less; rectangular fits joist bays and tight ceilings. Match by equivalent diameter, not by cross-section area alone.
Is flex duct as good as metal?
Properly stretched flex performs close to metal in short branches. Long, saggy or compressed flex can double friction — the leading cause of weak airflow in newer homes.