How duct size is really determined
Ductwork is sized to airflow, not to whatever duct happened to be in the wall before. First comes the airflow each run must carry, measured in CFM - roughly 400 CFM for every ton of cooling. Each duct is then sized so air moves through it at a comfortable velocity: fast enough to be efficient, slow enough to stay quiet. Push too much air through too small a duct and you get noise, high static pressure, and a struggling blower.
Why the return matters as much as the supply
Homeowners obsess over supply registers and forget the return - but a system can only push out as much air as it can pull back. Undersized or too-few returns are one of the most common reasons a home feels stuffy or uneven even with a properly sized AC. Returns should be sized as generously as the supply, at a low velocity that keeps them quiet.
Round duct size and airflow
A quick reference for how much air a round duct carries at a typical supply-trunk velocity. Flexible duct should be sized one step larger.
| Round duct | Approx airflow |
|---|---|
| 6 inch | ~175 CFM |
| 8 inch | ~300 CFM |
| 10 inch | ~490 CFM |
| 12 inch | ~700 CFM |
| 14 inch | ~950 CFM |
| 16 inch | ~1,250 CFM |
| 18 inch | ~1,600 CFM |
| 20 inch | ~1,950 CFM |
Airflow at roughly 900 FPM (a quiet supply-trunk velocity). Branch runs and returns use lower velocities.
Sizing is only half of it. Duct layout, total run length, and fittings all add resistance. We confirm every duct system with an ACCA Manual D design, included with a duct replacement or new system estimate.
Duct Sizing Questions, Answered
How do I calculate duct size?
Duct size follows airflow. Size the system's airflow first - about 400 CFM per ton, so a 3-ton system moves roughly 1,200 CFM - then size each duct to carry its share at a comfortable velocity: about 700 to 900 feet per minute for a supply trunk and 500 to 700 for branch runs and returns. The calculator above turns CFM into a round diameter and a rectangular equivalent instantly, and a full Manual D design confirms it for your exact layout.
What size duct do I need for a 3-ton system?
A 3-ton system moves about 1,200 CFM, which needs roughly a 16-inch round main trunk (or about a 20 by 10 inch rectangular trunk) in sheet metal, with 6- to 8-inch branch runs to individual rooms. Flexible duct should be sized one step larger because of its higher internal friction. Enter your airflow above for an exact size.
What happens if my ducts are too small?
Undersized ducts raise static pressure, the resistance the blower fights against. That shows up as weak airflow at the registers, rooms that never reach temperature, whistling or roaring noise, higher energy bills, and premature wear on the blower motor and compressor. Right-sizing the ductwork is often the cheapest way to fix an uneven, noisy system.
Is flexible duct sized the same as sheet metal?
No. Flexible duct has more internal friction than smooth rigid metal, so it needs to be one nominal size larger to move the same air quietly. A run that would be 6-inch metal is typically 7- or 8-inch flex. The calculator above upsizes automatically when you choose flexible duct.
How big should the return duct be?
Return ducts should be sized at least as large as the supply side, and often larger, because a starved return chokes the whole system. Returns are sized for low velocity, around 500 to 700 FPM, to stay quiet. Many comfort and noise complaints trace back to undersized or too-few returns rather than the equipment itself.
What is the rule of thumb for duct sizing?
The common rules of thumb are about 1 CFM of airflow per square foot of home and roughly 400 CFM per ton of cooling. From there, each duct is sized to move its share at a comfortable velocity - about 700 to 900 feet per minute for a supply trunk and 500 to 700 for branches and returns. Rules of thumb get you close; a Manual D design accounts for run length and fittings.
How many CFM can a 12x12 duct carry?
A 12 by 12 inch duct, about a 13 to 14 inch round equivalent, carries roughly 900 CFM at a typical supply-trunk velocity - enough for a 2 to 2.5 ton system's main trunk. Longer runs and more fittings lower that, so treat it as a planning figure rather than a hard maximum.
What size duct do I need for 2,000 CFM?
For 2,000 CFM, plan on about a 20-inch round trunk, or a large rectangular equivalent, in sheet metal. Flexible duct should be sized one step larger because of its higher friction. Enter your airflow in the calculator above for the exact size and a rectangular option.
Get a Duct System Designed Right.
We size every run with a real Manual D calculation and give you a flat-rate, written quote - no pressure, no obligation.