Find a round duct diameter and an equivalent rectangular size from airflow and design velocity.
Area = airflow ÷ velocity. 400 CFM at 700 fpm needs 0.571 sq ft, or 82.3 sq in.
Round diameter = √(4 × area ÷ π), converted to inches.
Rectangular equivalent uses the equal-friction formula De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. A rectangle with the same friction needs more area than the round duct, because corners carry air less efficiently.
A supply trunk must carry 400 CFM at 700 fpm. The round diameter is 10.2 inches, so the next standard size is 11 inches in this simple check (check whether your standard allows rounding down). If ceiling space limits the width to 10 inches, the equal-friction rectangle is about 10 × 9 inches.
Illustration only. Confirm sizes with your design method and local code.
Technicians and estimators use a quick sizing check in the field: is this run big enough for the airflow it needs to carry? This calculator gives the answer from two numbers, airflow and the velocity your design standard allows.
It is a quick check, not a full design. Proper residential and commercial duct design also accounts for friction rate, fittings, total external static pressure and noise. Follow the design method required in your area, such as ACCA Manual D for residential systems.
This table shows how much air standard round sizes carry at two velocities. It is handy for a field check before running the calculator for the exact number.
| Diameter | Area (sq ft) | CFM at 600 fpm | CFM at 900 fpm |
|---|---|---|---|
| 6 in | 0.196 | 118 | 177 |
| 8 in | 0.349 | 209 | 314 |
| 10 in | 0.545 | 327 | 491 |
| 12 in | 0.785 | 471 | 707 |
| 14 in | 1.069 | 641 | 962 |
| 16 in | 1.396 | 838 | 1,257 |
The numbers are area times velocity and nothing more. Your design method sets the velocity and friction limits for each part of the system, and fittings change the picture further.
The International Residential Code, adopted with local changes in most states, requires duct systems to be sized under ACCA Manual D (section M1601.1) and the equipment to be sized from a Manual J load and Manual S selection (section M1401.3). Inspectors in many jurisdictions ask to see those reports for new systems and replacements. A velocity check like this one is useful in the field, but it does not stand in for the design documents a permit may need.
Fictional example: a 12-inch branch on a remodel must carry 300 CFM. At 600 fpm, 300 CFM needs 0.5 sq ft, a little under the 0.545 sq ft of a 10-inch duct, so the existing 12-inch run has room to spare and the low airflow complaint is more likely a damper, a crushed flex section or a dirty coil.
Use the value from the design method or standard that applies to your job. Velocity affects noise and pressure drop, and limits differ for supply trunks, branches and returns.
Rectangular ducts have more surface per unit of area, so they need more cross-section to carry the same air at the same friction.
No. Airflow per room comes from a heat load calculation. This tool only sizes the duct for a given airflow.
Flex duct has higher friction than metal duct, so a simple velocity check will undersize it. Use manufacturer data.
A common rule of thumb is about 400 CFM per ton, but equipment data and the Manual S selection give the real design figure, which can be lower in humid climates.
Not necessarily. Returns are often sized for lower velocity to reduce noise, which makes them larger for the same airflow. Follow your design method.
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