Duct Size Calculator
A duct size calculator finds area as CFM ÷ velocity × 144 sq in, then diameter √(area ÷ 0.7854): 400 CFM at 700 ft/min needs 82.3 sq in, a 10.2 in round duct.
What size duct do I need?
A duct size calculator divides the airflow in cubic feet per minute (CFM) by the speed you want the air to move, in feet per minute. That gives the duct area in square feet. Multiply by 144 for square inches, then find the round diameter with the circle formula. 400 CFM at 700 ft/min needs 82.3 square inches, which is a 10.2-inch round duct. In practice you pick the next standard size, 10 or 12 inches, depending on how quiet you want it.
Enter the airflow and a design velocity. The calculator returns the diameter and the area.
The formula in plain words
- Area (sq ft) = CFM ÷ velocity
- Area (sq in) = area (sq ft) × 144
- Round diameter = square root of (area in sq in ÷ 0.7854)
The 0.7854 is π ÷ 4, which turns a circle’s area back into its diameter. This is the velocity method. Professional duct design (ACCA Manual D) sizes each run by friction loss and the blower’s available static pressure, which can change the result on long runs.
Worked examples
All three match the calculator.
- Supply branch for a living room, 400 CFM at 700 ft/min
- Area: 400 ÷ 700 × 144 = 82.3 sq in
- Diameter: √(82.3 ÷ 0.7854) = 10.2 in
- Standard size: 10 in runs slightly faster than 700; 12 in runs slower and quieter
- Bedroom branch, 100 CFM at 600 ft/min
- Area: 100 ÷ 600 × 144 = 24.0 sq in
- Diameter: √(24.0 ÷ 0.7854) = 5.5 in
- Standard size: 6 in flex or rigid
- Main trunk for a 3-ton system, 1,200 CFM at 900 ft/min
- Area: 1,200 ÷ 900 × 144 = 192.0 sq in
- Diameter: 15.6 in, so a 16 in round trunk or an equivalent rectangle
Round duct capacity by velocity
Airflow each size carries at three speeds. Area = π × diameter² ÷ 4.
| Round duct | Area | CFM at 600 ft/min | CFM at 700 ft/min | CFM at 900 ft/min |
|---|---|---|---|---|
| 5 in | 19.6 sq in | 82 | 95 | 123 |
| 6 in | 28.3 sq in | 118 | 137 | 177 |
| 7 in | 38.5 sq in | 160 | 187 | 241 |
| 8 in | 50.3 sq in | 209 | 244 | 314 |
| 10 in | 78.5 sq in | 327 | 382 | 491 |
| 12 in | 113.1 sq in | 471 | 550 | 707 |
| 14 in | 153.9 sq in | 641 | 748 | 962 |
| 16 in | 201.1 sq in | 838 | 977 | 1,257 |
Flexible duct has a rough inner wall and more resistance than smooth metal, especially when it sags or is pulled short. Many installers size flex one step larger than rigid for the same airflow.
Rectangular equivalents
Many trunks are rectangular so they fit between joists. A round duct of diameter D carries about the same air as a rectangle whose equivalent diameter matches D, which is always a bit bigger in area. For the 10.2-inch branch in example 1, a common choice is a 12 × 8 in rectangle, whose equivalent diameter is about 10.7 inches.
How to get the inputs
- Room airflow. Start from the room’s cooling load. The BTU calculator gives a quick estimate; a Manual J gives the real one.
- System airflow. Use the blower table for your furnace or air handler, or about 400 CFM per ton as a first pass.
- Velocity. Choose lower speeds near bedrooms and offices where noise matters, higher in trunks hidden in basements.
- Existing ducts. Measure inside diameter. For rectangular duct, measure inside width and height and use an equivalent-diameter chart.
Common mistakes
- Sizing by area alone for rectangles. A flat 4 × 20 in duct has 80 sq in but carries noticeably less than a 10-inch round.
- Ignoring the return side. Undersized returns choke the whole system no matter how good the supply is.
- Using one velocity for everything. Trunks and branches run at different speeds by design.
- Forgetting fittings. Every elbow and takeoff adds resistance that the velocity method does not count.
Related calculators
Size heating equipment with the furnace size calculator and check winter loads with the heat loss calculator. Ductless rooms are covered by the mini-split sizing calculator. For a new or modified system, have a licensed HVAC contractor run Manual J and Manual D before cutting metal.
Frequently asked questions
What does duct size have to do with airflow and velocity?
Airflow (CFM) equals duct area in square feet times air speed in feet per minute. Fix the airflow and choose a speed, and the area follows. 400 CFM at 700 ft/min needs 400 ÷ 700 = 0.571 sq ft, which is 82.3 sq in, or a 10.2-inch round duct.
What design velocity should I use for residential ductwork?
Velocity-method guides commonly use roughly 700 to 900 ft/min for residential supply trunks and about 500 to 700 ft/min for branch runs to rooms, with returns kept lower for quiet. Lower speed means bigger, quieter ducts; higher speed means smaller ducts with more noise and pressure loss.
How much airflow does a system need?
A common starting rule is about 400 CFM per ton of cooling, so a 3-ton system moves around 1,200 CFM. At 900 ft/min that trunk needs 192 sq in, a 15.6-inch round duct. The equipment's blower data and a Manual J room load set the real numbers.
Can I use this formula for rectangular ducts?
The required area is the same, but a rectangle of equal area carries less air than a circle because of friction in the corners. Designers convert using an equivalent-diameter chart or formula, so the rectangle ends up a little larger than the bare area suggests. An 8 × 10 in duct has 80 sq in but performs like a round duct of about 9.8 inches, not 10.1.
What happens if my duct is sized too small?
Air has to move faster, so static pressure rises, the blower works harder, and registers whistle. Rooms at the end of long runs get weak airflow. Too large is a problem too: slow air delivers poorly through long runs and costs more in metal and space.
Estimates only. Verify quantities with your supplier before purchasing.