Duct Size Calculator — Online Ductulator
Size a duct the way the wheel does: enter the airflow and a friction rate, and get the round duct diameter, the next stocked size up, the real air velocity, and whether that velocity will be heard in the room. Rectangular mode solves the second side for a depth you have room for, and the equivalent-diameter mode converts either way. The friction equation is the same one printed on a ductulator, so the answers match the chart.
Private by design. Airflow, friction and every sizing calculation run entirely in your browser — nothing is uploaded to a server.
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Round duct capacity and velocity at common friction rates
Airflow a galvanized round duct carries at three friction rates, plus the velocity it runs at when sized on the 0.1 in. wg per 100 ft line. Every figure comes from the same equation the calculator uses — the standard fit to the ASHRAE friction chart, Δp = 0.109136 × Q^1.9 ÷ D^5.02, for standard air in galvanized duct — so the table and the tool cannot disagree.
| Round duct | CFM at 0.06 | CFM at 0.08 | CFM at 0.10 | Velocity at 0.10 |
|---|---|---|---|---|
| 4" | 28 | 33 | 37 | 426 FPM |
| 6" | 83 | 97 | 109 | 553 FPM |
| 8" | 178 | 207 | 232 | 666 FPM |
| 10" | 320 | 372 | 419 | 768 FPM |
| 12" | 518 | 603 | 678 | 863 FPM |
| 14" | 779 | 906 | 1,019 | 953 FPM |
| 16" | 1,108 | 1,290 | 1,450 | 1,039 FPM |
| 18" | 1,513 | 1,760 | 1,980 | 1,120 FPM |
| 20" | 1,999 | 2,325 | 2,615 | 1,199 FPM |
| 22" | 2,571 | 2,991 | 3,364 | 1,274 FPM |
| 24" | 3,235 | 3,764 | 4,233 | 1,348 FPM |
- Recommended velocities for a residential low-velocity system: supply trunks 700–900 FPM, supply branches about 600 FPM, return trunks 600–700 FPM, return branches about 600 FPM. Notice that duct sized on the 0.1 line passes 900 FPM somewhere around 14 inches — above that size, noise, not friction, is what limits the airflow.
- Circular equivalent of a rectangular duct: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. A 10 × 8 is a 9.8" round; a 12 × 12 is a 13.1" round; the same 144 square inches stretched into a 24 × 6 slot is worth only a 12.4" round, because a slot rubs more air against more metal.
- Friction rate from the blower, ACCA Manual D style: FR = 100 × available static pressure ÷ total effective length. Half an inch of available static across a 500 ft effective run is 0.1 — which is why 0.1 became the default in the first place.
- This table is for galvanized duct and standard air. Flexible duct, especially flex that is not pulled tight, loses substantially more; manufacturers publish their own charts, and a compressed flex run can lose several times the figure above.
How to size a duct with the equal-friction method
- Enter the airflow. The CFM this duct has to carry. For a room, that is the design airflow from the load calculation; for a trunk, the total of everything downstream of it.
- Pick a friction rate. 0.1 in. wg per 100 ft is the textbook default. If you know the blower's available static pressure and the total effective length of the longest run, the Manual D helper works out the real rate: 100 × ASP ÷ TEL.
- Say what the duct is. Supply trunk, supply branch, return trunk or return branch. That sets the velocity band the result is checked against, because noise limits are different for each.
- Read the size and the velocity. The exact equal-friction diameter comes with the next stocked size up, its actual friction loss and its actual velocity. If the velocity flags red, go up a size or split the airflow between two runs.
Frequently Asked Questions
- What size duct do I need for 400 CFM?
- At the standard 0.1 in. wg per 100 ft friction rate, 400 CFM needs 9.8 inches of round duct, so you would buy 10 inch — it moves that air at 733 FPM, comfortably inside the 700–900 FPM band for a residential supply trunk. An 8 inch duct only carries about 230 CFM at that friction rate; pushing 400 CFM through it means 1,146 FPM and a duct you can hear.
- What is a friction rate and what should I use?
- It is how much static pressure the duct is allowed to burn per 100 feet of length, in inches of water gauge. Sizing every duct in a system for the same rate is the "equal friction" method, and it is what makes branches roughly self-balance. 0.1 is the classic default; a real Manual D job often lands between 0.06 and 0.08 once the equivalent length of every elbow and boot is counted.
- How do I convert a rectangular duct to round?
- Through the ASHRAE circular equivalent: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. A 10 × 8 inch duct is equivalent to a 9.8 inch round. That means the two carry the same airflow against the same friction loss — it does not mean they have the same free area or the same air speed, which is why this calculator shows both velocities.
- Why does my duct whistle even though the size is right?
- Because friction and noise are two different limits, and noise usually binds first. The recommended residential velocities are 700–900 FPM in a supply trunk and about 600 FPM in a branch. A duct sized on friction alone at a high friction rate can be well inside its pressure budget and still be moving air fast enough to hear at the register.
- Can a duct be too big?
- Yes, in three ways: it costs more metal and insulation, it is harder to fit in a joist bay, and supply air moving too slowly can dump out of the register instead of throwing across the room. The velocity check flags a duct that has fallen below its usual band as well as one that has gone above it.
- How flat can a rectangular duct be?
- Keep the aspect ratio at or below 4:1, and aim for 3:1. A flatter duct has far more surface area for the same free area, so it needs more sheet metal, more hangers and more fan power than the round duct it replaced. Past 4:1 this calculator tells you the shape is uneconomic rather than quietly returning a number.
Use Cases
- Size supply and return trunks from a room-by-room CFM list
- Convert an existing rectangular duct to its round equivalent before a retrofit
- Fit a duct into a joist bay by fixing the depth and solving the width
- Check whether an existing duct is oversized or is moving air fast enough to be heard
- Turn a blower's available static pressure into a friction rate with the Manual D formula