Duct Size Calculator: Equal Friction and Velocity Method
Size round, rectangular and flat oval ducts from the airflow in L/s, m³/h or CFM, by equal friction or by velocity. You get the standard size, velocity, friction rate and velocity pressure, with noise and aspect-ratio checks. Need the load first? Use the AC size calculator.
Round, rectangular, flat ovalL/s, m³/h or CFMColebrook friction factorEN 1506 and inch sizesVelocity and aspect checks
Airflow
Metric: mm, Pa/m, m/s. Imperial: inches, in.wg/100 ft, fpm.
Sizing method
Typical low-pressure design: 0.8–1.0 Pa/m (0.08–0.12 in.wg/100 ft).
The duct is rounded up, so the actual velocity is at or below this.
Duct
Imperial sizes use 1 in steps up to 24 in, then 2 in steps.
Keep to 4 : 1 or less where you can.
Set by the ceiling void. The width is solved.
Roughness categories from the ASHRAE Handbook—Fundamentals, Duct Design chapter.
Velocity check
Air conditions
Standard air is 20 °C at sea level (1.204 kg/m³).
Recommended duct size
Enter the airflow to see the result.
Straight-duct friction only. Fittings, dampers, grilles and coils add pressure loss that this tool does not include. Velocity limits are typical values for noise; your project specification, SMACNA or local code may set others. Check the final design with a full duct system calculation.
Need HVAC Revit families or an MEP model? We build them to your standards.
Request a projectSize a duct in three steps
Enter the airflow
Type the airflow in L/s, m³/h or CFM and choose metric or imperial units for the sizes and results.
Pick the method and shape
Size by a target friction rate such as 0.8–1.0 Pa/m (0.08–0.12 in.wg/100 ft), or by a target velocity, for a round, rectangular or flat oval duct in galvanised steel, duct board or flex.
Read the size and checks
The calculator rounds up to a standard size, draws the cross-section to scale and checks the velocity against typical limits for your building and the aspect ratio against 4 : 1.
Duct friction and equivalent diameter formulas
V is the mean velocity, ρ the air density (1.204 kg/m³ for standard air at 20 °C), Re = V·D/ν the Reynolds number and ε the absolute roughness: 0.09 mm for galvanised steel, 0.9 mm for fibrous glass duct board and about 3 mm for fully stretched flexible duct (ASHRAE Handbook—Fundamentals, Duct Design). A rectangular or flat oval duct has the same friction as a round duct of diameter De carrying the same airflow. Velocity pressure is ρV²/2.
Worked example: 1,000 CFM (472 L/s) in galvanised steel at 0.1 in.wg/100 ft (0.82 Pa/m). The exact round diameter is 13.65 in (347 mm) at about 983 fpm; the ASHRAE chart curve fit Δp = 0.109136·q1.9/D5.02 gives 13.9 in. Rounded up to a 14 in duct, the velocity is 935 fpm (4.75 m/s) and the friction 0.089 in.wg/100 ft. A rectangular duct with about 2 : 1 aspect ratio works out at 16 × 10 in.
Straight duct only: add fittings, dampers, grilles and coils separately to find the fan pressure.
Duct sizes for common airflows and velocity limits
| Airflow | Exact round | Standard round | Velocity | Rectangular, about 2 : 1 |
|---|---|---|---|---|
| 200 CFM | 7.5 in | 8 in | 573 fpm | 8 × 6 in |
| 400 CFM | 9.7 in | 10 in | 733 fpm | 14 × 6 in |
| 600 CFM | 11.3 in | 12 in | 764 fpm | 14 × 8 in |
| 800 CFM | 12.6 in | 13 in | 868 fpm | 18 × 8 in |
| 1,000 CFM | 13.7 in | 14 in | 935 fpm | 16 × 10 in |
| 1,500 CFM | 15.9 in | 16 in | 1,074 fpm | 22 × 10 in |
| 2,000 CFM | 17.7 in | 18 in | 1,132 fpm | 24 × 12 in |
Galvanised steel, standard air, 0.1 in.wg/100 ft. Round sizes in 1 in steps, rectangular in 2 in steps.
| Building | Main supply | Branch supply | Main return |
|---|---|---|---|
| Homes and apartments | 5.1 m/s (1,000 fpm) | 3.0 m/s (600 fpm) | 4.1 m/s (800 fpm) |
| Hotel and hospital rooms | 7.6 m/s (1,500 fpm) | 6.1 m/s (1,200 fpm) | 6.6 m/s (1,300 fpm) |
| Offices, shops, restaurants | 10.2 m/s (2,000 fpm) | 8.1 m/s (1,600 fpm) | 7.6 m/s (1,500 fpm) |
| Auditoriums and theatres | 6.6 m/s (1,300 fpm) | 5.1 m/s (1,000 fpm) | 5.6 m/s (1,100 fpm) |
| Industrial buildings | 15.2 m/s (3,000 fpm) | 11.2 m/s (2,200 fpm) | 9.1 m/s (1,800 fpm) |
Typical maximum velocities for low-velocity systems (Engineering ToolBox). Your specification may set lower limits for quiet spaces.
Duct sizing questions
Choose one friction rate for the whole system, typically 0.8–1.0 Pa/m (0.08–0.12 in.wg/100 ft) for low-pressure supply ducts. For each section, find the round diameter that carries its airflow at that friction rate, round up to a standard size, and convert to a rectangular size with the same equivalent diameter if needed. The calculator does each step with the Darcy–Weisbach and Colebrook equations.
At 0.1 in.wg/100 ft in galvanised steel, 1,000 CFM needs a 13.7 in round duct, so a 14 in duct, running at about 935 fpm. A rectangular duct of 16 × 10 in carries the same airflow at the same friction. Flexible duct is rougher and needs a larger size.
Pick a friction rate or a velocity. By velocity, the area is A = Q ÷ V: 800 CFM at 800 fpm needs 1 ft², about a 13.5 in round duct. By friction, solve the friction equation for the diameter. Then round up to the next standard size, which slightly lowers the velocity and friction.
Use the Huebscher equivalent diameter, De = 1.30 (a·b)0.625 / (a + b)0.25. For a 24 × 12 in duct, De = 18.3 in, so it has the same friction rate as an 18.3 in round duct at the same airflow. Note the velocity in the rectangular duct is lower, because its area is larger than the round equivalent.
Typical maximum velocities in low-velocity systems are about 5 m/s (1,000 fpm) for main supply ducts in homes and 3 m/s (600 fpm) for branches, and about 10 m/s (2,000 fpm) for main ducts in offices and shops. Higher velocities make noise at grilles and raise the fan pressure.
Keep it at 4 : 1 or less where the ceiling space allows. A flat duct uses more sheet metal and has more friction than a squarer duct with the same airflow, so the calculator warns above 4 : 1 and lets you fix the height to suit a shallow ceiling void.
Need a custom tool, Dynamo script or Revit family?
Tell us about your project and we will send you a quote before any work starts.