Enter the airflow in CFM and either a friction rate or a maximum velocity. Get the round duct diameter, the velocity and friction loss, the next stock size, and equivalent rectangular duct sizes. A ductulator in your browser, free, no account, nothing uploaded.
An Android app with saved jobs and duct fitting take-offs is coming.
Friction loss uses the Darcy-Weisbach equation with the friction factor from the Colebrook equation, as given in the ASHRAE Handbook, Fundamentals, duct design chapter. It assumes standard air at 0.075 lb/ft3, kinematic viscosity 1.62e-4 ft2/s and galvanized steel with a roughness of 0.0003 ft (0.09 mm), the same basis as most printed ductulators. The round diameter is found by solving that equation for the friction rate you enter.
Rectangular sizes use the Huebscher equivalent diameter, De = 1.30 (a b)^0.625 / (a + b)^0.25, which gives the round duct with the same friction loss at the same airflow. Widths are rounded up to the next 2 inches (50 mm), so the real friction is at or below your target. A rectangular duct with an aspect ratio over 4 to 1 costs more and is noisier.
| Term | Meaning |
|---|---|
| CFM | Cubic feet of air per minute. 1 CFM is 0.4719 L/s. |
| Friction rate | Pressure lost per 100 ft of straight duct, in inches of water gauge. 0.1 in.wg per 100 ft is 0.817 Pa/m. |
| Velocity | Airflow divided by duct cross-section area, in feet per minute. Higher velocity means more noise. |
| Equivalent diameter | The round duct that loses the same pressure per foot as the rectangular duct at the same CFM. |
Many residential and light commercial designs use a friction rate of 0.08 to 0.1 in.wg per 100 ft and keep supply branches near 600 to 900 fpm and mains below about 1,200 to 1,500 fpm. Your code, equipment data and project specification come first.
A ductulator is the slide-rule style duct sizing tool, a printed version of the friction chart. This page does the same job: pick CFM and friction rate or velocity, read off the round size, then look up the rectangular equivalent.
The equal friction method is the usual starting point, because it keeps the system roughly balanced. Use the velocity option when noise limits the duct, for example near occupied rooms.
No. The result is for straight, smooth galvanized duct. Fittings add pressure loss, and flexible duct has a higher friction than metal. Add allowances for both.
No. The calculation runs in your browser and nothing is uploaded or stored.
This is a design aid, not a substitute for a full load and duct design. Check results against your code, manufacturer data and ACCA or SMACNA procedures.