Free HVAC duct calculator. Enter any two of airflow, friction rate, velocity and diameter to size a round or rectangular air duct, in US or metric units, using the Darcy-Weisbach and Colebrook equations from the ASHRAE duct design method.
In equal friction duct design, you typically select a friction target (pressure loss per length) and size ducts so pressure drop stays uniform throughout the system. This helps balancing, keeps fan static predictable, and makes duct sizing consistent from main to branches.
The calculator uses the Darcy-Weisbach equation with the Colebrook friction factor, the method in the ASHRAE Handbook duct design chapter. Rectangular ducts are matched to round by the ASHRAE equivalent diameter, De = 1.30 × (a × b)0.625 / (a + b)0.25, so both have the same friction loss at the same airflow.
At a friction rate of 0.10 in.wg per 100 ft, 1,000 CFM calculates to about 13.7–13.9 in diameter (depending on the roughness setting), so you would use a 14 in round duct, which runs at about 935 fpm. If the ceiling void allows only 10 in of height, enter 10 in as the height and the calculator returns a width of about 16 in. The aspect ratio of 1.6:1 is comfortably below 4:1.
| Duct section | Friction rate | Velocity |
|---|---|---|
| Commercial supply main | 0.08–0.10 in.wg/100 ft (0.65–0.85 Pa/m) | 1,000–1,500 fpm (5–7.5 m/s) |
| Branch supply | 0.08–0.10 in.wg/100 ft | 600–900 fpm (3–4.5 m/s) |
| Return duct | 0.05–0.08 in.wg/100 ft | 600–1,000 fpm (3–5 m/s) |
| Residential main | 0.08–0.10 in.wg/100 ft | 700–900 fpm (3.5–4.5 m/s) |
Need standard duct sizes, a CFM to duct size chart or air density correction? Use the Online Ductulator. For the total fan pressure, see the Fan Static Pressure Calculator. Printable tables are in the Duct Size Chart PDF, and the two sizing methods are compared in Equal Friction vs Velocity Method.
A duct calculator sizes HVAC air ducts from the airflow and a design limit, either a friction rate (pressure loss per length) or a maximum velocity. Enter any two of airflow, friction, velocity and diameter and it returns the other two, plus the equivalent rectangular size.
Choose a friction rate, typically 0.08 to 0.10 in.wg per 100 ft for supply ducts, then find the diameter whose friction loss at that airflow matches it. For example, 1,000 CFM at 0.10 in.wg per 100 ft calculates to about 13.7 to 13.9 inches, so a 14 inch round duct (about 935 fpm) or a 16 by 10 inch rectangular duct is used.
0.08 to 0.10 in.wg per 100 ft (about 0.65 to 0.85 Pa/m) for supply ducts and 0.05 to 0.08 for returns in typical comfort systems. Use a lower rate on long runs to keep fan static pressure down.
Typical design velocities are 700 to 900 fpm for residential mains, 1,000 to 1,500 fpm for commercial supply mains and 600 to 900 fpm for branches. Use the lower end near noise-sensitive rooms.
Both size ducts with the same physics. This duct calculator solves any two of four values and shows Reynolds number and friction factor. The Online Ductulator adds standard size selection, a CFM to duct size chart, air density correction and rectangular to round converters.
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