HVAC Duct Sizing Calculator
Enter the airflow you need and the friction rate for your system to get the minimum duct diameter, air velocity, and noise rating. Add multiple branch runs for a complete duct schedule. Nothing uploaded.
Duct size
Reference table
Rectangular equivalents
Branch run schedule
| Run | CFM | Diameter | Velocity | Noise |
|---|
Learn more: duct sizing and friction rate
How the round duct size is found
The calculator sizes a round duct for a given airflow in CFM and friction rate in inches of water per 100 feet. Engineering ToolBox gives the standard equation for a circular duct as friction = 0.109136 x q^1.9 / d^5.02, with q in CFM and d the diameter in inches. The calculator solves it for the diameter.
For 400 CFM at 0.1 in.wg per 100 ft, the round duct is 9.8 inches (250 mm), with air moving at about 759 feet per minute. At 600 CFM and 0.08 it is 12 inches at about 767 FPM, and at 200 CFM and 0.1 it is 7.6 inches at about 642 FPM.
Where the friction rate comes from
The friction rate should come from your equipment. Take the static pressure the blower has available for ducts, after the filter, coil and fittings, and divide it by the total effective length of the run in hundreds of feet. Entering 0.1 without checking only gives a placeholder, and a wrong rate gives ducts that are too small, noisy or short of airflow.
Velocity and rectangular ducts
The calculator checks the velocity for the chosen size. Above 900 FPM it flags the duct as noisy, and below 500 FPM as oversized for the airflow. Those cutoffs are the calculator's own, so check the limits for your application.
For rectangular ducts it lists the closest sizes using the equivalent diameter 1.3 x (w x h)^0.625 / (w + h)^0.25. For a 250 mm round duct the best match is 150 by 350 mm, which has an equivalent diameter of 245 mm. Flexible duct has more friction, so the calculator offers a diameter increase of 15% as a rough allowance.
FAQ
What friction rate should I use if I do not know my blower's pressure?
Look up the air handler's rated external static pressure, subtract the filter, coil and fittings, and divide by the effective length. A value of 0.1 in.wg per 100 ft is only a placeholder until you have that number.
Why is there a size increase for flex duct?
Flexible duct is rougher than rigid metal, so it loses more pressure at the same airflow. The calculator adds 15% to the diameter as a simple allowance, which is its own assumption and not a published figure.
Is a bigger duct always safer?
No. A duct that is too large moves air slowly, which the calculator flags as oversized below 500 FPM. Size from airflow and friction rate and check the velocity.