Duct Size Calculator

1. Enter the required CFM. This is the airflow the duct run needs to deliver. Use a CFM calculator or Manual D worksheet to determine this value for each room or branch.

2. Select duct material and shape. Sheet metal is the baseline. Flex duct automatically upsizes by 1 inch due to higher friction. Choose rectangular if you have limited clearance in ceiling cavities or soffits.

3. Set the friction rate. The default of 0.08 in WC per 100 ft works for most residential systems. Commercial systems may use 0.1 or higher. Lower friction rates mean larger ducts but quieter, more efficient airflow.

4. Review the results. Check the velocity and noise indicators. Green velocity means quiet operation. If the velocity is red, consider upsizing the duct to reduce noise and turbulence.

There are two primary methods for sizing ducts: the friction rate method and the velocity method. This calculator uses the friction rate method, which is the standard approach in ACCA Manual D.

The friction rate method sizes ducts so that each 100 feet of duct creates the same pressure drop. This keeps the system balanced and ensures the blower can deliver the designed airflow to every room. A typical residential friction rate of 0.08 in WC per 100 ft balances duct size against noise and energy use.

The velocity method caps air speed to control noise. Residential supply ducts should stay below 900 FPM for quiet operation. Return ducts can tolerate slightly higher velocities because they are typically located in less noise-sensitive areas. Trunk lines serving multiple branches can run up to 1,000 FPM.

Undersized ducts create high velocity, noise, and excessive pressure drop that starves downstream rooms. Oversized ducts waste material and space but deliver quieter airflow. The goal is the smallest duct that keeps velocity within acceptable limits.

New construction duct design. After calculating room-by-room CFM loads, technicians size each duct run. Trunk lines carry the total system airflow and branch ducts split off to individual rooms. Getting the sizes right here prevents callbacks for comfort complaints.

Duct replacement and retrofit. When replacing old ductwork or converting from one material to another, you need to re-verify sizes. Swapping sheet metal for flex duct at the same diameter will choke airflow because flex has higher friction loss.

Troubleshooting airflow problems. If a room is not getting enough air, check whether the duct is undersized for the required CFM. A 6-inch flex duct trying to deliver 200 CFM will create noise, turbulence, and starve the room of conditioned air.