Duct Size Calculator
Size your ductwork correctly to ensure efficient airflow. Enter the required CFM and air velocity to determine round duct diameter or rectangular duct dimensions.
Duct Sizing Fundamentals
Duct sizing balances airflow volume (CFM), air velocity (FPM), and duct cross-sectional area. The relationship is straightforward: Area = CFM / (Velocity / 60). The division by 60 converts velocity from feet per minute to feet per second to match CFM units. For example, 400 CFM at 900 FPM requires 400 / 15 = 26.7 square inches of duct area.
Round ducts use the circle area formula: A = ΟrΒ². Solving for diameter gives D = β(4A/Ο). A 26.7-square-inch area needs a 5.8-inch diameter, which rounds to a standard 6-inch duct. Rectangular ducts distribute area across width and height, with common aspect ratios of 2:1 or 3:1 to fit framing bays. A 26.7-square-inch area fits a 4Γ7-inch or 3Γ9-inch rectangular duct.
Standard duct sizes increment by 1 inch for round (4, 5, 6, 7, 8, 10, 12 inches) and by specific dimensions for rectangular (4Γ8, 6Γ10, 8Γ12, etc.). Always round up to the next standard size. A calculated 7.3-inch duct becomes an 8-inch duct to ensure adequate airflow and prevent velocity spikes.
Velocity Guidelines and Noise Control
Air velocity directly affects noise levels and energy consumption. Residential main supply trunks target 700-900 FPM. Branch ducts serving individual rooms run 500-700 FPM for quieter operation. Return air ducts operate even slower at 500-600 FPM because they span larger areas and low velocity prevents pulling air through wall cavities.
Commercial systems tolerate higher velocities: 1,200-1,800 FPM in main ducts and 800-1,000 FPM in branches. Offices and retail spaces have higher ambient noise that masks duct whoosh. Industrial facilities push 2,000+ FPM because noise isn't a concern and compact ducts save space.
Excessive velocity causes problems beyond noise. Friction increases exponentially with speed, so doubling velocity quadruples pressure drop. Blower motors work harder, using more electricity and shortening equipment life. High-velocity turns and elbows generate turbulence, further reducing efficiency. Acoustic lining or flex duct can dampen noise but they also reduce effective diameter, so factor in a 10-15% size increase if using them.
Trunk and Branch Duct Design
HVAC duct systems split into two types: trunk-and-branch and radial. Trunk-and-branch uses a large main duct (trunk) that shrinks as branches peel off to serve rooms. The trunk starts at the air handler at full system CFM, then reduces after each branch takeoff. If a 1,200 CFM system splits 400 CFM to three zones, the trunk starts at 1,200 CFM (14-inch round), drops to 800 CFM after the first branch (12-inch), and ends at 400 CFM (8-inch).
Each branch duct sizes independently for its room CFM. A 150 CFM bedroom needs a 6-inch round duct at 800 FPM. A 300 CFM living room needs an 8-inch duct. Reducing trunk size saves material and keeps velocity consistent. Oversized trunks slow airflow, allowing dust settlement and reducing throw distance from registers.
Radial duct systems run individual ducts from a central plenum directly to each room, like spokes on a wheel. Each duct carries its room's CFM at optimal velocity. Radial designs simplify balancing and reduce leakage because there are no trunk joints, but they use more linear feet of duct and require more space in the air handler area. This calculator handles individual duct segments; run it separately for each trunk section and branch to build a complete duct layout.
Frequently Asked Questions
What is CFM in HVAC?
CFM (Cubic Feet per Minute) measures airflow volume. A 3-ton air conditioner typically moves 1,200 CFM, or 400 CFM per ton. Proper duct sizing ensures this airflow reaches rooms without excessive noise or pressure drop.
What is FPM and why does it matter?
FPM (Feet per Minute) is air velocity inside the duct. Residential supply ducts use 700-900 FPM; faster air creates noise. Return ducts run slower at 600-700 FPM. Too slow means oversized ducts.
Which is better, round or rectangular ducts?
Round ducts have less surface area for the same airflow, reducing friction and energy loss. Rectangular ducts fit better in tight spaces like floor joists and wall cavities. Round is more efficient; rectangular is more practical.
How do I measure duct size?
For round ducts, measure the outside diameter and subtract twice the metal thickness (usually 0.5 inches total). For rectangular ducts, measure the inside width and height. Use a tape measure at the widest points.
What happens if ducts are too small?
Undersized ducts restrict airflow, causing high velocity noise, increased pressure drop, reduced equipment efficiency, and uneven room temperatures. The blower works harder and uses more energy.