Duct Size Calculator
Calculate optimal duct dimensions for HVAC airflow.
Required Duct Size:
Rectangular Equivalents (HxW):
HCAV Duct Sizing Guide
Proper duct sizing is critical for an HVAC system's efficiency, longevity, and comfort. Ducts that are too small create high static pressure, noise, and reduced airflow, stressing the blower motor. Ducts that are too large lose velocity, making it difficult to push conditioned air to distant rooms.
The Velocity Method
This calculator uses the Velocity Reduction Method, which is a common rule-of-thumb approach for simple residential branch runs. The formula relies on the relationship:
Q = A × V
Where:
- Q: Airflow in Cubic Feet per Minute (CFM)
- A: Cross-sectional Area in Square Feet
- V: Velocity in Feet per Minute (FPM)
Recommended Velocities (Residential)
- Supply Trunks: 700 - 900 FPM
- Supply Branches: 500 - 700 FPM
- Return Grilles: 400 - 600 FPM (Lower velocity reduces noise)
Round vs. Rectangular Ducts
Round ducts are the most efficient for airflow because they have the least surface area for friction. However, rectangular ducts are often necessary to fit inside walls or joist bays. This calculator provides the required round diameter first, and then offers several rectangular options that provide equivalent cross-sectional area (Note: For precise friction equivalence, aspect ratios should be kept as close to 1:1 as possible).