Duct Sizing Methodology
Core Engineering Principles
The HVAC Duct Calculator utilizes the Equal Friction Method, which is the industry standard for sizing residential and light commercial ductwork. This method involves sizing the duct system such that the pressure loss per unit length is constant throughout the system.
Our calculations are derived from the Darcy-Weisbach equation and the Colebrook equation, simplified for air at standard atmospheric pressure and temperature (0.075 lb/ft³).
Primary Formulas
Airflow and Velocity
Where Q is airflow in Cubic Feet per Minute (CFM), A is the cross-sectional area of the duct in square feet, and V is the velocity in Feet per Minute (FPM).
Round Duct Diameter (Ductulator Formula)
Where Hf is the friction loss (inches of water per 100 feet), Q is the airflow (CFM), and D is the diameter (inches).
Rectangular to Round Equivalent
Where De is the circular equivalent diameter, and a and b are the sides of the rectangular duct.
Assumptions and Constants
- • Air Density: Assumed at 0.075 lb/ft³ (Standard Air).
- • Roughness Factor: 0.0003 ft for galvanized sheet metal; 0.003 ft for flexible ductwork.
- • Flex Duct Adjustment: The calculator applies a 1.5x resistance multiplier for flexible ducting to account for typical installation compression and internal wire helix turbulence.
Limitations
While this tool provides high-precision estimates based on engineering theory, it does not account for specific local conditions such as high altitude (which reduces air density) or specific equipment fan curves. A full Manual D calculation remains the gold standard for final system design and permit applications.