Duct Size Calculator
Calculate ductwork accurately and easily using our free Duct Size Calculator to ensure the correct velocity and pressure drop.
Calculate duct size in seconds
Choose circular, square, or rectangular ducts
Use metric or imperial units
Download a professional PDF report with your results
What Is Duct Size & Why Does It Matter?
Getting your duct sizing right is critical for HVAC system efficiency and performance. Whether you're designing residential or commercial systems, ductwork that is too small causes high air velocities and noise, while oversized ducts can waste space and materials.
By using a duct size calculator, engineers and contractors can achieve optimal air distribution, ensure code compliance, and avoid costly redesigns. Therefore, it's an essential step in modern HVAC design, especially when you're working with varying flow rates and velocity limits.
Duct Size Variables
Duct Type
Your choice of circular, square, or rectangular duct directly impacts the area available for airflow and the final dimensions.
Flow Rate
Flow rate, usually measured in CFM (cubic feet per minute) or L/s, is the volume of air that needs to pass through the duct.
Maximum Velocity
Setting an air velocity limit helps reduce noise and friction loss while maintaining a balanced HVAC system.
Duct Sizing Calculation
As the calculation can be measured in terms of feet/s and meter/s, there are two different calculation methods to establish duct size:
Imperial Equation
Following the formula below, you can calculate the duct size:
Components:
A = Duct cross-sectional area (ft²)
Q = Flow rate (CFM)
V = Velocity (FPM)
In imperial units, duct size is calculated by dividing the airflow in CFM by the air velocity in FPM. This helps ensure the duct area supports efficient air delivery without excess noise or pressure loss.
Metric Equation
Following the formula below, you can calculate the duct size:
Components:
A = Duct cross-sectional area (m²)
Q = Flow rate (L/s)
V = Velocity (m/s)
In metric units, duct size is calculated by dividing the airflow in L/s by the velocity in m/s. As a result, you can accurately size ductwork for balanced air distribution and performance.
Duct Sizing In h2x
h2x speeds up duct sizing with built-in engineering logic and a user-friendly interface.
See why contractors, consultants, and engineers trust h2x for HVAC system design:
Smart Layouts: Draw and adjust systems quickly with auto-connected components.
Verified Sizing: Create accurate, CIBSE-verified calculations that update instantly.
Easy Exports: Generate branded PDFs or AutoCAD and Revit files in one click.
Detailed Reports: Get ductwork summaries and bills of materials for easy reviews.
