Duct Size Calculator
Use our free HVAC duct calculator to find the duct size for your airflow and maximum velocity, for round, square or rectangular ducts.
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.
An air duct calculator turns an airflow and a velocity limit into a duct area, then into dimensions for the shape you choose. That gives you a starting size in seconds. For the full method, including the friction check this calculator doesn't run, see how to calculate duct size.
This calculator sizes by velocity. To compare sizes at a set friction rate instead, use our duct sizing chart.
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
The calculation is the same in US imperial and metric units, A = Q / V. Only the units change:
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. It has been used to size thousands of ducts across the world. Here's what it adds to duct design:
Smart Layouts: Draw and adjust systems quickly with auto-connected components.
Accurate Sizing: CIBSE-verified ventilation calculations, which recalculate as you change the design.
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.
