ASHRAE Duct Sizing Guide
Good duct sizing protects comfort, noise control, and fan energy. A small duct raises velocity and static pressure. A large duct may waste space and material. This calculator helps compare both sides before drawings are finalized.
Why Airflow Matters
Airflow is the starting point. Each room load creates a required supply or return volume. The tool converts cubic feet per minute into cubic feet per second. It then uses that flow with your selected method. You can size by friction rate, by target velocity, or by known dimensions.
Friction And Velocity Balance
Common practice checks pressure loss per one hundred feet. Lower friction usually means larger ducts and quieter systems. Higher friction can reduce first cost, but it may require more fan power. Velocity is also important. Excess speed can create noise at grilles, elbows, and dampers. The calculator reports velocity, velocity pressure, friction loss, fitting loss, and total estimated loss.
Round And Rectangular Choices
Round ducts are efficient because their perimeter is low for the area provided. Rectangular ducts fit tight ceiling spaces better. Their aspect ratio should stay reasonable. Very flat ducts add friction and may be harder to balance. This tool uses hydraulic diameter for rectangular duct checks. It also gives width and height from the chosen aspect ratio.
Advanced Inputs
Air density, roughness, fittings, parallel runs, and safety factor affect the final result. Density changes with temperature and elevation. Roughness changes with duct material and age. Fittings add dynamic loss. Safety factor allows design margin when loads are uncertain. Parallel runs divide the design airflow between identical sections.
Use Results Carefully
The output is a planning estimate, not a stamped design. Real projects need coordination with equipment data, leakage class, fire dampers, acoustic limits, and local code. Use the result to compare options, prepare schedules, and discuss tradeoffs. Then confirm final sizes with approved standards, manufacturer data, and professional judgment.
Document every assumption. Keep airflow sources traceable. Record grille selections, coil pressure, filter pressure, and branch damper positions. These details make later balancing easier. They also help another designer audit the system. When a value seems unusual, change one input at a time. This shows which parameter drives the result most clearly.