Engineering Tools
Duct Sizing Calculator
Enter airflow and velocity to instantly calculate round duct diameter and recommended rectangular duct dimensions. Suitable for HVAC ventilation system duct design. Results are for preliminary reference only.
Duct Size Calculation
Calculation Results
Calculation Formulas
This tool calculates duct cross-section dimensions based on design airflow and velocity, suitable for preliminary HVAC ventilation system duct design.
Airflow unit conversion to m³/s:
CMH: Q = Q_cmh ÷ 3600
CMM: Q = Q_cmm ÷ 60
CFM: Q = Q_cfm × 0.000472
Round duct diameter:
D = √(4 × Q ÷ (π × V)) × 1000 (unit: mm)
Where Q = airflow (m³/s), V = velocity (m/s).
Rectangular duct equivalent cross-section area:
A = Q ÷ V (unit: m²)
Multiple width-height (W × H) combinations are generated from the cross-section area, rounded to the nearest 50mm, with aspect ratio limited to ≤ 4:1.
Rectangular duct equivalent diameter:
De = 1.3 × (W × H)0.625 ÷ (W + H)0.25 (unit: mm)
The equivalent diameter is used to compare the air resistance characteristics of rectangular ducts with round ducts. Values closer to the round duct diameter indicate higher efficiency.
Frequently Asked Questions
What are the design standards for duct air velocity?
Generally, main ducts are recommended at 5~8 m/s, branch ducts at 3~5 m/s, and supply outlets at 2~3 m/s. Excessively high velocities increase noise and pressure loss, while excessively low velocities require larger duct sizes.
How do I choose between round and rectangular ducts?
Round ducts have lower air resistance and are simpler to fabricate, making them suitable for straight runs and spaces with ample room. Rectangular ducts allow flexible aspect ratio adjustments to fit ceiling spaces, making them ideal for limited plenum heights.
Why shouldn't the aspect ratio of rectangular ducts be too large?
The recommended aspect ratio for rectangular ducts should not exceed 4:1. A higher ratio increases the duct surface area, raises friction losses and material costs, and reduces the equivalent diameter, negatively affecting system performance.
Professional Consultation
Need Professional HVAC Engineering?
Online tools provide preliminary estimates for reference. Actual engineering design requires consideration of many additional professional factors. Contact our engineering team for precise HVAC system planning.