Duct Size Calculator
Use this free HVAC duct size calculator to estimate round and rectangular duct dimensions from airflow and design velocity. The tool helps engineers, contractors, and technicians quickly check duct area, equivalent round size, air velocity, and approximate friction loss for preliminary HVAC design.
Duct Size Calculator
Calculate round and rectangular duct sizes from airflow and design velocity. Includes duct area, equivalent round diameter, actual velocity, and approximate friction loss.
Input Data
Calculated Results
Engineering Notes
This calculator is intended for preliminary HVAC duct sizing. Final duct design should consider project specifications, noise criteria, available static pressure, duct routing, fittings, balancing dampers, insulation, leakage class, and local authority requirements.
Calculation Basis
- Air quantity is converted to m³/s.
- Duct area = airflow ÷ velocity.
- Round duct diameter is calculated from equivalent area.
- Rectangular size is calculated using the selected aspect ratio.
- Friction loss is an approximate straight-duct estimate using air density of 1.2 kg/m³.
How to Use the Duct Size Calculator
Use this duct size calculator to estimate round and rectangular duct dimensions based on airflow and design velocity. Enter the required airflow, select the unit, enter the target velocity, choose the aspect ratio, and review the calculated duct area, equivalent round diameter, suggested rectangular duct size, actual velocity, and approximate friction loss.This tool is intended for preliminary HVAC design checks and quick engineering calculations. Final duct sizing should always be verified against project requirements, system static pressure limitations, noise criteria, routing constraints, fittings, and applicable standards.You may also find our HVAC Unit Converter and Heat Load Calculator useful when preparing HVAC design calculations.
Duct Size Calculator Formula
This duct size calculator is based on the basic airflow relationship:
Duct Area = Airflow / Velocity
Where:
Airflow is converted into a consistent unit Velocity is the selected design air speed Equivalent round diameter is calculated from the required cross-sectional area Rectangular duct dimensions are estimated using the selected aspect ratio Friction loss is shown as an approximate value for straight duct only.
For final design, the results should be checked against recognized references such as ASHRAE and SMACNA.
