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.
Frequently Asked Questions
What does this duct size calculator calculate?
It estimates the required duct area, the equivalent round diameter, a suggested rectangular size at your chosen aspect ratio, the actual velocity that results, and an approximate friction loss for straight duct.
What air velocity should I use for duct sizing?
It depends where the duct runs and what noise criteria apply. Commercial supply mains commonly sit around 5 to 8 m/s, branch ducts around 3 to 5 m/s, and runouts near occupied spaces lower still, because duct-borne noise rises steeply with velocity. Residential systems run lower throughout. Check the project noise criteria and the fan static you actually have available before settling on a figure.
What is equivalent round diameter?
It is the round duct that can stand in for a rectangular one. There are two definitions in use: equal cross-sectional area, and equal friction loss at the same airflow. ASHRAE uses the friction-based definition, De = 1.30 (ab)^0.625 / (a+b)^0.25, which returns a slightly smaller figure than the equal-area result, roughly 4 to 5 percent for a 2:1 duct. This calculator derives the round size from equal area, so treat it as a preliminary conversion and use the ASHRAE relation where friction matters.
Does duct aspect ratio matter?
Yes, more than people expect. For a fixed cross-sectional area, a flatter duct has a longer perimeter, so it carries more friction and needs more sheet metal to move the same air. A square duct is the most efficient shape available. By 4:1 the penalty is meaningful in both pressure drop and material cost, and ratios beyond that are usually avoided unless the ceiling void leaves no choice.
Can I use this for round and rectangular ducts?
Yes. The tool gives both the equivalent round diameter and a suggested rectangular size at the aspect ratio you select, so you can compare the two before deciding what fits the route.
Why is the friction loss approximate?
The figure covers straight duct only, calculated at an air density of 1.2 kg/m³. Real system pressure drop also depends on elbows, transitions, dampers, branches, grilles, filters and coils. On a fitting-heavy layout those can rival the straight-duct loss, so the number here is a starting point rather than a total.
Is this suitable for a final HVAC submission?
No. It is built for preliminary design, quick checks and teaching. Final duct design needs to account for project specifications, routing, available static pressure, fittings, noise limits, leakage class, insulation and local authority requirements. Verify against recognised references such as the ASHRAE Handbook and SMACNA before issue.
Related Engineering Tools
Duct sizing is one part of a wider calculation. These cover the airflow that feeds into it and the waterside equivalents.
