Airflow
Q = A × V
Q is volumetric flow rate, A is duct cross-sectional area and V is average air velocity.
Existing engineering calculator
Duct Flow Calculator
Calculate airflow, air velocity or the required duct size for circular and rectangular ducts. Use it for preliminary HVAC, industrial ventilation, dust-collection and process-air checks.
Calculator inputs and result
Select the unknown quantity, choose the duct shape and enter the remaining values. Results update as valid values are entered.
Engineering knowledge
Duct airflow, velocity and area
Duct airflow is the volume of air that passes through a duct in a given time. The same air quantity can be expressed in m³/s, m³/hr or CFM. For a given flow, a smaller duct area produces a higher velocity; a larger duct area produces a lower velocity. These relationships are central to preliminary ventilation and duct-sizing work.
Detailed engineering guide
Review duct area, airflow, velocity, pressure loss, practical velocity selection and common design checks before treating a preliminary result as a final design.
Read Duct Flow Rate, Velocity and Area →Input and result definitions
Calculation modeSelect flow rate, velocity or duct size according to the quantity that is unknown.
Duct geometryChoose a rectangular duct, defined by width and height, or a circular duct, defined by internal diameter.
Result valuesThe output reports the selected unknown in the units built into the existing calculator.
Formula and calculation method
The existing calculator applies the following area and continuity relations:
Rectangular area
A = W × H
Width and height are converted from millimetres to metres before calculation.
Circular area
A = πD² / 4
D is internal diameter in metres. The size mode rearranges these existing relations to find the missing dimension.
Assumptions and limitations
- The result is a geometric airflow, velocity or size calculation; it does not calculate pressure loss, noise, fan static pressure or system resistance.
- Use actual internal duct dimensions and a consistent unit basis.
- Air density, temperature, elevation, leakage, fittings, dampers, filters and branch distribution are outside the implemented calculation.
- For final duct design, use the applicable project criteria, pressure-loss method, duct construction standard and qualified engineering review.
Illustrative use case
A rectangular duct that is 600 mm wide by 400 mm high has an area of 0.24 m². At 10 m/s, the existing calculation gives 2.40 m³/s, or 8,640 m³/hr. This is a useful first check, but detailed design must also examine velocity limits, fitting losses, noise and the complete system pressure requirement.
Common industrial applications
- Preliminary HVAC supply, return and exhaust duct sizing.
- Industrial ventilation, dust collection and air-pollution-control duct systems.
- Process-air, combustion-air and general ventilation capacity checks.
- Early fan-selection inputs before static-pressure and system-resistance calculations.
Frequently asked questions
Does this calculator calculate duct pressure loss?
No. It calculates airflow, velocity and duct geometry only. Pressure loss requires duct length, roughness, fittings, flow conditions and a suitable loss method.
Can the result be used for fan selection?
It can provide a preliminary airflow input. Final fan selection also requires total static pressure, operating density, system resistance and manufacturer performance data.
Which duct shape should I choose?
Choose the geometry that represents the actual duct. Use internal dimensions when the calculated area is intended to represent the flow passage.
References
- ASHRAE. ASHRAE Handbook—Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2021.
- Çengel, Y. A. and Cimbala, J. M. Fluid Mechanics: Fundamentals and Applications. 4th ed. McGraw-Hill Education. 2018.
This is an original educational summary. It does not reproduce book wording, tables or figures. Use current approved project data where an engineering decision depends on the result.
Review information
Content type: existing engineering calculator with knowledge module.Interface reviewed: 26 August 2026. Technical review is required before project use, procurement, construction, operation, compliance or safety decisions.
Engineering disclaimer
Educational and preliminary engineering-reference use only.This calculator does not replace project specifications, detailed design, manufacturer information, applicable standards, safety requirements or review by a qualified engineer. Verify all values, assumptions and decisions for the actual service conditions.