Calculation sequence
Step-by-step method
- Divide airflow by target velocity for clear area.
- Solve the circular area relationship for diameter.
- Convert diameter to millimetres or inches.
MechanicalBeginnerDuct Systems
Round Duct Sizing Calculator
Calculate a preliminary round duct diameter from airflow and target average velocity.
Why it matters: Duct area and velocity affect pressure loss, acoustics, coordination, and the quality of airflow measurements.
Calculated result
Engineering evaluation
Preliminary round duct diameter (D)24.72 in.
- Required clear area (A)
- 3.333 ft²
- Entered target velocity (V)
- 1,500 fpm
- Maximum acceptable velocity (Vmax)
- 2,500 fpm
Engineering interpretation
The entered airflow and velocity require 3.333 ft², equivalent to a preliminary clear round diameter of 24.72 in..
Important limitations
- Select an available duct size and then recalculate actual velocity, friction, fittings, leakage, acoustics, and construction requirements.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
A = Q/V
D = 12√(4A/π)
criterion V≤Vmax
Current substitution
A = 5000/1500 = 3.333 ft²; D = 24.72 in.; 1500 ≤ 2500 fpm.
Equation legend
Inputs and calculated quantities
- Q
- Airflow — cfm or m³/s
- V
- Target average velocity — fpm or m/s
- Vmax
- Maximum acceptable velocity — fpm or m/s
- D
- Preliminary round duct diameter — in.
- A
- Required clear area — ft²
- V
- Entered target velocity — fpm
- Vmax
- Maximum acceptable velocity — fpm
Applications
How this calculation is used
Building design
Screen a round duct diameter before detailed friction and acoustic review.
Construction
Check whether an installed clear diameter can carry the intended flow at the entered velocity.
Commissioning
Relate measured airflow and average velocity to the required clear round area.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe one stable operating condition and use consistent measurement boundaries.
- Fluid or air properties and geometric dimensions are representative of the evaluated system.
Limitations
- This is a preliminary evaluation and field-verification tool, not a final sizing or code-compliance determination.
- Manufacturer data, adopted codes, project criteria, and professional engineering review remain controlling.
Field use
Verification procedure
- Confirm the approved reference, measurement plane, and instrument calibration.
- Stabilize the system and record dimensions and operating data.
- Compare the calculated result with project documents and manufacturer data before disposition.
Common engineering mistakes
- Using outside dimensions rather than clear inside dimensions.
- Combining values from different loads, device positions, or measurement conditions.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsCurrent handbook chapters covering duct design, fluid flow, heat transfer, insulation, and measurement; verify the licensed edition during professional review.
- SMACNA Technical StandardsIndustry duct-design, construction, leakage, and air-system reference context; protected tables are not reproduced.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement