Calculation sequence
Step-by-step method
- Calculate required clear area from airflow and target velocity.
- Solve height from area and entered width-to-height ratio.
- Multiply height by aspect ratio for width.
MechanicalBeginnerDuct Systems
Rectangular Duct Sizing Calculator
Calculate preliminary rectangular duct dimensions from airflow, target velocity, and width-to-height aspect ratio.
Why it matters: The same area can be arranged in many shapes, but high aspect ratios affect friction, construction, coordination, and acoustics.
Calculated result
Engineering evaluation
Preliminary duct width (W)30.98 in.
- Preliminary duct height (H)
- 15.49 in.
- Required clear area (A)
- 3.333 ft²
- Entered aspect ratio (AR)
- 2 W/H
- Maximum acceptable velocity (Vmax)
- 2,500 fpm
- Maximum acceptable aspect ratio (ARmax)
- 4 W/H
Engineering interpretation
The preliminary clear dimensions are 30.98 × 15.49 in., with area 3.333 ft² and aspect ratio 2.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
A = Q/V
AR = W/H
H = √(A/AR)
W = AR·H
criteria V≤Vmax and AR≤ARmax
Current substitution
A = 5000/1500 = 3.333; H = √(3.333/2) = 15.49 in.; W = 30.98 in.; V ≤ Vmax and AR ≤ ARmax.
Equation legend
Inputs and calculated quantities
- Q
- Airflow — cfm or m³/s
- V
- Target average velocity — fpm or m/s
- AR
- Width-to-height aspect ratio — ratio
- Vmax
- Maximum acceptable velocity — fpm or m/s
- ARmax
- Maximum acceptable aspect ratio — ratio
- W
- Preliminary duct width — in.
- H
- Preliminary duct height — in.
- A
- Required clear area — ft²
- AR
- Entered aspect ratio — W/H
- Vmax
- Maximum acceptable velocity — fpm
- ARmax
- Maximum acceptable aspect ratio — W/H
Applications
How this calculation is used
Building design
Screen rectangular dimensions before equal-friction and fitting review.
Construction
Check clear dimensions within available coordination space.
Commissioning
Convert measured airflow and target velocity to a geometry check.
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