Duct Size Calculator
Estimate preliminary round and rectangular duct dimensions from airflow and target velocity, plus duct surface area and material cost.
Default project preview
14 in diameter
Required flow area
1 ft²
Rectangular duct
17 × 9 in
Raw round diameter
13.5 in
Duct surface area
130 ft²
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Design airflow
Enter this project assumption in CFM. The current imperial default is 800 CFM.
Target air velocity
Enter this project assumption in ft/min. The current imperial default is 800 ft/min.
Rectangular width-to-height ratio
Enter this project assumption in :1. The current imperial default is 2 :1. Minimum: 1.
Round-size increment
Enter this project assumption in in. The current imperial default is 1 in.
Rectangular-size increment
Enter this project assumption in in. The current imperial default is 1 in.
Duct length
Enter this project assumption in ft. The current imperial default is 30 ft.
Duct material price
Enter this project assumption in per ft². The current imperial default is 4.5 per ft². Minimum: 0.
How to read the outputs
Required flow area
airflow ÷ velocity. Default preview: 1 ft².
Rectangular duct
2:1 target ratio. Default preview: 17 × 9 in.
Raw round diameter
before size rounding. Default preview: 13.5 in.
Duct surface area
rectangular section only. Default preview: 130 ft².
Equivalent length
straight material length. Default preview: 30 ft.
Material cost
surface material only. Default preview: $585.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Flow area = volumetric airflow ÷ velocity. Round diameter = √(4 × area ÷ π). Rectangular width and height use the entered aspect ratio and round up.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
This calculator relates airflow, duct area and air velocity for preliminary round or rectangular duct dimensions. It does not perform a complete pressure-loss, noise, fitting or balancing design.
Calculation steps
- 1Determine design airflow from room or system calculations.
- 2Choose a target velocity appropriate to the duct location and noise goal.
- 3Calculate required cross-sectional area.
- 4Select a standard duct size and then check pressure loss and fittings.
Worked numerical example
Delivering 800 cfm at 800 feet per minute requires 1.0 ft² of duct area, equivalent to a round diameter of about 13.5 in before standard-size selection.
Purchasing and rounding
Duct is ordered in standard dimensions and fittings. Round up to an available size only after checking how the change affects velocity and pressure loss.
Interpret the result
Area and velocity are only one part of duct design. A duct that carries the airflow geometrically may still have excessive static pressure or noise.
Page-specific assumptions
- Steady airflow.
- Uniform velocity across the section.
- No fitting losses in the size calculation.
- Air density near normal building conditions.
Common mistakes
- Sizing from equipment tonnage alone.
- Ignoring equivalent length of fittings.
- Using inside and outside dimensions interchangeably.
- Choosing velocity without considering acoustics.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Duct Size Calculator questions
Does this calculate duct pressure loss?+
No. Pressure loss depends on duct roughness, length, fittings, transitions, filters and airflow.
Why round duct dimensions up?+
Rounding up preserves at least the calculated flow area at the selected velocity.
Can high air velocity cause noise?+
Yes. Velocity, fittings, grilles and duct construction all affect noise and comfort.