Duct Air Velocity Calculator
Calculate average air velocity in a rectangular duct from airflow and inside dimensions, with comparison to an editable target velocity.
Default project preview
720 ft/min
Duct cross-section
1.25 ft²
Target area requirement
1.286 ft²
Area difference
-0.036 ft²
Equivalent round diameter
15.1 in
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.
Airflow
Enter this project assumption in cfm. The current imperial default is 900 cfm.
Inside duct width
Enter this project assumption in in. The current imperial default is 18 in.
Inside duct height
Enter this project assumption in in. The current imperial default is 10 in.
Planning target velocity
Enter this project assumption in ft/min. The current imperial default is 700 ft/min.
Duct run length
Enter this project assumption in ft. The current imperial default is 45 ft. Minimum: 0.
How to read the outputs
Duct cross-section
inside width × inside height. Default preview: 1.25 ft².
Target area requirement
airflow ÷ target velocity. Default preview: 1.286 ft².
Area difference
less area than target requires. Default preview: -0.036 ft².
Equivalent round diameter
same cross-sectional area. Default preview: 15.1 in.
Velocity ratio
actual ÷ planning target. Default preview: 103%.
Air travel time
45 ft run. Default preview: 3.75 s.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Duct area = inside width × inside height. Average velocity = airflow ÷ cross-sectional area. Required area at the target velocity = airflow ÷ target velocity.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Use the Duct Air Velocity Calculator to estimate a planning quantity from verified airflow and clear duct dimensions. It supports scope comparison, purchasing and operating-cost planning, but it does not select equipment, approve code compliance or replace project-specific design.
Calculation steps
- 1Measure or obtain each design input from the actual project and keep one consistent unit system.
- 2Enter airflow and clear duct dimensions using product data or approved design information rather than generic nominal values.
- 3Review the unrounded engineering or usage result before applying allowances, duty cycle, efficiency or tariff assumptions.
- 4Apply each adjustment once, then round only the final purchasing, equipment-count or scheduling output.
Worked numerical example
Delivering 800 cfm through a 12 in × 18 in clear duct area of 1.5 ft² produces an average velocity of about 533 fpm.
Purchasing and rounding
Use actual internal duct dimensions after liner thickness is considered. Select duct and fittings from an airflow/pressure design; velocity alone is not a purchasing specification.
Interpret the result
Average velocity is airflow divided by clear cross-sectional area. It does not show local turbulence or pressure loss, and acceptable velocity depends on noise, friction and system design criteria.
Page-specific assumptions
- Inputs describe the actual connected system or measured project scope.
- One operating condition, product rating and unit system are used per calculation.
- Allowances and efficiency losses are not already embedded in the entered values.
- Code, health, structural, electrical and hydraulic requirements are verified independently.
Common mistakes
- Using nominal equipment ratings instead of delivered performance at design conditions.
- Mixing instantaneous flow or power with daily or annual energy.
- Applying the same allowance more than once.
- Treating a quantity or cost estimate as equipment selection or professional approval.
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 Air Velocity Calculator questions
Why use inside duct dimensions?+
Air flows through the clear internal cross-section, which may be smaller than the outside dimensions because of insulation or lining.
Does lower velocity always mean better performance?+
No. Lower velocity can reduce noise and friction but requires larger ducts and must still provide adequate air distribution.
Is equivalent round diameter a full conversion?+
It only matches cross-sectional area. Equal-friction conversion for rectangular ducts uses a different relationship.