Sizing result
Alternative size comparison
Nearby standard sizes help compare velocity and estimated pressure loss.
| Option | Size | Area | Velocity | Friction rate | Total loss | Comment |
|---|---|---|---|---|---|---|
| Calculate to populate this table. | ||||||
Branch duct schedule
| Branch | Airflow | Suggested round size | Velocity |
|---|---|---|---|
| Use Generate branch schedule. | |||
Formula used
The calculator converts airflow to cubic meters per second. Velocity sizing uses area = airflow ÷ velocity. Round diameter follows from the circular area. Rectangular equal-friction sizing uses the common equivalent-diameter relationship. Pressure loss uses Darcy-Weisbach with an estimated air density and surface roughness. Fitting losses use K × dynamic pressure. These calculations support preliminary HVAC sizing and comparison work.
How to use this calculator
Choose your unit system, airflow, duct shape, and sizing method. Equal friction uses the friction-rate target to find an equivalent round diameter. Velocity sizing uses your target air speed. Manual mode evaluates dimensions you enter. Add straight length, fittings, material, temperature, altitude, and any extra component allowance. Press Calculate duct size. Review velocity, friction rate, effective length, and pressure loss before choosing a final duct size.
Duct sizing guide
Correct duct sizing balances airflow delivery, pressure loss, available space, and sound. Small ducts increase velocity and friction. That can raise fan power and create objectionable noise. Oversized ducts reduce resistance but consume more building space and material. Designers often begin with a target friction rate or velocity, then check the entire duct path.
Round ducts usually provide efficient airflow for a given cross-sectional area. Rectangular ducts fit shallow ceilings and wall cavities more easily. Their aspect ratio still matters. Very flat rectangular ducts can increase surface contact, resistance, and installation difficulty. Keeping the aspect ratio moderate is usually preferable.
Fittings can contribute a large share of system pressure loss. Elbows, tees, dampers, grilles, filters, and transitions all disturb airflow. This tool estimates those effects with common loss coefficients and equivalent-length allowances. Real equipment data should replace generic assumptions whenever available.
Temperature and altitude influence air density. Lower density reduces dynamic pressure at the same velocity. Flexible duct also behaves differently from smooth metal duct because corrugations increase resistance. Installation quality therefore matters as much as nominal size.
Use the comparison table instead of selecting dimensions from one number alone. A nearby standard size may produce a better compromise between velocity and pressure loss. Final systems should be verified against project requirements, equipment fan data, local codes, and accepted HVAC design practice. This calculator is best used for preliminary design, education, and quick duct schedule checks.
Typical design reference
| Application | Typical velocity range | Typical friction target | Notes |
|---|---|---|---|
| Residential supply | 600–900 ft/min | 0.06–0.10 in. w.g./100 ft | Balance noise and compact routing. |
| Residential return | 500–700 ft/min | 0.05–0.08 in. w.g./100 ft | Lower velocity often reduces noise. |
| Commercial supply | 900–1,500 ft/min | 0.08–0.15 in. w.g./100 ft | Project standards may vary widely. |
| Exhaust | 700–1,200 ft/min | 0.08–0.12 in. w.g./100 ft | Contaminant transport may set minimums. |
Frequently asked questions
What is duct sizing?
Duct sizing selects dimensions that carry required airflow while controlling velocity, pressure loss, noise, and space usage.
What CFM should a duct carry?
Required CFM comes from room loads, ventilation requirements, equipment capacity, and the airflow assigned to each branch.
What duct velocity is recommended?
Recommended velocity depends on building type and location. Main commercial ducts often run faster than residential branches.
How is rectangular duct size calculated?
A designer can size rectangular ducts from required area or match an equivalent round diameter while controlling aspect ratio.
What is equivalent duct diameter?
Equivalent diameter represents a round duct expected to produce similar friction behavior to a rectangular duct at comparable airflow.
Why does duct pressure loss matter?
Pressure loss determines how much static pressure the fan must overcome to move design airflow through the system.
Is larger ductwork always better?
No. Larger ducts reduce resistance but cost more, occupy more space, and may create low velocities in some applications.