Duct Size Calculator
Calculate the duct cross-sectional area and recommended size from airflow and design velocity.
Why duct sizing matters
Duct sizing is one of the most important parts of HVAC design. If ducts are too small, air moves too fast, noise increases, and static pressure rises. If ducts are too large, installation cost goes up and equipment may not control airflow as designed. A good duct size balances efficiency, comfort, sound, and cost.
This duct size calculator gives a fast first-pass answer using the classic airflow and velocity relationship. It is perfect for planning, estimating, and sanity checking your layout before detailed design.
How this duct size calculator works
1) Calculate required duct area
The base relationship is:
Area = Airflow / Velocity
When airflow is in CFM and velocity is in FPM, area comes out in square feet. The calculator also converts to square inches for easier fabrication sizing.
2) Convert area to physical dimensions
- Round duct: computes diameter from area using circle geometry.
- Rectangular duct: uses your width:height ratio to produce matching dimensions.
- Equivalent round: gives a round comparison for pressure drop context.
3) Suggest a practical size
The raw calculated size is often not a standard manufactured size. This tool also suggests the next standard round diameter to help you move from theory to installable dimensions.
Typical velocity guidelines
Velocity targets vary by project type. Lower velocities are quieter, while higher velocities reduce duct size but can increase noise and pressure losses.
| Application | Typical Velocity Range | Notes |
|---|---|---|
| Residential branch runs | 500–800 FPM | Prioritizes comfort and low noise |
| Residential trunk lines | 700–1,000 FPM | Common compromise for size and sound |
| Commercial supply ducts | 900–1,500 FPM | Depends on acoustics and ceiling constraints |
| Industrial/high-velocity systems | 1,500+ FPM | Usually needs careful noise control |
Round vs rectangular duct
Round duct
- Lower pressure drop for the same area
- Often quieter at equal airflow
- Good for long runs and energy efficiency
Rectangular duct
- Fits tight ceiling or wall spaces
- Easy to coordinate around structural constraints
- Can have higher friction and more noise if too flat
When using rectangular duct, avoid extreme aspect ratios where possible. Very wide-flat ducts usually increase friction and can worsen airflow distribution.
Quick example
Suppose you need 1,200 CFM at 900 FPM:
- Required area = 1200 / 900 = 1.33 ft²
- In square inches: 1.33 × 144 = 192 in²
- Equivalent round diameter is about 15.6 in, so a standard 16 in round duct is typically selected.
Important design limitations
This tool provides a practical sizing estimate, not a complete Manual D or full duct design. Final design should also check:
- Total external static pressure (TESP)
- Friction rate and fitting losses
- Duct length, transitions, elbows, dampers, and terminals
- Balancing, zoning, and sound attenuation
- Local code requirements and manufacturer data
Final thoughts
A reliable duct size calculator helps you make faster and better HVAC decisions. By combining airflow targets with realistic velocity goals, you can quickly estimate duct dimensions that improve comfort, reduce noise, and support efficient system operation.