Enter Round Duct Data
Formula Used
- Round duct area: A = πD² ÷ 4
- Equal area size: H = √(A ÷ r), and W = rH
- Equivalent round diameter: De = 1.30(WH)0.625 ÷ (W + H)0.25
- Equal friction by aspect ratio: H = D(r + 1)0.25 ÷ [1.30r0.625], and W = rH
- Hydraulic diameter: Dh = 4A ÷ P
- Air velocity: V = Q ÷ A
- Velocity pressure estimate: VP = (V ÷ 4005)²
Use the same length unit through the duct formulas. Airflow is converted to CFM for velocity checks.
How to Use This Calculator
- Enter the round duct diameter.
- Select the dimension unit.
- Choose equal friction, equal area, fixed size, or custom size mode.
- Enter an aspect ratio when the mode needs it.
- Add airflow to check duct velocity.
- Add an allowance for insulation or clearance when needed.
- Press the calculate button.
- Download the result as CSV or PDF.
Example Data Table
| Round Diameter | Method | Aspect Ratio | Rectangular Size | Planning Note |
|---|---|---|---|---|
| 8 in | Equal area | 1.5 : 1 | 8.68 in × 5.79 in | Useful for quick area matching. |
| 10 in | Equal friction | 2 : 1 | 13.13 in × 6.56 in | Better for equivalent diameter checks. |
| 12 in | Equal area | 2 : 1 | 15.04 in × 7.52 in | Keeps the same cross sectional area. |
| 16 in | Equal friction | 3 : 1 | 26.28 in × 8.76 in | Good for shallow ceiling spaces. |
| 20 in | Equal friction | 2.5 : 1 | 29.67 in × 11.87 in | Check velocity before final selection. |
Round Duct to Rectangular Conversion Guide
Basic Purpose
Round duct is common in small systems. Rectangular duct is common above ceilings. A conversion helps when space is limited. It also helps when a project needs flat, wide, or shallow duct runs. The key is to keep airflow steady. You should also control velocity and pressure loss.
Why Equivalent Size Matters
A duct is not only a shape. It is also a path for moving air. If the new rectangular duct is too small, velocity rises. Noise can increase. Static pressure can rise. Fans may work harder. If the duct is too large, material costs can rise. Air balance may also change. A good conversion gives a practical size with similar flow behavior.
Choosing A Method
The equal area method keeps the same cross sectional area. It is simple and useful for quick planning. The equal friction method uses an equivalent diameter equation. It often gives a rectangular size that behaves closer to the original round duct. Fixed width and fixed height modes are helpful in real buildings. They let you check a size that fits a ceiling, wall, chase, or equipment opening.
Using Aspect Ratio
Aspect ratio is width divided by height. A low ratio is usually better for airflow. Very wide ducts can have more surface contact. That may increase friction. Many designers try to keep ratios moderate. This calculator allows larger values for testing. It also reports velocity and area change, so you can judge the result.
Airflow And Velocity Checks
Airflow is optional, but it improves the result. Enter CFM, liters per second, or cubic meters per hour. The tool converts flow and estimates air speed. High velocity can create noise. Low velocity can require more space. The warning limit helps you compare the result against your project target.
Practical Notes
Always confirm final duct sizes with project standards. Add room for insulation, hangers, turning vanes, dampers, and access panels. Round elbows and rectangular fittings can create extra loss. Use the calculator for planning and comparison. Final design should follow local codes and professional HVAC practice. Save each trial so future layout choices stay easy to review and share.
FAQs
What does this calculator convert?
It converts a round duct diameter into rectangular duct dimensions. It can use equal area, equal friction, fixed width, fixed height, or custom size checking.
Which method should I use?
Use equal area for quick sizing. Use equal friction when you want a rectangular duct that better matches the round duct’s equivalent flow behavior.
What is aspect ratio?
Aspect ratio is width divided by height. A 20 inch by 10 inch duct has a 2:1 aspect ratio. Lower ratios are often better.
Can I enter a fixed width?
Yes. Select fixed width mode. The calculator will solve the required height using equal area. This helps with ceiling or chase limits.
Can I check airflow velocity?
Yes. Enter airflow in CFM, L/s, or m³/h. The calculator reports round velocity and rectangular velocity for comparison.
What is equivalent round diameter?
It is the round duct diameter that gives similar flow behavior to a rectangular duct. The calculator uses a standard equivalent diameter equation.
Does allowance change airflow size?
No. Allowance only adds extra outer space for insulation or clearance. It does not change the internal airflow dimensions.
Is this enough for final duct design?
Use it for planning and comparison. Final duct design should also consider fittings, pressure loss, balancing, noise, codes, and professional project standards.