Duct Size Calculator
Formula Used
Area: A = Q ÷ V
Round diameter: D = √(4A ÷ π)
Rectangular height: H = √(A ÷ ratio)
Rectangular width: W = H × ratio
Equivalent diameter: De = 1.30 × (W × H)0.625 ÷ (W + H)0.25
Velocity pressure: VP = (V ÷ 4005)²
Friction rate: FR = 0.109136 × Q1.9 ÷ D5.02
Total pressure drop: ΔP = FR × total equivalent length ÷ 100
How to Use This Calculator
- Enter the required airflow for the duct section.
- Select the airflow unit used by your design data.
- Enter a target duct velocity.
- Choose round or rectangular duct sizing.
- Add duct length and fitting equivalent length.
- Set friction and velocity limits for checking.
- Press the calculate button.
- Review the result above the form.
- Download CSV or PDF for your records.
Example Data Table
| Airflow | Target Velocity | Approximate Area | Approximate Round Diameter | Common Use |
|---|---|---|---|---|
| 400 CFM | 700 FPM | 0.571 sq ft | 10.2 in | Small branch duct |
| 800 CFM | 800 FPM | 1.000 sq ft | 13.5 in | Medium branch duct |
| 1200 CFM | 900 FPM | 1.333 sq ft | 15.6 in | Main supply run |
| 2000 CFM | 1000 FPM | 2.000 sq ft | 19.2 in | Larger trunk duct |
Duct Sizing Guide
A duct size calculator helps plan air movement before metal is cut. It turns airflow and velocity targets into usable dimensions. The main idea is simple. Airflow needs enough open area. Smaller ducts raise speed. Larger ducts lower speed. Good sizing balances comfort, noise, pressure loss, and available space.
Why Duct Size Matters
Ducts are part of the air delivery path. A weak size choice can reduce room airflow. It can also increase fan load. High velocity may create noise at grilles and branches. Low velocity may waste ceiling space and raise material cost. This tool gives quick design numbers, so early decisions become easier. It does not replace a project specification. It gives a clear mathematical starting point.
Round and Rectangular Options
Round ducts are efficient because their boundary is smooth and compact. Rectangular ducts fit better in tight chases, walls, and ceilings. The calculator estimates both forms. For rectangular ducts, the aspect ratio controls width compared with height. A high ratio makes the duct wide and shallow. A low ratio keeps it closer to square. Equivalent diameter helps compare rectangular and round airflow behavior.
Friction and Velocity Checks
Friction loss is important for fan selection. The calculator estimates pressure drop for a straight duct length. It also includes extra equivalent length for elbows, transitions, dampers, and fittings. The result can be compared with a target friction rate. A warning appears when velocity or friction is higher than your preferred limit. These checks help identify ducts that may need a larger size.
Using the Results
Start with required airflow. Choose the unit that matches your design data. Enter target velocity and limits. Select duct shape. Add length and fitting allowance. Press calculate. Review area, diameter, sides, velocity pressure, total equivalent length, and pressure drop. Export the result when you need a record. Use the example table to compare common airflow ranges. For final construction, confirm values with local codes, equipment data, and a qualified professional. Site conditions, insulation, leakage, grille losses, and balancing methods can change the best final size. Keep records for each revision. Compare several velocities before choosing one. This habit helps spot practical layouts and supports clearer conversations with installers during final reviews.
FAQs
What is a duct size calculator?
It is a tool that estimates duct area, diameter, rectangular sides, velocity, and pressure loss from airflow and design limits.
Which airflow unit can I use?
You can enter airflow in CFM, liters per second, or cubic meters per second. The calculator converts values internally.
How is round duct diameter calculated?
The tool divides airflow by velocity to find area. It then converts that area into an equal round diameter.
How is rectangular duct size calculated?
The calculator uses the selected aspect ratio. It finds height from area, then multiplies height by the ratio for width.
What is equivalent diameter?
Equivalent diameter compares a rectangular duct with a round duct. It helps estimate friction behavior for rectangular sections.
Why does velocity matter?
Velocity affects noise, pressure loss, and comfort. Very high velocity can create noise and raise fan energy demand.
What is fitting equivalent length?
It is an added length allowance for elbows, transitions, dampers, and other fittings that create extra resistance.
Can this replace professional duct design?
No. It gives mathematical estimates. Final duct design should consider codes, equipment data, balancing, leakage, and site conditions.