Use this calculator to estimate practical round or rectangular duct sizes for everyday ventilation projects. Enter airflow, target velocity, shape preference, and duct details to see required area, likely dimensions, velocity, and pressure loss in one place.
Duct size input form
Example data table
| Case | Airflow | Target Velocity | Preferred Shape | Suggested Size | Use Case |
|---|---|---|---|---|---|
| 1 | 120 CFM | 500 FPM | Round | 180 mm round | Quiet bathroom exhaust |
| 2 | 250 CFM | 700 FPM | Rectangular | 275 × 150 mm | Small room supply run |
| 3 | 600 CFM | 900 FPM | Round | 300 mm round | Main branch for living area |
Formula used
Area = Airflow ÷ Target Velocity
Diameter = √(4 × Area ÷ π)
Height = √(Area ÷ Aspect Ratio), Width = Height × Aspect Ratio
Dh = 2ab ÷ (a + b)
ΔP/L = f × (ρV² ÷ 2) ÷ Dh, using Reynolds number and roughness for the friction factor.
How to use this calculator
- Enter the airflow needed for the room, fan, or branch run.
- Choose the airflow and velocity units you normally use.
- Select round or rectangular duct as your preferred shape.
- For rectangular ducts, enter a practical width-to-height ratio.
- Add duct length, roughness, density, viscosity, and any airflow safety margin.
- Press Calculate duct size to see the results above the form.
- Review size, area, actual velocity, Reynolds number, and estimated pressure loss.
- Use the CSV and PDF buttons to save the result summary.
Frequently asked questions
1. What does this duct size calculator estimate?
It estimates required duct area, likely round or rectangular size, actual velocity, Reynolds number, friction factor, and pressure drop for a simple ventilation layout.
2. Should I choose round or rectangular ducts?
Round ducts usually deliver lower resistance and less noise. Rectangular ducts fit tight spaces better, especially near ceilings, soffits, and boxed wall runs.
3. Why does velocity matter?
Velocity affects noise, resistance, and comfort. Higher velocity can shrink duct size, but it often raises pressure loss and can make the system louder.
4. What is a good aspect ratio for rectangular ducts?
Many practical layouts stay near 1:1 to 4:1. Very flat ducts can fit tight spaces, but resistance usually increases compared with round ducts.
5. Why is pressure drop shown?
Pressure drop helps you judge how hard the fan must work. Lower values generally support quieter, more efficient airflow through the duct run.
6. Can I use this for home exhaust and supply runs?
Yes. It works well for quick checks on bathroom exhaust, kitchen branches, utility rooms, and simple supply or return air runs.
7. Does insulation change the duct size result?
The airflow math stays the same, but insulation can affect outside dimensions, installation clearance, condensation risk, and delivered air temperature.
8. Are these results final for every project?
No. Use them as a design estimate. Final sizing should also consider fittings, elbows, terminals, fan curves, codes, and balancing needs.