Input design conditions
Enter the system airflow and target air velocity. Select whether you want a round or rectangular duct. For rectangular ducts, choose an aspect ratio (width:height).
Sizing results
Run a calculation to see duct dimensions, velocities, and dynamic pressures. CSV and PDF export will be enabled when results are available.
| Parameter | Value | Unit | Notes |
|---|
Example duct sizing scenarios
These sample scenarios illustrate typical airflow levels, velocities, and approximate duct sizes for small comfort air-distribution systems. Use them to sanity-check your own designs.
| Case | Airflow | Velocity | Duct type | Approximate duct size |
|---|---|---|---|---|
| Small office branch | 150 CFM | 700 fpm | Round | ≈ 6 in diameter |
| Bedroom supply branch | 300 CFM | 800 fpm | Rectangular | ≈ 10 in × 5 in |
| Living room branch | 600 CFM | 900 fpm | Round | ≈ 11 in diameter |
| Main trunk section | 1200 CFM | 900 fpm | Rectangular | ≈ 22 in × 9 in |
Recommended velocities for typical duct sections
Supply branches for comfort cooling generally run between 600–900 fpm, main trunks between about 900–1200 fpm, and return or exhaust ducts somewhat lower. Use this duct size calculator together with our Airflow CFM Calculator to confirm each section’s target volume and speed.
Balancing airflow between rooms
If one room’s duct is undersized, it will receive less airflow than intended while nearby branches may be over-supplied. Combine this tool with room-by-room load estimates or an Air Changes Per Hour Calculator so every space receives the correct proportion of system airflow.
Friction rate and pressure loss
Each duct section contributes pressure loss that the fan must overcome. Higher velocities and rougher interiors increase friction. After sizing here, estimate pressure drop using an HVAC Friction Loss Calculator and confirm that fan total static pressure remains within your equipment’s capabilities.
Impact of duct size on noise
High velocities through small ducts can create turbulence, grille noise, and whistling at dampers. Oversizing reduces noise but may require more space and sheet metal. Choose sizes that balance comfort, acoustic limits, and constructability for the building you are designing.
Using rectangular ducts effectively
When architectural constraints force rectangular ducts, choose aspect ratios that avoid very flat or very tall shapes. Extreme aspect ratios can increase resistance and be harder to fabricate. This calculator reports physical dimensions and an equivalent diameter for friction and balancing checks.
Coordinating ducts with building structure
Sized ducts must still pass through joists, beams, and ceiling voids without weakening the structure. Once the calculator suggests dimensions, round to the nearest practical size that fits openings, allows insulation, and maintains acceptable velocity and friction along the full route.
Formula used
The calculator applies standard ventilation and fluid-mechanics relationships to determine the required duct size from airflow and velocity.
- Continuity equation: \( Q = V \times A \) where Q is airflow (m³/s or CFM), V is air velocity (m/s or fpm), and A is duct area (m² or ft²).
- Round duct area: \( A = \dfrac{\pi D^2}{4} \) where D is the internal diameter of the duct.
- Rectangular duct area: \( A = W \times H \) where W is width and H is height of the duct.
- Rectangular aspect ratio: \( \text{Aspect ratio} = \dfrac{W}{H} \). Given A and aspect ratio, the calculator solves for W and H.
-
Hydraulic equivalent diameter for rectangular ducts:
De = 1.30 × ( (W × H)0.625 / (W + H)0.25 )
where W and H are in metres. The equivalent diameter allows use of round-duct friction charts. - Dynamic pressure: \( p_d = \dfrac{1}{2} \rho V^2 \) using an assumed air density ρ ≈ 1.2 kg/m³, reported in Pascals and inches of water gauge.
How to use this calculator
- Decide the design airflow for the duct section. Enter the numeric value and choose the matching airflow unit (CFM, m³/h, or L/s).
- Select a design velocity appropriate for the duct type. Quieter residential branches use lower velocities; main trunks can tolerate higher values.
- Choose whether the duct will be round or rectangular. Round ducts have inherently lower pressure losses.
- For rectangular ducts, specify the desired aspect ratio. An aspect ratio of 2.0 produces a duct twice as wide as it is high.
- Click Calculate duct size. The results table will show airflow, velocity, required area, and recommended dimensions in both metric and imperial units.
- Review the hydraulic equivalent diameter and dynamic pressure. Compare velocities against your project criteria or local design guidelines to confirm suitability.
- Use the CSV button to export the table for spreadsheets, or the PDF button to create a simple report for documentation and submittals.
Frequently asked questions
What inputs do I need for this duct size calculator?
You only need three main inputs: airflow, design velocity, and duct shape. For rectangular ducts, add an aspect ratio. The calculator converts units consistently and returns area, dimensions, velocities, and dynamic pressure for the selected configuration.
What velocity is suitable for quiet supply ducts?
Comfort cooling supply branches are often sized around 600–900 fpm, with main trunks between about 900–1200 fpm. For very quiet spaces, target the lower end. Always confirm velocities against local guides and manufacturer data for diffusers and grilles.
Can I use this calculator for return and exhaust ducts?
Yes. The same continuity and area relationships apply to supply, return, and exhaust ducts. Use the airflow required for that section and a velocity appropriate to noise and contamination. For greasy or dusty exhaust, consult codes and specialist design references.
What is hydraulic equivalent diameter and why is it important?
Hydraulic equivalent diameter represents a rectangular duct as an “equivalent” round duct with similar friction characteristics. Using this value lets you apply round-duct friction charts or ductulator tools to rectangular sections without complex manual adjustments or separate lookup tables.
How accurate are these results compared with manual ductulators?
The calculator follows standard ventilation equations and unit conversions, so it matches manual ductulators when the same airflow, velocity, and friction assumptions are used. Differences usually come from rounding to nominal duct sizes or choosing different friction rates along each run.
Does duct size affect energy consumption and running cost?
Oversized ducts reduce velocity and friction, which can lower fan static pressure and energy use, but increase material cost. Undersized ducts raise pressure losses and fan power. To estimate bill impact, combine airflow estimates with our Electricity Cost Calculator.
Can I use this tool for flexible ducting layouts?
Yes, but flexible duct has higher friction and should be kept as straight and short as possible. Size the equivalent rigid duct here, then apply manufacturer guidance on maximum lengths, bends, and stretching to avoid excessive pressure drop, noise, and installation issues.