Metric: A(m²)=(m³/h)/(3600×m/s), then convert to mm².
| Airflow (CFM) | Velocity (FPM) | Area (ft²) | Round Dia (in) | Rect Size (in) |
|---|---|---|---|---|
| 600 | 700 | 0.86 | 12.5 | 15.7 × 7.9 |
| 1000 | 800 | 1.25 | 15.1 | 19.0 × 9.5 |
| 1500 | 900 | 1.67 | 17.5 | 21.9 × 11.0 |
| 2500 | 1000 | 2.50 | 21.4 | 26.8 × 13.4 |
| 4000 | 1200 | 3.33 | 24.7 | 31.0 × 15.5 |
- Select a unit system that matches your project drawings.
- Enter total airflow for the run and your target velocity.
- Choose the duct shape and, if rectangular, a sizing method.
- Optionally add leakage and safety allowances for design margin.
- Pick rounding rules, then press Calculate.
- Use the CSV or PDF buttons to save the result sheet.
Airflow and velocity targets
Airflow is usually established from room loads, ventilation rates, or equipment schedules. Once total flow is known, velocity targets control noise and friction. Many projects keep main supply runs around 900–1400 FPM, branches 600–1000 FPM, returns 500–900 FPM, and exhaust 800–1500 FPM. Higher velocities reduce duct size but increase pressure drop and breakout noise.
Allowances that protect performance
The calculator applies leakage and safety allowances to form an effective airflow. A 5% leakage allowance can be practical on long runs with multiple joints, while a 5–15% safety factor is often used to cover future tenant changes or conservative air balance. Using both allowances compounds the flow, so keep them intentional and documented.
Choosing round or rectangular geometry
Round ducts provide more area per perimeter, typically improving energy efficiency for the same airflow and velocity. Rectangular ducts fit tight plenums and shafts, but very flat sections can raise friction and require reinforcement. Aspect ratios near 1.5:1 to 3:1 are commonly selected to balance constructability and pressure loss; extreme ratios should be avoided unless space demands it.
Rounding and standard availability
Exact calculated dimensions rarely match stocked sizes. Rounding up by an increment (for example 1 inch or 25 mm) maintains capacity and keeps fabrication simple. Selecting the next standard round size is useful when spiral duct catalogs govern procurement. After rounding, review the achieved velocity to ensure it remains within your target range.
Interpreting results and field checks
Use the required area as a quick sanity check across alternatives. If achieved velocity is much lower than intended, the duct may be oversized and harder to balance; if much higher, expect higher fan power and potential noise. Confirm clearances, insulation thickness, and fitting transitions, and coordinate final sizes with dampers, diffusers, and access panels before issuing shop drawings. Where codes specify minimum ventilation, confirm outside air fractions and economizer modes. For long ducts, consider velocity-pressure losses in fittings and use balancing dampers near terminals as required for verification.
FAQs
1) What airflow unit should I use?
Use the same unit system as your project. Imperial expects CFM and FPM. Metric expects m³/h and m/s. Switching units only changes inputs and display; the sizing logic remains consistent.
2) Why does the calculator show achieved velocity?
Rounding to practical sizes changes area. Achieved velocity confirms whether the selected size still meets your design intent. If it is far above target, expect higher noise and pressure loss.
3) When should I add leakage allowance?
Add leakage allowance when long duct runs, many joints, or lower sealing classes are expected. Keep the value modest and based on your specification so the allowance does not hide real leakage issues.
4) Is round always better than rectangular?
Round often has lower friction for the same area, but rectangular may be required for space constraints. Choose the shape that fits clearances, insulation, and access needs, then confirm pressure loss using your duct design method.
5) What aspect ratio is recommended for rectangular ducts?
Many teams aim near 1.5:1 to 3:1 for a practical balance. Very flat ducts can increase friction, noise, and reinforcement needs. Use the aspect ratio option to explore alternatives quickly.
6) Do I still need a full duct pressure calculation?
Yes. This tool sizes by airflow and velocity only. Final design should include fittings, transitions, elbows, and terminal losses, plus fan selection and balancing strategy to meet total external static pressure.