Duct reference
HVAC duct size chart
Round duct face-area lookup values for preliminary airflow and velocity checks. The chart is geometry, not a final duct-selection table.
Round duct area lookup
| Diameter (in) | Area (in²) | Area (ft²) | Velocity at 400 CFM (FPM) |
|---|---|---|---|
| 8 | 50.3 | 0.349 | 1,146 |
| 10 | 78.5 | 0.545 | 734 |
| 12 | 113.1 | 0.785 | 509 |
| 14 | 153.9 | 1.069 | 374 |
| 16 | 201.1 | 1.396 | 286 |
| 18 | 254.5 | 1.767 | 226 |
Formula and example
Area (ft²) = π × (diameter in ÷ 12)² ÷ 4
Velocity (FPM) = CFM ÷ area (ft²)
Velocity (FPM) = CFM ÷ area (ft²)
For a 12 in round duct, area is about 0.785 ft². At 400 CFM, velocity is 400 ÷ 0.785 = about 509 FPM. The table’s velocity column is an arithmetic example only; it is not a universal velocity recommendation.
How to use the chart
- Start with assigned airflow for the specific run.
- Use the selected diameter to obtain area.
- Calculate actual velocity, then check straight-run friction and fittings.
- Check the complete pressure path and actual fan performance before finalizing a size.
Limits
Final duct design includes fittings, transitions, terminals, filters, coils, leakage, sound, available fan pressure, and balancing. A round-area chart cannot replace an equal-friction, static-regain, or other documented system-design method.
FAQ
Is this a final sizing chart?
No. It is a geometry lookup. Final design also considers pressure loss, fittings, sound, fan performance, and system criteria.