HVAC Duct Size Calculator
Size round HVAC ducts from CFM airflow and velocity. Find the minimum diameter and next standard duct size in inches.
HVAC Duct Size Calculator
Typical: 600-900 FPM (supply), 400-600 FPM (return)
Min Diameter
6.8"
Std Duct Size
7"
Capacity at 7"
214
CFM
200 CFM at 800 FPM → 6.77" min → use 7" duct
Standard Round Duct Capacity (at 800 FPM)
| Diameter | Area (in²) | Max CFM | Rooms |
|---|---|---|---|
| 4" | 12.6 | 70 | ~1 rooms |
| 5" | 19.6 | 109 | ~1 rooms |
| 6" | 28.3 | 157 | ~2 rooms |
| 7" | 38.5 | 214 | ~2 rooms |
| 8" | 50.3 | 279 | ~3 rooms |
| 9" | 63.6 | 353 | ~4 rooms |
| 10" | 78.5 | 436 | ~4 rooms |
| 12" | 113.1 | 628 | ~6 rooms |
| 14" | 153.9 | 855 | ~9 rooms |
| 16" | 201.1 | 1117 | ~11 rooms |
| 18" | 254.5 | 1414 | ~14 rooms |
| 20" | 314.2 | 1745 | ~17 rooms |