The formula for determining the area of a circle is A = n.r2 Below for your convenience is a quick reference chart listing the equivalent square inches for round duct.
| Round Duct Diameter | Equivalent Square Inches |
| 3 | 7.07 |
| 4 | 12.57 |
| 5 | 19.64 |
| 6 | 28.27 |
| 7 | 38.48 |
| 8 | 50.26 |
| 9 | 63.62 |
| 10 | 78.54 |
| 12 | 113.1 |
| 14 | 153.94 |
| 16 | 201.06 |
| 18 | 254.47 |
| 20 | 314.16 |
| 22 | 380.13 |