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Engineering ToolBox > Rectangular and Circular HVAC Ducts - Equivalent Diameter
The table below can be used to compare equivalent diameters for rectangular and round circular ducts. The table is based on the ducts friction loss formula.
The rectangular dimensions and the air flow volume are adapted to the equal friction loss method of sizing ventilation duct systems. An approximate friction loss of 0.8 inches water gauge per 100 ft duct (6.6 Pa/m) is used.
| Air flow - q - (Cubic Feet per Minute, cfm) (m3/s) | Rectangular Duct Sizes (Inches) | Equivalent Diameter Round Duct Sizes - de - (Inches) | Velocity - v - (ft/min) (m/s) | Friction Loss (inches water gauge per 100 ft duct) |
| 200 (0.09) | 3 x 7 4 x 5 | 4.9 4.9 | 1527 (7.8) | 0.88 |
| 300 (0.14) | 4 x 7 5 x 6 | 5.7 6.0 | 1635 (8.3) | 0.82 |
| 400 (0.19) | 4 x 9 5 x 7 6 x 6 | 6.4 6.4 6.6 | 1736 (8.8) | 0.80 |
| 500 (0.24) | 6 x 7 | 7.1 | 1819 (9.2) | 0.78 |
| 750 (0.35) | 5 x 12 6 x 10 7 x 8 | 8.3 8.4 8.2 | 1996 (10.1) | 0.77 |
| 1000 (0.47) | 7 x 10 8 x 9 | 9.1 9.3 | 2166 (11) | 0.79 |
| 1250 (0.59) | 8 x 10 9 x 9 | 9.8 9.8 | 2386 (12.1) | 0.88 |
| 1500 (0.71) | 8 x 12 10 x 10 | 10.7 10.9 | 2358 (11.9) | 0.77 |
| 1750 (0.83) | 8 x 14 9 x 12 10 x 11 | 11.5 11.3 11.5 | 2469 (12.5) | 0.78 |
| 2000 (0.94) | 8 x 15 10 x 12 | 11.8 12.0 | 2589 (13.2) | 0.81 |
| 2500 (1.2) | 10 x 14 12 x 12 | 12.9 13.1 | 2712 (13.8) | 0.8 |
| 3000 (1.4) | 12 x 14 | 14.1 | 2767 (14.1) | 0.75 |
| 3500 (1.7) | 12 x 15 | 14.6 | 3010 (15.3) | 0.84 |
| 4000 (1.9) | 10 x 22 14 x 15 | 15.9 15.8 | 2938 (14.9) | 0.73 |
| 4500 (2.1) | 12 x 19 14 x 16 | 16.4 16.4 | 3068 (15.6) | 0.76 |
| 5000 (2.4) | 10 x 25 12 x 20 15 x 16 | 16.9 16.8 16.9 | 3248 (16.5) | 0.82 |
| 6000 (2.8) | 14 x 20 15 x 18 | 18.2 17.9 | 3358 (17.1) | 0.8 |
| 7000 (3.3) | 12 x 26 16 x 20 | 19.0 19.5 | 3482 (17.7) | 0.8 |
| 8000 (3.8) | 12 x 30 14 x 25 | 20.2 20.2 | 3595 (18.3) | 0.8 |
| 9000 (4.3) | 12 x 34 15 x 25 | 21.4 21.0 | 3671 (18.6) | 0.78 |
| 10000 (4.7) | 12 x 36 16 x 25 20 x 20 | 21.9 21.7 21.9 | 3858 (19.6) | 0.83 |
| 12500 (5.9) | 12 x 45 16 x 30 20 x 24 | 24.1 23.7 23.9 | 4012 (20.4) | 0.8 |
| 15000 (7.1) | 16 x 36 18 x 30 23 x 25 | 24.7 25.2 26.2 | 4331 (22) | 0.87 |
| 17500 (8.3) | 16 x 40 20 x 32 25 x 25 | 27.0 27.5 27.3 | 4337 (22) | 0.79 |
| 20000 (9.4) | 20 x 35 25 x 28 | 28.6 28.9 | 4483 (22.8) | 0.79 |
| 25000 (11.8) | 16 x 55 20 x 43 25 x 38 | 31.0 31.5 33.5 | 4709 (23.9) | 0.78 |
| 30000 (14.2) | 20 x 50 30 x 32 | 33.7 33.9 | 4815 (24.5) | 0.74 |
| 35000 (16.5) | 20 x 55 30 x 35 | 35.2 35.4 | 5179 (26.3) | 0.81 |
| 40000 (18.9) | 25 x 48 30 x 40 | 37.4 37.8 | 5243 (26.6) | 0.77 |
| 45000 (21.2) | 32 x 40 | 39.1 | 5397 (27.4) | 0.77 |
| 50000 (23.6) | 32 x 45 35 x 40 | 41.3 40.9 | 5222 (26.5) | 0.66 |