Duct System Design Basics
Duct systems are designed to properly distribute air throughout a building. Ducts that are not well designed result in discomfort, high energy costs, bad air quality, and increased noise levels.
Choosing the right-sized duct is critical to maintain proper air flow in the system.
Below is a simple model for determining duct sizes. The model applies to system up to 60 feet from unit to register. Because of the complexity of designing duct systems, we highly recommend using the ACCA "Manual D" for sizing ductwork, or consulting a professional HVAC contractor. Refer to the links on the left for more information.
Determine your duct size based on the required CFM (cubic feet per minute).
| C.F.M. | Heating BTU | Cooling BTU | Round Size (in.) | Rectangular Size (in.) |
| 60 | 4,400 | 1,820 | 5 | |
| 70 | 5,180 | 3,030 | 6 | |
| 100 | 7,400 | 3,640 | 6 | 3 1/4x10 |
| 120 | 8,900 | 4,390 | 7 | 3 1/4x12 |
| 145 | 10,700 | 5,450 | 7 | 3 1/4x14 |
| 180 | 13,300 | 6,360 | 8 | 6x8 |
| 210 | 15,600 | 8,200 | 8 | 6x8 |
| 270 | 20,000 | 8,800 | 9 | 8x8 |
| 290 | 21,500 | 9,100 | 9 | 8x8 |
| 300 | 22,200 | 11,200 | 9 | 8x8 |
| 370 | 27,400 | 11,800 | 10 | 10x8 |
| 390 | 28,900 | 13,900 | 10 | 10x8 |
| 460 | 34,000 | 17,000 | 10 | 12x8 |
| 560 | 41,500 | 18,800 | 12 | 14x8 |
| 620 | 45,900 | 20,000 | 12 | 14x8 |
| 660 | 48,900 | 24,200 | 12 | 16x8 |
| 800 | 59,300 | 27,300 | 12 | 18x8 |
| 900 | 66,700 | 28,200 | 14 | 20x8 |
| 930 | 68,900 | 30,300 | 14 | 20x8 |
| 1000 | 74,100 | 33,300 | 14 | 22x8 |
| 1100 | 81,500 | 36,400 | 14 | 24x8 |
| 1200 | 88,900 | 39,400 | 16 | 26x8 |
| 1300 | 96,300 | 42,400 | 16 | 28x8 |
| 1400 | 103,700 | 45,500 | 17 | 30x8 |
| 1500 | 111,100 | 51,500 | 17 | 24x10 |
| 1700 | 128,900 | 54,500 | 18 | 26x10 |
| 1800 | 133,300 | 57,600 | 18 | 26x10 |
| 1900 | 140,700 | 60,600 | 18 | 28x10 |
| 2000 | 148,100 | 66,700 | 18 | 30x10 |
The model assumes 0.1 FPM (friction rate per 100 feet of duct) for supply and return, and an average number of fittings. For one or two long branch lines, we suggest one size larger branch pipe.
