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

Choosing the right-sized duct is critical to maintain proper air flow in the system.

Above 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).