Air flow Rectangular Duct Sizes

(Inches) Round Duct Sizes

(Cubic Feet per Minute, cfm)

(Inches)

200 (0.09)3 x 7 4.9

4 x 5 4.9

300 (0.14)4 x 7 5.7

5 x 6 6

4 x 9 6.45 x 7 6.4

6 x 6 6.6

6 x 7 7.1

5 x 12 8.36 x 10 8.4

7 x 8 8.2

7 x 10 9.1

8 x 9 9.3

8 x 10 9.8

9 x 9 9.8

8 x 12 10.7

10 x 10 10.9

8 x 14 11.59 x 12 11.3

10 x 11 11.5

8 x 15 11.8

10 x 12 12

10 x 14 12.9

12 x 12 13.1

12 x 14 14.1

12 x 15 14.6

10 x 22 15.9

14 x 15 15.8

12 x 19 16.4

14 x 16 16.4

10 x 25 16.912 x 20 16.8

15 x 16 16.9

14 x 20 18.2

15 x 18 17.9

12 x 26 19

16 x 20 19.5

12 x 30 20.2

Equivalent Diameter

- q - 

- de -

(m3/s)

400  (0.19)

500  (0.24)

750  (0.35)

1000  (0.47)

1250  (0.59)

1500  (0.71)

1750 (0.83)

2000 (0.94)

2500 (1.2)

3000 (1.4)

3500 (1.7)

4000 (1.9)

4500 (2.1)

5000 (2.4)

6000 (2.8)

7000 (3.3)

8000 (3.8)

14 x 25 20.2

12 x 34 21.4

15 x 25 21

12 x 36 21.916 x 25 21.7

20 x 20 21.9

12 x 45 24.116 x 30 23.7

20 x 24 23.9

16 x 36 24.718 x 30 25.2

23 x 25 26.2

16 x 40 2720 x 32 27.5

25 x 25 27.3

20 x 35 28.6

25 x 28 28.9

16 x 55 3120 x 43 31.5

25 x 38 33.5

20 x 50 33.7

30 x 32 33.9

20 x 55 35.2

30 x 35 35.4

25 x 48 37.4

30 x 40 37.8

32 x 40 39.1

32 x 45 41.3

35 x 40 40.9

8000 (3.8)

9000 (4.3)

10000 (4.7)

12500 (5.9)

15000 (7.1)

17500 (8.3)

20000 (9.4)

25000 (11.8)

30000 (14.2)

35000 (16.5)

40000 (18.9)

45000 (21.2)

50000 (23.6)

(ft/min)

(m/s)

15270.88

-7.8

16350.82

-8.3

1736

0.8-8.8

18190.78

-9.2

1996

0.77-10.1

21660.79

-11

23860.88

-12.1

23580.77

-11.9

2469

0.78-12.5

25890.81

-13.2

27120.8

-13.8

27670.75

-14.1

30100.84

-15.3

29380.73

-14.9

30680.76

-15.6

3248

0.82-16.5

33580.8

-17.1

34820.8

-17.7

35950.8

Velocity in equivalent round Duct

Friction Loss 

- v - (Inches water gauge per 100 ft

duct)

-18.30.8

36710.78

-18.6

3858

0.83-19.6

4012

0.8-20.4

4331

0.87-22

4337

0.79-22

44830.79

-22.8

4709

0.78-23.9

48150.74

-24.5

51790.81

-26.3

52430.77

-26.6

53970.77

-27.4

52220.66

-26.5