DrFixIt! Equivalent HVAC Duct Sizes By CFMA table showing typical minimum rectangular and round HVAC duct dimensions required to deliver specific airflows. 

Cubic Feet per Minute 
Rectangular Duct Sizes Side Dimensions in Inches 
Round Duct Sizes Diameter in Inches 
100  3 X 4  4 
200  3 X 7 4 X 5 
5 
300  4 X 7 5 X 6 
6 
400  4 X 9 5 X 7 6 X 6 
7 
500  6 X 7  8 
750  5 X 12 6 X 10 7 X 8 
9 
1000  7 X 10 8 X 9 
10 
1250  8 X 10 9 X 9 
10 
1500  8 X 12 10 X 10 
12 
1750  8 X 14 9 X 12 10 X 11 
12 
2000  8 X 15 10 X 12 
12 
2500  10 X 14 12 X 12 
14 
3000  12 X 14  14 
3500  12 X 15  15 
4000  10 X 22 14 X 15 
16 
4500  12 X 19 14 X 16 
17 
5000  10 X 25 12 X 20 15 X 16 
17 
6000  14 X 20 15 X 18 
19 
7000  12 X 26 16 X 20 
20 
8000  12 X 30 14 X 25 
21 
9000  12 X 34 15 X 25 
22 
10000  12 X 36 16 X 25 20 X 20 
23 
12500  12 X 45 16 X 30 20 X 24 
24 
15000  16 X 36 18 X 30 23 X 25 
26 
17500  16 X 40 20 X 32 25 X 25 
28 
20000  16 X 45 20 X 35 25 X 28 
30 
25000  16 X 55 20 X 43 25 X 38 
32 
30000  20 X 50 30 X 32 
34 
35000  20 X 55 30 X 35 
36 
40000  25 X 48 30 X 40 
38 
45000  25 X 25 32 X 40 
40 
50000  32 X 45 35 X 40 
42 