Duct Size Chart
Round duct diameter and velocity for 25 to 5,000 CFM at four friction rates, with rectangular equivalents.
How to use ↓Duct size chart
Round duct, galvanized sheet metal, standard air
ductchart.com
Round diameter and velocity by CFM
| CFM | 0.05 in. w.g./100 ft | 0.08 in. w.g./100 ft | 0.10 in. w.g./100 ft | 0.15 in. w.g./100 ft | ||||
|---|---|---|---|---|---|---|---|---|
| Dia in. | fpm | Dia in. | fpm | Dia in. | fpm | Dia in. | fpm | |
| 25 | 4.0 | 284 | 3.6 | 345 | 3.5 | 378 | 3.2 | 446 |
| 50 | 5.2 | 341 | 4.7 | 413 | 4.5 | 453 | 4.1 | 534 |
| 75 | 6.0 | 379 | 5.5 | 459 | 5.2 | 503 | 4.8 | 593 |
| 100 | 6.7 | 408 | 6.1 | 494 | 5.8 | 541 | 5.4 | 638 |
| 150 | 7.8 | 453 | 7.1 | 548 | 6.8 | 601 | 6.2 | 708 |
| 200 | 8.7 | 487 | 7.9 | 590 | 7.5 | 646 | 6.9 | 761 |
| 250 | 9.4 | 516 | 8.6 | 624 | 8.2 | 684 | 7.5 | 805 |
| 300 | 10.1 | 540 | 9.2 | 654 | 8.8 | 716 | 8.1 | 843 |
| 400 | 11.2 | 581 | 10.2 | 703 | 9.8 | 769 | 9.0 | 906 |
| 500 | 12.2 | 614 | 11.1 | 743 | 10.6 | 813 | 9.8 | 958 |
| 600 | 13.1 | 643 | 11.9 | 778 | 11.4 | 851 | 10.5 | 1002 |
| 800 | 14.6 | 691 | 13.2 | 836 | 12.7 | 914 | 11.7 | 1076 |
| 1000 | 15.8 | 730 | 14.4 | 883 | 13.8 | 966 | 12.7 | 1137 |
| 1200 | 17.0 | 764 | 15.4 | 924 | 14.8 | 1011 | 13.6 | 1190 |
| 1500 | 18.5 | 808 | 16.8 | 976 | 16.0 | 1068 | 14.8 | 1257 |
| 2000 | 20.6 | 867 | 18.7 | 1048 | 17.9 | 1146 | 16.5 | 1348 |
| 2500 | 22.4 | 916 | 20.4 | 1107 | 19.5 | 1210 | 17.9 | 1424 |
| 3000 | 24.0 | 958 | 21.8 | 1157 | 20.8 | 1265 | 19.2 | 1488 |
| 4000 | 26.7 | 1027 | 24.3 | 1241 | 23.2 | 1357 | 21.4 | 1596 |
| 5000 | 29.1 | 1085 | 26.5 | 1310 | 25.3 | 1433 | 23.3 | 1684 |
Rectangular equivalents (equal friction)
| Round in. | Width (in.) for a height of | |||||
|---|---|---|---|---|---|---|
| 4 in. | 6 in. | 8 in. | 10 in. | 12 in. | 14 in. | |
| 4 | 4 | 3 | 2 | 2* | 2* | 2* |
| 5 | 6 | 4 | 3 | 3 | 3 | 2* |
| 6 | 8 | 6 | 4 | 4 | 3 | 3* |
| 7 | 11 | 7 | 6 | 5 | 4 | 4 |
| 8 | 15 | 10 | 7 | 6 | 5 | 5 |
| 9 | 20* | 12 | 9 | 7 | 6 | 6 |
| 10 | 26* | 15 | 11 | 9 | 8 | 7 |
| 12 | 40* | 23 | 16 | 13 | 11 | 9 |
| 14 | 59* | 33* | 22 | 17 | 14 | 12 |
| 16 | 83* | 45* | 30 | 23 | 19 | 16 |
| 18 | 113* | 60* | 40* | 30 | 24 | 20 |
| 20 | 148* | 78* | 51* | 37 | 30 | 25 |
| 24 | 239* | 125* | 80* | 58* | 45 | 37 |
* Aspect ratio over 4:1. Widths are rounded up to the next whole inch. Round diameters are exact, not rounded to stock sizes. Velocity is at that diameter.
Calculated at build time from the equations, not copied from another chart: Darcy-Weisbach friction with the Colebrook friction factor (ASHRAE Duct Systems Design Guide), galvanized roughness 0.0005 ft, standard air 0.075 lb/ft³. Rectangular sizes by the Huebscher equal-friction equation (ASHRAE Fundamentals). Estimates only: they can differ from your chart or the manufacturer's data by 5% or more (flex by 2 times or more), so verify accuracy for your own application against the design documents, manufacturer data and local code.
How to use
- Find your CFM down the left side.
- Read across to your friction rate. The first number is the round diameter in inches, the second is the velocity in fpm.
- For a rectangular duct, find the round size in the second table and read the width under your height.
- Press Print chart. It prints on one page, Letter or A4.
About this duct size chart
This duct sizing chart lists the round diameter that carries each airflow at friction rates of 0.05, 0.08, 0.10 and 0.15 in. w.g. per 100 ft, for galvanized sheet metal, along with the velocity in that diameter. Diameters are exact, so round up to the next stock size on the job. For anything between the rows, or for duct board and flex, use the duct size calculator.
The numbers are generated from the same equations as the calculator: Darcy-Weisbach with the Colebrook friction factor, galvanized roughness 0.0005 ft and standard air. They are not copied from a printed chart, so they can differ a little from the ductulator in your truck. Results can differ from another chart by 5 percent or more either way.
The second table gives equal-friction rectangular sizes. Pick the height your space allows and read off the width, rounded up to a whole inch. An asterisk marks sizes over 4:1, which cost more sheet metal and tend to leak and make noise. For another height or the reverse conversion, use the round to rectangular converter.
Example: 400 CFM at 0.08 in. w.g. per 100 ft needs a 10.2 in. round duct, moving 703 fpm. The next stock size is 12 in. For a rectangular run with an 8 in. height, the second table gives an 11 in. width.
FAQ
Which friction rate does the chart use?
It shows four: 0.05, 0.08, 0.10 and 0.15 in. w.g. per 100 ft. Lower rates give bigger, quieter ducts. See the friction rate guide for how to choose.
Is the rectangular size the same speed as the round?
No. The equivalent size matches friction at the same airflow, not velocity. The rectangle has more area, so the air moves slower than in the round duct.
Does the chart work for flex or duct board?
No, it is for galvanized sheet metal. Duct board and flex are rougher, so they need a bigger diameter for the same friction rate. Use the flex duct calculator or the main calculator.