Duct Sizing Chart
Round and rectangular duct airflow at 0.08 and 0.10 in. wg per 100 ft.
Each figure is the most air the duct carries at or below the friction rate at the top of its column, from the Duct Size Calculator's own model, rounded down to the nearest 5 cfm. Velocity in fpm is under or beside each airflow.
Basis: standard air, inside dimensions, straight duct, friction per 100 ft. Fittings, joints, liner and installation are not included. Flex that sags or is compressed carries less air than shown. For any other size, airflow, friction rate or material, use the calculator.
Round duct
| Round duct | Metal | Flex | ||
|---|---|---|---|---|
| 0.08 in. wg | 0.10 in. wg | 0.08 in. wg | 0.10 in. wg | |
| 4 in. | 30 (344) | 35 (401) | 20 (229) | 25 (286) |
| 5 in. | 60 (440) | 65 (477) | 40 (293) | 50 (367) |
| 6 in. | 95 (484) | 110 (560) | 70 (357) | 80 (407) |
| 7 in. | 145 (543) | 165 (617) | 110 (412) | 120 (449) |
| 8 in. | 210 (602) | 240 (688) | 155 (444) | 175 (501) |
| 9 in. | 290 (656) | 330 (747) | 215 (487) | 240 (543) |
| 10 in. | 385 (706) | 435 (798) | 290 (532) | 320 (587) |
| 12 in. | 630 (802) | 710 (904) | 470 (598) | 525 (668) |
| 14 in. | 950 (889) | 1075 (1006) | 710 (664) | 795 (744) |
| 16 in. | 1360 (974) | 1535 (1099) | 1015 (727) | 1140 (816) |
| 18 in. | 1860 (1053) | 2095 (1186) | 1390 (787) | 1560 (883) |
| 20 in. | 2460 (1128) | 2775 (1272) | 1845 (846) | 2065 (947) |
| 22 in. | 3165 (1199) | 3570 (1352) | 2375 (900) | 2660 (1008) |
| 24 in. | 3990 (1270) | 4495 (1431) | 2995 (953) | 3350 (1066) |
| 26 in. | 4930 (1337) | 5555 (1507) | 3705 (1005) | 4145 (1124) |
| 28 in. | 5995 (1402) | 6755 (1580) | 4505 (1054) | 5045 (1180) |
| 30 in. | 7195 (1466) | 8105 (1651) | 5410 (1102) | 6055 (1234) |
| 32 in. | 8535 (1528) | 9605 (1720) | 6420 (1149) | 7185 (1286) |
| 36 in. | 11640 (1647) | 13105 (1854) | 8770 (1241) | 9815 (1389) |
Rectangular metal duct
Find the short side down the left and the long side across the top. A rectangle is charted at its circular equivalent (Huebscher), which carries the same air at the same friction rate; it is not the rectangle of the same area, which would be too small. Shapes over 4:1 are left blank.
| Short × long, in. | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 24 | 30 | 36 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 6 | 125500 | 180540 | 240576 | 305610 | 365626 | 430645 | 495660 | 560672 | 690690 | ||
| 8 | 270608 | 360648 | 460690 | 555714 | 655737 | 760760 | 860774 | 1065799 | 1380828 | ||
| 10 | 490706 | 625750 | 765787 | 905815 | 1045836 | 1190857 | 1485891 | 1935929 | 2385954 | ||
| 12 | 800800 | 980840 | 1165874 | 1355903 | 1545927 | 1935968 | 25351014 | 31351045 | |||
| 14 | 1205885 | 1440926 | 1680960 | 1920987 | 24151035 | 31701087 | 39401126 | ||||
| 16 | 1725970 | 20101005 | 23101040 | 29101091 | 38401152 | 47901198 | |||||
| 18 | 23551047 | 27051082 | 34251142 | 45401211 | 56751261 | ||||||
| 20 | 31151121 | 39551187 | 52601262 | 65951319 | |||||||
| 24 | 50501263 | 67551351 | 85101418 |
| Short × long, in. | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 24 | 30 | 36 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 6 | 140560 | 205615 | 275660 | 345690 | 415711 | 485728 | 560747 | 630756 | 780780 | ||
| 8 | 305686 | 410738 | 520780 | 630810 | 740833 | 855855 | 970873 | 1205904 | 1555933 | ||
| 10 | 555799 | 705846 | 860885 | 1020918 | 1180944 | 1345968 | 16751005 | 21801046 | 26901076 | ||
| 12 | 905905 | 1105947 | 1315986 | 15301020 | 17451047 | 21851093 | 28551142 | 35351178 | |||
| 14 | 1360999 | 16251045 | 18951083 | 21651113 | 27201166 | 35751226 | 44401269 | ||||
| 16 | 19451094 | 22701135 | 26001170 | 32801230 | 43301299 | 53951349 | |||||
| 18 | 26551180 | 30501220 | 38601287 | 51151364 | 63901420 | ||||||
| 20 | 35101264 | 44551337 | 59251422 | 74251485 | |||||||
| 24 | 56901423 | 76051521 | 95851598 |
How to read it
Choose the friction rate first. 0.08 to 0.10 in. wg per 100 ft is common for low-velocity comfort ductwork, but noise or the fan's available static pressure often governs. Then check the velocity: the design criteria on the calculator page give the usual ranges for supply, return and main trunks.
These charts are an estimating aid and do not replace engineering judgment.