Reference
Duct Size Chart (CFM by Round and Rectangular Duct)
How much air each common duct size carries, from the same ASHRAE friction relation the duct calculators use, with rectangular equivalents and the fitting allowances.
How the chart is calculated
Capacities are for smooth galvanized round duct, from the ASHRAE friction fit used by all of this site’s duct calculators. Pick the friction rate from the available static pressure and the equivalent length of the longest run, then check the velocity: the duct must satisfy both.
Flex duct has more friction: count its length at 1.5 times, and keep it stretched and supported.
Round duct capacity
| Round duct | Area | CFM at 0.08 in wc per 100 ft | CFM at 0.10 in wc per 100 ft | CFM at 700 FPM (branch limit) | CFM at 900 FPM (trunk limit) |
|---|---|---|---|---|---|
| 4 in | 12.6 sq in | 33 | 37 | 61 | 79 |
| 5 in | 19.6 sq in | 60 | 67 | 95 | 123 |
| 6 in | 28.3 sq in | 97 | 109 | 137 | 177 |
| 7 in | 38.5 sq in | 145 | 163 | 187 | 241 |
| 8 in | 50.3 sq in | 207 | 232 | 244 | 314 |
| 9 in | 63.6 sq in | 282 | 317 | 309 | 398 |
| 10 in | 78.5 sq in | 372 | 419 | 382 | 491 |
| 12 in | 113.1 sq in | 603 | 678 | 550 | 707 |
| 14 in | 153.9 sq in | 906 | 1,019 | 748 | 962 |
| 16 in | 201.1 sq in | 1,290 | 1,450 | 977 | 1,257 |
| 18 in | 254.5 sq in | 1,760 | 1,980 | 1,237 | 1,590 |
| 20 in | 314.2 sq in | 2,325 | 2,615 | 1,527 | 1,963 |
| 22 in | 380.1 sq in | 2,991 | 3,364 | 1,848 | 2,376 |
| 24 in | 452.4 sq in | 3,764 | 4,233 | 2,199 | 2,827 |
A duct’s capacity is the smaller of its friction figure and its velocity figure. Up to 10 in, friction usually limits; from 12 in up, velocity does.
Rectangular equivalents
| Rectangular | Equivalent round | CFM at 0.08 | Rectangular | Equivalent round | CFM at 0.08 |
|---|---|---|---|---|---|
| 3¼ × 10 in (wall stack) | 6.0 in | 97 | 8 × 20 in | 13.5 in | 820 |
| 3¼ × 14 in (wall stack) | 6.9 in | 142 | 8 × 24 in | 14.6 in | 1,015 |
| 6 × 8 in | 7.6 in | 178 | 10 × 10 in | 10.9 in | 471 |
| 6 × 10 in | 8.4 in | 235 | 10 × 12 in | 12.0 in | 598 |
| 6 × 12 in | 9.1 in | 294 | 10 × 16 in | 13.7 in | 861 |
| 6 × 14 in | 9.8 in | 353 | 10 × 20 in | 15.2 in | 1,133 |
| 8 × 8 in | 8.7 in | 261 | 10 × 24 in | 16.5 in | 1,409 |
| 8 × 10 in | 9.8 in | 350 | 12 × 12 in | 13.1 in | 763 |
| 8 × 12 in | 10.7 in | 441 | 12 × 16 in | 15.1 in | 1,108 |
| 8 × 14 in | 11.5 in | 534 | 12 × 20 in | 16.8 in | 1,467 |
| 8 × 16 in | 12.2 in | 628 | 12 × 24 in | 18.3 in | 1,834 |
Velocity limits
| Location | Limit |
|---|---|
| Supply trunk | 900 FPM |
| Supply branch | 700 FPM |
| Return duct | 600 FPM |
| Return grille face | 400 to 500 FPM (300 with a filter in the grille) |
| Supply register face | 500 to 700 FPM |
Fitting equivalent lengths
| Fitting | Equivalent length |
|---|---|
| 90° elbow, metal | 15 ft |
| 90° elbow, flex | 30 ft |
| 45° elbow | 10 ft |
| Trunk takeoff | 35 ft |
| Register boot | 35 ft |
| Return grille box | 30 ft |
| Roof or wall cap | 30 ft |
These are indicative values in the range of the ACCA Manual D fitting groups; a Manual D design uses the specific fitting.
Airflow rules of thumb
- 400 CFM per ton of cooling (350 in humid climates, 450 in dry ones).
- About 1.67 to 2 sq ft of return grille per ton at 300 to 400 FPM.
- One supply register for each 100 to 180 CFM, depending on size and throw.
Calculators that use this chart
- Airflow: duct size, duct velocity, equivalent length, CFM per room, return grille size, register size
- Costs and installation: duct leakage loss, filter cost, ductwork cost, replacement cost
- Planning: zoning planner
The guide to duct sizing basics walks through a full sizing.