Duct Sizing Chart — CFM by Round Duct Size & Friction Rate
How much air a rigid round metal duct carries at each design friction rate — the duct-calculator wheel as a table. Find your available friction rate, read down to the first size that covers the airflow, and sanity-check the resulting velocity. Computed from the Darcy-Weisbach equation at ASHRAE standard air in galvanized duct; rectangular ducts convert via the equal-friction equivalents chart, and flex duct has its own, lower numbers.
Round galvanized duct — CFM capacity by friction rate
| Duct Ø | 0.05"/100 ft | 0.06"/100 ft | 0.08"/100 ft | 0.10"/100 ft | 0.15"/100 ft | 0.20"/100 ft | FPM @ 0.10 |
|---|---|---|---|---|---|---|---|
| 4" | 25 | 30 | 35 | 35 | 45 | 55 | 425 |
| 5" | 45 | 50 | 60 | 70 | 85 | 100 | 495 |
| 6" | 75 | 85 | 100 | 110 | 140 | 160 | 565 |
| 7" | 115 | 125 | 150 | 170 | 210 | 245 | 625 |
| 8" | 165 | 180 | 210 | 240 | 300 | 350 | 685 |
| 9" | 225 | 250 | 290 | 330 | 410 | 480 | 745 |
| 10" | 300 | 330 | 385 | 435 | 545 | 635 | 800 |
| 12" | 485 | 535 | 625 | 710 | 880 | 1,030 | 900 |
| 14" | 735 | 810 | 945 | 1,070 | 1,330 | 1,550 | 1,000 |
| 16" | 1,045 | 1,155 | 1,350 | 1,525 | 1,895 | 2,210 | 1,090 |
| 18" | 1,430 | 1,580 | 1,845 | 2,080 | 2,585 | 3,015 | 1,180 |
| 20" | 1,895 | 2,090 | 2,440 | 2,750 | 3,420 | 3,985 | 1,260 |
| 22" | 2,440 | 2,690 | 3,145 | 3,540 | 4,400 | 5,125 | 1,340 |
| 24" | 3,075 | 3,390 | 3,955 | 4,460 | 5,535 | 6,445 | 1,420 |
| 26" | 3,800 | 4,190 | 4,890 | 5,510 | 6,835 | 7,960 | 1,495 |
| 28" | 4,625 | 5,100 | 5,950 | 6,700 | 8,310 | 9,675 | 1,565 |
| 30" | 5,550 | 6,120 | 7,140 | 8,040 | 9,970 | 11,605 | 1,640 |
How to use this chart
Work out the friction rate your system can afford first: the fan's rated external static pressure, minus the pressure drops of everything that isn't duct — coil, filter, balancing dampers, registers — divided by the total effective length of the longest run (fittings counted as equivalent length), times 100. Then read this chart at that rate. The duct size calculator runs the same math for exact flows, the friction loss chart answers the inverse question (what a known duct loses at a known flow), and the duct equivalent chart converts each round size to rectangular options.
Keep an eye on the velocity column: capacity at 0.20 in/100 ft may be inside the friction budget yet loud in a ceiling. Recommended velocities by application are on the air velocity chart.
What this chart assumes — and when it stops being valid
Every cell is computed, not transcribed: Darcy-Weisbach with the Colebrook friction factor, galvanized duct at 0.0003 ft absolute roughness, standard air at 0.075 lb/ft³. That matches the classic duct-calculator within a few percent — published tables vary slightly with the roughness and joint assumptions each publisher used. It stops being valid for flex duct and ductboard (rougher), high altitude or hot airstreams (lighter air), and fittings (their losses are separate, as equivalent lengths). Verify final designs against the equipment data and your mechanical code.
Common questions
How many CFM can a 6 inch duct handle?
Rigid 6" round metal duct carries about 110 CFM at the classic 0.10 in. w.g. per 100 ft design rate (about 565 FPM), or roughly 100 CFM at 0.08. Flex duct carries meaningfully less at the same rate — use the flex duct chart, not this one.
What friction rate should I size ducts at?
The 0.10 in/100 ft column is the classic duct-calculator default and a fair commercial starting point. Real design derives the rate from what's available: take the fan's rated external static, subtract component losses (coil, filter, registers), and divide by the effective run length — many residential systems end up sizing at 0.05–0.08, which is why the chart carries those columns.
What size duct do I need for 1000 CFM?
At 0.10 in/100 ft, 1,000 CFM wants a duct between 12" (710 CFM) and 14" (1,070 CFM) — so 14" round, running about 940 FPM. At a tighter 0.06 rate it takes 16". Rectangular equivalents come from the duct equivalent chart, not from matching area.
Is this chart valid for flex duct?
No. This chart is computed for rigid galvanized round duct at standard air. Flex duct — even fully stretched — has a rougher wall and carries less air at the same friction rate, and compressed flex is drastically worse. Ductboard and lined duct also run higher friction. Size those from their own data.
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