CFM Duct Sizing Chart
At the standard residential friction rate of 0.1 in. w.g. per 100 ft, 100 CFM needs a 6 inch round duct, 400 CFM needs 10 inch, 1000 CFM needs 14 inch, and 2000 CFM needs 18 inch. The full duct CFM chart is below, with rectangular equivalents and the actual velocity and friction for each size so you can see how much headroom you have.
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Duct size by CFM
Assumptions, so you can check this against whatever you normally use:
- Friction rate 0.1 in. w.g. per 100 ft, equal friction method
- Galvanized steel, absolute roughness 0.0003 ft
- Standard air, 70°F at sea level
- Round sizes limited to what is actually stocked: every inch to 10, even inches above that
- Rectangular sizes are Huebscher equivalents, never smaller than required, 4:1 aspect ratio or less
| CFM | Round | Velocity fpm |
Friction in. w.g./100 ft |
Rectangular | Low profile |
|---|---|---|---|---|---|
| 25 | 4" | 286 | 0.049 | 4 x 4 | — |
| 50 | 5" | 367 | 0.057 | 6 x 4 | 8 x 4 |
| 75 | 6" | 382 | 0.049 | 6 x 6 | 12 x 4 |
| 100 | 6" | 509 | 0.082 | 6 x 6 | 12 x 4 |
| 125 | 7" | 468 | 0.058 | 8 x 8 | 16 x 4 |
| 150 | 7" | 561 | 0.081 | 8 x 8 | 16 x 4 |
| 175 | 8" | 501 | 0.056 | 8 x 8 | 16 x 4 |
| 200 | 8" | 573 | 0.071 | 8 x 8 | 16 x 4 |
| 250 | 9" | 566 | 0.060 | 10 x 10 | 18 x 6 |
| 300 | 9" | 679 | 0.083 | 10 x 10 | 18 x 6 |
| 350 | 10" | 642 | 0.066 | 10 x 10 | 24 x 6 |
| 400 | 10" | 733 | 0.084 | 10 x 10 | 24 x 6 |
| 450 | 12" | 573 | 0.043 | 12 x 12 | 24 x 6 |
| 500 | 12" | 637 | 0.052 | 12 x 12 | 24 x 6 |
| 600 | 12" | 764 | 0.073 | 12 x 12 | 24 x 6 |
| 700 | 12" | 891 | 0.096 | 12 x 12 | 24 x 6 |
| 800 | 14" | 748 | 0.058 | 14 x 14 | 30 x 8 |
| 900 | 14" | 842 | 0.072 | 14 x 14 | 30 x 8 |
| 1000 | 14" | 935 | 0.087 | 14 x 14 | 30 x 8 |
| 1200 | 16" | 859 | 0.063 | 16 x 16 | 30 x 8 |
| 1400 | 16" | 1003 | 0.084 | 16 x 16 | 30 x 8 |
| 1600 | 18" | 905 | 0.060 | 18 x 18 | 30 x 10 |
| 1800 | 18" | 1019 | 0.075 | 18 x 18 | 30 x 10 |
| 2000 | 18" | 1132 | 0.091 | 18 x 18 | 30 x 10 |
| 2500 | 20" | 1146 | 0.082 | 20 x 20 | 30 x 12 |
| 3000 | 22" | 1136 | 0.072 | 22 x 22 | 30 x 14 |
| 3500 | 22" | 1326 | 0.096 | 22 x 22 | 30 x 14 |
| 4000 | 24" | 1273 | 0.081 | 22 x 22 | 30 x 18 |
How to read this
Find your CFM in the first column and take the round size next to it. That is the smallest stocked size that stays at or under the friction rate, so it is the cheapest duct that does the job rather than the biggest one that fits.
The velocity column is the one worth a second look. Anything much over 900 fpm in a house starts to be audible in a quiet room, so if a run passes a bedroom and the number is high, go up one size. The friction column shows how close that row is to the 0.1 limit, which tells you whether you have room to add a fitting or two before you are over.
Rectangular duct CFM chart
The two rectangular columns are different answers, not the same answer twice. The first is the closest to square, which is quieter and uses less metal. The second is the flattest size that still moves the air, for when joist depth decides the question for you.
Take the 10 inch row. Its two rectangular answers are 10 x 10 and 24 x 6. Both carry the same air. The second one is 4.0:1 instead of 1.0:1, so it fits a shallower joist bay and costs you more sheet metal and a little more noise for the privilege.
Do not size rectangular duct by equal area
A rectangular duct has more wall for the same amount of air inside it, so it drags more. The right conversion is the Huebscher equivalent diameter, which matches friction and capacity. Matching cross-sectional area instead gives you a duct that is too small, every time.
Here is what that costs. A 10 inch round duct has a cross section of about 79 square inches. A 10 x 8 rectangle is 80 square inches, near enough the same box. But its equivalent diameter is only 9.8 inches, not 10. Size it that way and the run is one size light before you have installed a single fitting. The rectangular columns in the chart above are never smaller than required, which is why they sometimes look generous.
Both columns stay at or under a 4:1 aspect ratio. Past that the surface area climbs fast enough that the friction numbers on this page stop describing the duct you actually built, and SMACNA and ACCA both stop at the same place.
Where these numbers come from
Every value is calculated, not copied. Friction comes from the Darcy-Weisbach equation with the Haaland friction factor, which is the explicit form of Colebrook used in ASHRAE Fundamentals Chapter 34. Rectangular equivalents use the Huebscher equation, the same relationship built into a physical Ductulator.
That matters because the duct charts floating around as supplier PDFs disagree with each other, usually because they assume different roughness or a different friction rate and do not say so. The assumptions for this one are listed above the table.
These charts are free and always will be. If you also need to turn the job into a signable quote and an invoice, that is the paid tool this site pays for itself with. See what it costs.
Duct sizing questions
What size duct do I need for 400 CFM?
A 10 inch round duct, or 10 x 10 rectangular, carries 400 CFM at 0.1 in. w.g. per 100 ft with a velocity of 733 fpm. If joist depth is tight, 24 x 6 has the same equivalent diameter in a lower profile.
How many CFM can a 6 inch duct handle?
About 100 CFM at the standard 0.1 in. w.g. per 100 ft friction rate, at 509 fpm. You can push more air through it, but the friction and the noise both climb quickly once you do.
What friction rate should I use for residential duct sizing?
0.1 in. w.g. per 100 ft is the everyday convention and it is what this chart uses. On a real system the correct rate comes from the available static pressure divided by the total effective length, which is what Manual D works out.
Why is the rectangular size not the same area as the round size?
A rectangular duct has more surface area than a round duct of the same cross section, so it has more friction. The sizes here use the Huebscher equivalent diameter, which matches friction and capacity rather than area. Sizing rectangular duct by equal area gives you a duct that is too small.
What size rectangular duct do I need for 1000 CFM?
14 x 14 at 0.1 in. w.g. per 100 ft, which is the 14 inch round size converted by equivalent diameter. If joist depth will not take it, 30 x 8 carries the same air in a flatter box at the cost of more metal and a little more noise.